Chapter 3: Pharmacokinetics & Pharmacodynamics Fundamentals

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Welcome back to the Deep Dive.

Today we are stripping away the external layers of health care, the bedside manner, the charting, the diagnosis, and we are going to look at the engine room.

We're talking about the invisible chemical narrative that takes place inside a patient every single time you hand them a paper cup with a pill in it.

It really is a narrative.

I love that you call it that.

It's the story of a journey.

Yeah.

And frankly, for a nurse, it is one of the most critical stories you need to understand because once that pill goes past the lips, you can't see it anymore.

It's gone.

It's gone.

But you are still responsible for what it does.

Exactly.

I want to set the scene for you listening.

We are specifically targeting that learner persona today, the nursing student or, you know, the clinician who wants to sharpen their saw.

Picture this.

You're standing at the bedside of a patient.

Let's call Mr.

Jones.

You have a tiny white tablet.

Mr.

Jones swallows it with a sip of water.

For him, the event is over.

Done.

But for the drug, the work is just beginning.

What happens in that black box between the moment he swallows and the moment his pain actually goes away or, heaven forbid, the moment he has a toxic reaction?

That mystery in the middle is exactly what we are decoding today.

We are analyzing Chapter 3, pharmacokinetics and pharmacodynamics from the text pharmacology,

a patient -centered nursing process approach.

These are two absolute giants of pharmacology and they sound intimidating.

Pharmacodynamics.

They do, but they are the pillars of safe practice.

I mean, if you don't respect these two giants, you are flying blind.

You really are.

So before we get lost in the weeds, let's strip these words down to their roots.

I always need a memory trick.

Me too.

Pharmacokinetics has kinetics in it, like kinetic energy.

Movement.

Precisely.

Pharmacokinetics is simply what the body does to the drug.

It's the movement of the drug through the system.

How does the body take it in, move it around, chew it up, and eventually spit it out?

We break that down into four specific phases.

Absorption, distribution, metabolism, and excretion.

Okay,

so pharmacokinetics is the body bullying the drug, pushing it around.

That's a great way to put it, yeah.

Then we have pharmacodynamics.

That has dynamics in it.

Power.

Change.

Right.

Pharmacodynamics is the opposite.

It's what the drug does to the body.

Once it arrives at its destination, how does it actually change the physiology?

How does it lower the heart rate?

How does it kill the bacteria?

That is the response.

Our mission today is to decode chapter three strictly.

We aren't bringing in outside theories.

We are looking at the text, the figures, and the tables provided to us.

That's right.

The goal is to move beyond just memorizing definitions.

We want you to understand the logic of this movement, because if you understand the logic, you can predict what happens when things go wrong.

And in nursing, things always go wrong.

Always.

Kidneys fail, livers get tired, diets change.

If you understand the mechanism, you can catch a medication error before it becomes a sentinel event.

Let's start at the starting line.

Phase one of pharmacokinetics.

Absorption.

Absorption.

This is the movement of the drug into the bloodstream after administration.

Now, the text highlights that about 80 % of all drugs are taken by mouth.

Enteral administration.

Enteral administration, right.

Which seems simple enough.

Swallow, dissolve, absorb.

But the text points out it's not just flop, fizz, relief.

Oh, far from it.

If you take a solid drug like a tablet or a capsule, it has to go through a dissolution phase.

Okay.

It must disintegrate into small particles and combine with liquid to form a solution.

If it doesn't dissolve, it can't cross the lining of the GI tract.

It just goes right through you.

It just passes right through and ends up in the toilet.

No effect whatsoever.

This is where we get into the actual ingredients of the pill.

I think a lot of people, maybe even new students, assume a 500 -milligram Tylenol is just a solid block of 500 milligrams of medicine.

Right.

It's just pure drug.

But it's not.

If it were, it might be a tiny pile of unstable dust.

Tablets contain excipients.

These are fillers and inert substances.

Things like simple syrup, vegetable gums, honey, elixirs.

They're used to give the drug size, shape, and stability.

It's like the delivery vehicle.

It's the car the drug rides in.

Exactly.

But here's where the chemistry gets really cool.

Sometimes those excipients are functional.

The text mentions penicillin.

Penicillin, in its raw form, is actually poorly absorbed in the GI tract.

Why is that?

Because gastric acid destroys it.

The stomach is a hostile environment.

It's an acid bath.

Very, very hostile.

So pharmaceutical companies add salts to the penicillin.

You'll see labels like penicillin potassium or penicillin sodium.

Oh, I've seen that.

Those ions, potassium or sodium, are excipients that buffer the drug, allowing it to be absorbable.

It protects it.

So the salt acts like a shield or a facilitator.

It helps it survive the acid and dissolve where it needs to.

Speaking of that acid bath, the acidity of the stomach plays a huge role in how fast things happen.

How so?

Generally, drugs disintegrate and absorb faster in acidic fluids.

We're talking a pH of one or two.

Which is a healthy, normal adult stomach.

Right.

But think about your patient population.

Who might not have a pH of one or two?

Hmm.

The text specifically mentions the very young and older adults.

Correct.

Both infants and older adults tend to have less gastric acidity.

Their stomach pH is higher, more alkaline.

So what does that mean in practice?

It means if you have an older adult patient,

their absorption of drugs that rely on that acid environment is going to be slower.

The drug just sits there longer, waiting to break down.

That's a really important variable for a nurse to keep in mind.

You're wondering, why hasn't the pain med kicked in yet?

Exactly.

Well, maybe their stomach pH is slowing down the disintegration.

It could be as simple as that.

Now on the flip side, sometimes we don't want the drug to dissolve in the stomach.

We have those shiny, hard -coated pills and taric -coated.

This is a massive safety concept in the chapter.

And taric -coated, or EC drugs, are designed with a specific armor, as you put it.

A little suit of armor.

Yep.

They resist disintegration in the harsh acid of the stomach.

They're designed to wait.

Wait for what?

They wait until they hit the alkaline environment of the small intestine to dissolve.

Why make them wait?

What's the point?

Two main reasons.

One, the drug might be really irritating to the stomach lining.

Lining aspirin is a classic example.

It can cause ulcers if it sits in the stomach.

And two, the stomach acid might destroy the drug before it can even work.

Okay, so this brings us to the safety alert that was practically blinking neon red in the text.

It should be.

Crushing pills.

Do not crush in taric -coated or sustained release capsules.

Period.

Full stop.

I feel like we need to unpack the why here, because in the real world, you have a patient who can't swallow well, and the instinct is, I'll just crush it and put it in applesauce.

Why is that so dangerous?

If you crush an enteric -coated pill,

you have physically destroyed that armor.

You are releasing the entire drug in the wrong place, the stomach, and at the wrong time.

So you could cause, what, gastric bleeding?

Severe gastric bleeding.

Or in the case of sustained release capsules, you are taking a dose meant to be released over 12 or 24 hours, and you are dumping it into the system all at once.

That's what they call dose dumping.

Exactly.

It leads to instant toxicity.

Yeah.

You have fundamentally changed the pharmacokinetics of the drug by crushing it.

You've broken the entire design.

So we've got the drug dissolving.

Now it needs to get into the blood.

The text compares the speed of absorption based on the route.

Right.

We know 4 is instant because you're putting it directly in the blood, but what about the others?

Let's follow the blood flow.

Blood flow is the highway for absorption.

The more blood flow to an area, the faster the absorption.

It's that simple.

So let's compare muscle versus fat.

Muscle wins every time.

Intramuscular or IM injections are generally faster than subcutaneous or SQ because muscles have significantly better blood flow.

Makes sense.

But even within muscles, location matters.

The text notes that the deltoid muscle in the arm has higher blood flow than the gluteus maximus in the buttocks.

So a shot in the arm hits faster than a shot in the rear.

Generally, yes.

And subcutaneous tissue fat has less blood flow than muscle, so it's slower.

Which isn't always a bad thing.

Not at all.

It makes absorption more predictable and steady, which is why we use it for things like insulin or heparin.

We want that slow, steady release.

And then there is the rectal route.

Usually the slowest of the bunch.

The rectum has a smaller surface area and no villi, those little finger -like projections that help absorption in the small intestine.

Right.

Plus the composition of the suppository base matters.

It's just not a reliable route for speed.

Okay, let's follow the oral drug again.

It survives the stomach, it gets to the small intestine, it crosses the membrane, it's in the bloodstream.

Is it home free?

Not yet.

And this is one of the most critical concepts in oral medication.

The first pass effect.

I like to think of this as the liver toll booth.

That's a great way to picture it.

It's perfect.

Anatomically, oral drugs travel from the intestinal lemon into the portal vein.

It acts like a detour.

It goes straight to the liver.

It does not go to the heart or the brain or the big toe yet.

It goes to the liver first.

And the liver is the body's detox center.

It is.

The liver treats the drug like a foreign invader.

It metabolizes or breaks down some of that drug into an inactive form before it lets it pass into the general circulation.

So if I take 100 milligrams of a drug, the liver might eat 20 milligrams of it right off the bat.

Exactly.

Or it might eat 80 milligrams.

This brings us to the concept of bioavailability.

Bioavailability.

It's the percentage of the administered drug that is actually available for activity.

So for IV drugs?

Bioavailability is 100 percent.

It bypasses the liver toll booth initially.

It goes straight into the systemic circulation.

But for oral drugs?

It is always less than 100 percent because of that first pass effect.

Always.

The text gives the really stark comparison here that I love.

Look at rosevastatin, a common cholesterol drug.

Its bioavailability is only 20 percent.

Wow.

Wait, so you swallow the pill and 80 percent is lost to first pass metabolism?

Roughly, yes.

Only 20 percent survives the toll booth to actually do the work.

Compare that to digoxin, which has a bioavailability of 75 to 95 percent.

Why does this matter to the nurse standing there with the cup of water?

It feels like a manufacturer problem, not my problem.

Ah, but it becomes a nurse problem when the patient's physiology changes.

Think about liver dysfunction.

If your patient has a sick liver cirrhosis, hepatitis,

the toll booth is unmanned.

The guard is asleep.

The guard is asleep.

The liver isn't breaking down the drug efficiently.

So let's play that out with rosevastatin.

If the liver is sick instead of destroying 80 percent.

Maybe it only destroys 40 percent or zero percent.

So instead of getting the expected 20 percent into the blood, the patient gets 60 percent or 100 percent.

Exactly.

You have effectively tripled or quintupled the dose without changing the pill count.

That is a huge clinical implication.

If the liver isn't working, bioavailability goes up and toxicity risk skyrocket.

Precisely.

That is why we check liver enzymes.

We aren't just being thorough.

We are checking if the toll booth is working.

Okay, let's move to phase two.

Distribution.

The drug has made it past the liver.

It's in the general circulation.

Now it needs to catch a ride to the tissues.

Distribution is the movement from circulation to body tissues.

And again, blood flow is king.

Organs with high perfusion, the heart, liver, kidneys, they get the drug fast.

And the others?

Muscle, fat, bone, they get us slower.

But there is a massive complication here.

The text talks about protein binding.

And I really want to dive into the analogy of the taxicabs here because it explains so much.

It's the perfect analogy.

In your plasma, you have protein circulating.

The big one is albumin.

Think of albumin as a fleet of yellow taxicabs constantly circulating in the blood.

Okay, I'm picturing the taxis cruising down the bloodstream.

Many drugs want a ride in the taxi.

When a drug molecule attaches to albumin, it is protein bound.

And here's the key point the text emphasizes.

If the drug is in the taxi, can it do any work?

No.

That is the crucial concept.

The portion of the drug bound to protein is inactive.

It's just a passenger.

It's stuck in the cab.

It cannot leave the blood vessel to get to the receptor.

It cannot exert a pharmacological effect.

So it's just circling the city.

Just circling.

Only the free drug, the drug that is not bound to the protein, is active.

That's the drug that can exit the blood vessel and do the job.

So different drugs have different affinities for these taxis.

Some like the taxi more than others.

Yes.

We have what the text calls highly protein bound drugs.

It lists diazepam, warfarin, ibuprofen.

These are drugs that are more than 90 % bound.

90%.

So if you take warfarin, 99 % of it is riding in the taxi.

Only 1 % is actually keeping your blood thin.

That's right.

That sounds incredibly inefficient, but the dosing accounts for it, right?

The 5 -milligram tablet assumes 99 % is wasted in the taxi.

The standard dose assumes that 99 % is bound, yes.

But here's where it gets dangerous.

What happens if you take 200 pily protein bound drugs at the same time?

They fight for the taxis.

They compete.

There aren't enough seats.

Let's say you have a patient on warfarin, which is 99 % bound, and you give them furosmide, which is 95 % bound.

They fight for the seat on the albumin.

So if warfarin gets kicked out of the taxi?

Suddenly you don't have 1 % free warfarin.

Maybe you have 2 % or 3%.

Which numerically sounds so small.

1 % to 2%.

Who cares?

But functionally, you have just increased the active potency by 100%.

Oh, wow.

You have doubled the effective dose instantly.

And now the patient is at risk for massive bleeding.

Not because they took more pills, but because the drugs wrestled for the albumin.

Exactly.

This is the competition effect.

And it's a huge source of drug interactions.

There is another scenario the text mentions, which ties back to our patient's overall health.

What if there aren't enough taxis to be dim with?

Hypoalbuminemia.

Low aldium.

And this happens in liver disease, kidney disease, or importantly in malnutrition.

Malnutrition.

If you have an older adult patient who basically survives on tea and toast,

their protein levels might be low.

Fewer taxis means?

Fewer places for the drug to bind.

Which means more free drug floating around.

And more free drug means higher risk of toxicity.

Correct.

So if you have a malnourished older adult taking a highly protein -bound drug, like warfarin,

you better be monitoring them closely.

A normal dose might be an overdose for them simply because they lack the protein to carry it safely.

That connects perfectly to the case study mentioned in the text.

The patient is on warfarin and then gets prescribed valproic acid.

Two highly protein -bound drugs.

Right.

The nursing problem there is the risk for injury due to bleeding, because those drugs are going to brawl for the albumin sites.

Exactly.

It's a chemical wrestling match in the blood and the patient is the one who gets hurt.

Before we leave distribution, we have to talk about the barriers.

The body has some gated communities where it doesn't want drugs to go.

The blood -brain barrier.

The BBB is a tight junction of cells lining the blood vessels in the brain.

It's designed to keep 98 % of foreign substances out.

It protects the brain from toxins.

So what gets in?

What's the secret password?

Highly lipid -soluble drugs and drugs with low molecular weight.

Benzodiazepines, for example.

They slide right through the lipid membrane.

And what bounces off?

Water -soluble drugs or protein -bound drugs.

They can't get through the gate.

This is great for protecting the brain, but it makes it tough if you're trying to treat a brain infection, I imagine.

Correct.

It makes treating CNS conditions like meningitis or brain cancer very challenging.

You have to pick a drug that has the key to the gate.

The other barrier isn't really a barrier at all.

The placenta.

Right.

We used to think the placenta was a shield.

It's not.

Drugs cross the placenta.

The text breaks down the risk by trimester, and this is vital for patient education.

Let's walk through the trimesters.

First trimester.

The risk is spontaneous abortion.

This is when the organs are forming organogenesis, so major structural damage can occur.

Second trimester.

Still risk of spontaneous abortion or teratogenesis, which is a fancy word for birth defects.

And third trimester.

Altered fetal growth and development.

The takeaway for the nurse is clear.

Teach breastfeeding or pregnant patients to consult their provider before taking anything.

Not just prescriptions.

No.

Even OTCs, even herbs.

The assumption should be everything crosses.

Okay.

Phase three.

Metabolism.

Also known as biotransformation.

The drug has done its job, or at least it's been distributed, and now the body wants to get rid of it.

We're back to the liver.

The liver is the primary site of metabolism.

We mentioned the first pass effect, but metabolism is ongoing.

How does it do it?

The liver uses this massive enzyme system called the cytochrome P450 system.

It sounds like a robot from a sci -fi movie.

It acts like a machine.

These enzymes convert drugs.

Usually they take a lipid soluble drug, which the body likes to hold onto in fat, and turn it into a water soluble substance.

Why water soluble?

Because the kidneys are the exit, and the kidneys can't excrete fats well.

They excrete water.

So the liver preps the trash for the kidneys to take out.

It makes the drug water soluble, so it can leave in the urine.

Now, usually metabolism breaks a drug down, makes it inactive, but there is a plot twist that blew my mind when I first read it.

Pro drugs.

Yes, pro drugs are fascinating.

These are drugs that are inactive until the liver metabolizes them.

So you swallow a useless chemical.

And the liver turns it into the medicine.

The text mentions codeine.

Right.

Codeine is a pro drug.

On its own, codeine actually has very little pain relieving activity.

But when the liver enzymes,

specifically an enzyme called CYP2D6 work on it, they convert codeine into morphine.

Wait, really?

The liver turns codeine into morphine?

It does.

And the morphine is what actually hits the receptors and stops the pain.

So if your liver isn't working, or if you genetically lack that specific enzyme.

Codeine might not work for you at all.

No conversion, no active drug.

It just floats around doing nothing.

Let's talk about time.

Half -life.

We see the symbol T1 half.

Half -life is the time it takes for the amount of drug in the body to be reduced by half.

It dictates how often we give a drug.

The text gives an ibuprofen example.

Let's walk through the math, because I think seeing the numbers drop helps visualize the curve.

Okay.

Ibuprofen has a half -life of about two hours.

Let's say you take 200 milligrams.

Okay.

In two hours, 50 % is gone.

You have 100 milligrams left.

Right.

In another two hours, so four hours total, 50 % of that is gone.

You have 50 milligrams left.

In another two hours, six hours total, you have 25 milligrams left.

And at eight hours,

12 .5 milligrams.

So it's not a straight line down.

It's a curve that flattens out.

It never quite hits zero mathematically.

It's a curve, exactly.

Yeah.

And generally, the rule of thumb is that it takes about four half -lives to clear a drug from the body significantly.

This also works in reverse for building up a drug, right?

The steady state.

Yes.

Steady state occurs when the amount of drug you take equals the amount you eliminate.

You want the level to be consistent.

A plateau where the therapeutic effect is constant.

And that also takes about four half -lives.

It also takes about four half -lives to reach that steady state.

But what if we can't wait?

What if the patient is seizing now?

That's when we use a loading dose.

Imagine a drug like phenytoin.

It has a half -life of 22 hours.

Wow.

If we waited for four half -lives to get to steady state, we'd be waiting almost four days.

You can't wait four days for seizure control.

No.

So you front load it.

You give a large initial dose, a loading dose, to fill up the tank immediately.

Get them in that therapeutic range quickly.

Then you switch to a smaller maintenance dose to keep it topped off.

Phase four, excretion, the exit strategy.

The main route is the kidneys.

They are the primary exit.

They filter out free drugs, water soluble drugs, and unchanged drugs.

The text brings up urine pH again here.

We talked about stomach pH for absorption, but urine pH matters for excretion.

It does.

It's all about chemistry, specifically something called ion trapping.

Acidic urine eliminates weak bases.

Alkaline urine eliminates weak acids.

The example given is aspirin toxicity.

Aspirin salicylate is a weak acid.

Right.

So if someone overdoses on aspirin, we want to get it out fast.

We can give them IV sodium bicarbonate.

Which does what?

This makes the urine alkaline a pH of eight or higher.

Because the urine is alkaline, it acts like a magnet for the acidic aspirin.

It pulls it out of the blood and traps it in the urine to be excreted.

The text says it promotes excretion at 18 times the normal rate.

That is a life -saving intervention based purely on simple acid -danes chemistry.

It's really cool.

How do we know if the kidneys are up to the task?

We checked the labs.

Traditionally, we looked at creatinine and BUN, which is blood urea nitrogen.

But the gold standard now is EGFR, estimated glomerular filtration rate.

Which is calculated using creatinine, age, body size, and sex.

And remember, EGFR is expected to be lower in older adults and females because of decreased muscle mass.

But if it's too low, the kidneys are failing.

And if the kidneys fail?

The drug stays in the body.

If the drug stays, toxicity happens.

You must check kidney function before dosing.

It's a fundamental safety check.

The text briefly categorizes kidney problems into pre -renal, intra -renal, and post -renal.

Let's just define those quickly so we have the vocabulary.

Pre -renal is before the kidney.

Think dehydration, hemorrhage, shock.

The kidney itself is fine, but it's not getting enough blood flow to filter.

Intra -renal.

Intra -renal is inside the kidney.

Chronic kidney disease, glomerulonephritis.

The filter itself is damaged.

And post -renal.

Post -renal is after a kidney.

Kidney stones, prostate hypertrophy.

The plumbing is clogged, so urine backs up.

All three result in drug accumulation.

Okay, we have successfully navigated pharmacokinetics.

The body has moved the drug.

Now we switch to pharmacodynamics.

The drug strikes back.

The effect.

What does it do?

We start with primary versus secondary effects.

This seems straightforward, but there is nuance.

A great example is diphenhydramine.

You know it as benadryl.

Benadryl?

The primary effect is what we usually want.

It stops the sneezing.

It treats the allergy symptoms.

The secondary effect is the side hustle.

Right.

CNS depression,

drowsiness.

And the text makes a good point.

Is the secondary effect good or bad?

It depends on the context.

If you were taking benadryl to sleep at night, the drowsiness is desirable.

You want it.

You want it.

If you were taking it before driving a truck, it's very undesirable.

It's the same chemical effect, but the clinical judgment changes based on the goal.

Lymph, talk about the dose -response relationship.

Potency and efficacy.

These are often confused.

They are.

Potency is about how much drug you need.

High potency means you only need a tiny bit to get a big effect.

Think fentanyl.

Micrograms give you massive pain relief.

And low potency.

Low potency means you need a lot.

Codeine.

You need milligrams.

But just because something is potent doesn't mean it's the best.

Right.

That brings us to maximal efficacy.

This is the sealing effect.

If you keep increasing the dose, eventually you hit a point where the response won't get any stronger.

You just get more toxicity.

So more isn't always better.

More is not always better.

Speaking of toxicity, we have to define the therapeutic index, the TI.

This is the safety margin.

It's the mathematical relationship between the therapeutic dose, or ED50, the dose that's effective in 50 % of people, and the toxic dose, or TD50, the dose that's toxic in 50 % of people.

And you want those numbers to be far apart.

You want them very far apart.

If the TI is wide, the drug is safe.

You have a lot of wiggle room between helping and hurting.

If the TI is narrow,

watch out.

Narrow therapeutic index.

The text lists warfarin,

degoxin, phenytoin.

The difference between the dose that helps and the dose that kills is razor thin.

These are the drugs where we draw blood levels constantly.

We measure the peak and the trough.

Let's define those clearly for the listener.

Onset peak duration.

Onset.

Time it takes to reach the minimum effective concentration, or MEC.

When does the patient start feeling relief?

Peak.

Peak is the highest concentration in the blood.

For oral drugs, usually two to three hours.

Four is much faster.

30 to 60 minutes.

Duration.

How long does the therapeutic effect last?

Pretty simple.

And the trough.

This is the one nurses specifically have to schedule.

The trough is the lowest point of concentration.

We draw this blood sample right before the next dose is due.

This tells us how well the body eliminated the last dose.

So if the trough is too high, the patient is retaining drug and is at risk for toxicity.

We need to lower the dose or space it out.

And if it's too low?

The drug isn't working.

The patient isn't covered.

Maybe the bacteria are growing back between antibiotic doses.

Now, how does the drug actually trigger the cell?

Receptor theory.

The lock and key.

The receptor is the lock on the cell surface.

The drug is the key.

Simple enough.

Agonists are drugs that fit in the lock, turn it, and open the door.

They activate the receptor.

They produce a response.

Epinephrine acts as an agonist.

And antagonists.

They fit in the lock, but they don't turn.

They just sit there.

They block the keyhole so the body's natural chemicals can't get in.

Like putting gum in the keyhole.

Exactly.

Beta blockers are a classic example.

They block the beta receptors so adrenaline can't get in to raise the heart rate.

Result.

Heart rate stays low.

There are four receptor families listed in the chapter.

We don't need to get into the physics of each right now.

But what I find tricky for students is the difference between non -specific and non -selective drugs.

This is a classic exam trap.

But more importantly, it's a clinical trap.

Let's break it down using the examples in the text.

Start with non -specific.

Non -specific means the drug targets one type of receptor.

But that receptor is located in many places.

The example is bethenicol.

Right.

Bethenicol stimulates cholinergic receptors.

We give it to help the bladder contract, say, for urinary retention.

We want to unlock the bladder.

But cholinergic receptors aren't just in the bladder.

They are in the heart, the eyes, the stomach, the lungs.

They're everywhere.

So you fix the bladder, but you also accidentally drop the heart rate, constrict the pupils, and constrict the bronchioles.

Exactly.

It's specific to the receptor key, but non -specific to the site in the body.

You get systemic side effects.

Okay.

And non -selective.

Non -selective means the drug affects multiple types of receptors.

It's like a master key that opens different kinds of locks.

A shotgun approach.

It's a shotgun approach.

The example is epinephrine.

It doesn't just hit one receptor.

It hits alpha one, beta one, and beta two.

Which do different things.

All different things.

It hits alpha one, which raises blood pressure, beta one, which raises heart rate, and beta two, which dilates the lungs.

Great for anaphylaxis because you need all those things to happen at once to save a life.

Bad if you just wanted one specific effect.

Let's move into the variations in response.

Side effects versus adverse reactions.

Side effects are predictable.

We know they might happen.

They're secondary.

Nausea with antibiotics is a side effect.

Right.

Adverse reactions are unintentional, unexpected, and always undesirable.

Anaphylaxis is an adverse reaction.

And toxicity is simply exceeding the therapeutic range.

Too much drug.

What about when a drug stops working?

Tolerance versus tachyphylaxis.

Tolerance is slow.

It happens over the course of therapy.

You take opioids for pain for a month and suddenly the same dose doesn't work.

The receptors have adapted.

You need more drug to get the same effect.

And tachyphylaxis.

Tachyphylaxis is fast.

It's an acute, rapid decrease in response.

It can happen after the very first dose.

The text mentions nitroglycerin patches and ranadine for this.

Yes.

If you leave a nitro patch on 24 -7, the receptors get exhausted immediately.

The drug stops working.

That's why we have drug -free intervals to let them reset.

And we can't ignore the placebo effect.

It's psychological in origin based on belief.

But the physiologic response is real.

Studies show heart rate can actually drop.

Pain can actually decrease.

It shows the power of the mind -body connection in pharmacology.

But as nurses, we focus on the chemical administration.

Section seven.

Drug interactions.

The text says 20 % of older adults take five or more meds.

Polypharmacy.

Polypharmacy.

That is a recipe for interaction soup.

It is.

We have pharmacokinetic interactions affecting absorption, metabolism, excretion.

And then we have pharmacodynamic interactions.

Let's hit the highlights.

Absorption interactions.

Laxatives.

They speed up the gut.

The drug goes through so fast it doesn't get absorbed.

Opposite of that.

Opioids.

They slow down the gut.

The drug sits there and gets absorbed too much.

Can lead to toxicity.

And the complexes.

This one is huge.

This is the tetracycline and ciprofloxacin rule.

If you take these antibiotics with dairy, antacids, or iron, they bind together chemically.

They form a chelate, basically a little rock.

And the body can't absorb a rock.

The body can't absorb a rock.

You must separate them by at least two hours.

Now metabolism interactions.

We are back to the CYP450 system.

This is the battleground.

Enzyme inducers versus enzyme inhibitors.

This is probably the hardest concept to grasp, so let's simplify the engine analogy.

Enzyme inducers.

These drugs, like phenobarbital rifampin, St.

John's wort, they tell the liver to build more workers.

They rev up the engine.

So the factory is running on overtime.

Exactly.

And if the liver engine is revving, it metabolizes other drugs faster.

So let's play out the scenario.

I'm on warfarin.

I start taking St.

John's wort, an inducer.

St.

John's wort tells the liver to speed up.

The liver eats the warfarin faster than expected.

The warfarin levels in your blood drop.

You are now at risk for a blood clot.

The drug fails.

Okay.

Now enzyme inhibitors.

These drugs, cimetidine, ritzomycin, grapefruit juice, tell the liver workers to take a break.

They inhibit the enzymes.

They slow the engine down.

So if I'm on warfarin and I drink a glass of grapefruit juice, an inhibitor.

The liver stops breaking down the warfarin.

The warfarin accumulates.

Your levels go up and up.

You are at risk for bleeding toxicity.

Inducers lead to loss of effect.

Inhibitors lead to toxicity.

That's the golden rule.

And there's a massive one here regarding lifestyle.

Tobacco.

Tobacco.

Tobacco is an inducer, specifically for an enzyme called CYP1A2.

If a patient smokes, they might need higher doses of drugs like theophylline or clozapine because their liver is burning through it so fast.

But the danger comes when they quit.

The patient comes into the hospital.

No smoking allowed.

The induction stops.

The liver slows down to its normal pace.

But if you keep giving them that high smoker's dose.

Instant toxicity.

Smoking cessation is great for health.

But from a pharmacology standpoint, you have to adjust the meds immediately.

That is a critical catch.

Let's look at pharmacodynamic interactions.

Additive, synergistic, antagonistic.

Additive is 1 plus 1 equals 2, aspirin plus codeine.

They work differently, but together they add up to better pain relief.

Or alcohol plus aspirin.

They both cause gastric bleeding, so the risk is added up.

Synergistic.

1 plus 1 equals 3.

The effect is greater than the sum of the parts.

Ampicillin plus solbactam.

The solbactam allows the ampicillin to work better than it ever could alone.

And antagonistic.

1 plus 1 equals 0.

One blocks the other.

Morphine, which is an agonist, plus naloxone, which is an antagonist.

The naloxone cancels the morphine.

We use this for overdoses.

Lastly, drug nutrient interactions.

We mentioned grapefruit, but what about MAOIs and cheese?

The cheese effect.

MAOIs are older antidepressants.

They inhibit the breakdown of a substance called tiramine.

And tiramine is in?

Tiramine is found in aged cheese, wine, cured meats.

If you eat those foods while on MAOIs, tiramine builds up and causes a massive release of norepinephrine.

Result?

Hypertensive crisis.

A mass, dangerous spike in blood pressure.

Stroke risk.

And photosensitivity.

Some drugs make your skin sensitive to the sun.

The book mentions two types.

Photoallergic is an immune response.

It's delayed.

Phototoxic is rapid, like a super sunburn within hours.

And the nursing implication is?

Teach your patients.

Sunscreen, hats, long sleeves.

We have covered the science.

Now, section 8 is clinical judgment.

How do we apply this?

The nursing process.

Assessment.

Don't just hand out the pills.

Look at the patient.

Check protein levels, like albumin, for distribution issues.

Check what else?

Check kidney function, your EGFR, for excretion issues.

And ask about herbs, like St.

John's wort, for metabolism issues.

It's a full picture.

Then planning and interventions.

Use your resources.

Use the RX list or WebMD interaction checkers.

No nurse memorizes every single interaction.

It's impossible.

Patient teaching is huge.

Don't crush this.

Don't drink grapefruit juice with this.

Don't stop smoking without telling us so we can adjust your dose.

And finally, evaluation.

Did it work?

Did it cause harm?

Monitor.

Check those peak and trough levels.

Watch for side effects.

It's a continuous cycle.

So let's recap.

We trace the pill.

It enters the mouth.

That's absorption and dissolution.

It faces the stomach acid.

It hits the liver toll booth.

That's the first pass effect.

It catches a taxi.

That's distribution and protein binding.

It avoids the barriers, like the BBB.

It finds the lock.

That's the receptor.

That's pharmacodynamics.

It gets broken down by the factory metabolism, the CYP450 system.

And it exits through the kidneys.

That's excretion.

It's an incredible journey.

It really is.

Why does this matter?

Why do we just spend an hour on this?

Because these aren't just abstract concepts.

They are the math and science that keeps the patient safe.

If you don't understand protein binding, you overdose the malnourished patient.

If you don't understand half -life, you mistime the doses.

Pharmacology isn't about memorizing drug names.

It's about respecting the body's physiology.

Here's a final thought to chew on.

We think of medicine as this precise, controlled science.

But consider how fragile it is.

A simple change in diet, a glass of grapefruit juice, or a habit change quitting smoking can flip a switch.

It can turn a life -saving medication into a toxic poison.

The doctor prescribes the dose, but the nurse sees the patient eating the grapefruit.

The nurse sees the patient stopping the cigarettes.

The power of observation is squarely in the nurse's hands.

You are the final safety barrier.

You are the one connecting all these dots at the bedside.

Thank you for diving deep with us today.

This has been the Last Minute Lecture Team, signing off.

Stay curious.