Pharmacology for Nurses · Drug Administration
Pharmacokinetics and Pharmacodynamics
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In 30 seconds
Every medication raises two paired questions: What does the body do to the drug? and What does the drug do to the body? Pharmacokinetics What the body does to a drug: ADME Full entry → (PK) answers the first — how a drug moves through the body, summarized as absorption, distribution, metabolism, and excretion (ADME). Pharmacodynamics What the drug does to the body Full entry → (PD) answers the second — how a drug produces effects at receptors and how effect size changes with dose. Together they explain why an IV dose acts faster than an oral one, why two people can respond differently to the same drug, and why some drugs need close monitoring. This topic builds the framework that later drug-class chapters use. Always verify actual drug facts against current references, the facility formulary, and prescriber orders.
Why this matters
PK and PD let you predict when a drug starts working, how hard it works, and how long it lasts — exactly what nurses monitor. They also explain clinical puzzles: why a drug stopped working (receptor tolerance), why a dose changes after a second drug is added (enzyme induction or inhibition), or why a person with kidney problems may accumulate a drug (impaired excretion). Exams favor these concepts because they test relationships, not recall; practice needs them because the same drug behaves differently in different people.
The college version
Core Concepts
Absorption
Absorption moves a drug from its administration site into the bloodstream. An IV drug skips absorption entirely, which is why IV onset is fastest. Oral drugs must dissolve and cross the gut lining, influenced by blood flow, surface area, solubility, and formulation (immediate vs. extended release). Bioavailability Fraction of a dose reaching general circulation Full entry → — the fraction of a dose reaching general circulation unchanged — is central. Oral drugs are especially affected by the First-pass effect Liver inactivates part of an oral dose Full entry →: blood from the gut passes through the liver first, where enzymes may inactivate much of the dose. That is why oral doses of some drugs exceed their IV doses, and why routes that bypass the liver (sublingual, buccal, IV) can work with smaller doses.
Distribution
Once in the blood, a drug distributes into tissues. Many drugs bind plasma proteins such as albumin; the bound fraction is inactive, and only free (unbound) drug produces effects. Bound drug acts as a reservoir, releasing as free drug is used. If two drugs compete for the same binding sites, one can push the other off the protein, transiently raising free levels. Barriers matter too: the blood-brain barrier, for example, lets lipid-soluble drugs cross more readily. Volume of distribution is the conceptual space a drug occupies — large for drugs that concentrate in tissues, small for those that stay in the blood.
Metabolism
Metabolism (biotransformation) converts a drug chemically, usually making it more water-soluble for elimination. The liver is the main site, with the CYP450 enzyme family doing much of the work. Two consequences matter. Some drugs are prodrugs — inactive until metabolized into the active form. And other drugs change enzyme speed: enzyme induction speeds metabolism (levels fall, effect weakens), while enzyme inhibition slows it (levels rise, effects and side effects intensify). Starting or stopping a second medication can therefore change the levels of the first. Age, genetics, and liver function also matter — one reason dosing is individualized.
Excretion
Excretion removes the drug and its metabolites, primarily through the kidneys. The key measure is Half-life Time for plasma level to fall by 50% Full entry →: the time for the plasma concentration to fall by half. Half-life drives dosing intervals and predicts steady state — the plateau where drug entering each interval equals drug eliminated — reached after roughly four to five half-lives. That is why some drugs take days to reach full effect and why dose changes take time to show up. When kidney function declines, half-life lengthens and drugs can accumulate to toxic levels, which is why renal status is part of pre-administration assessment. The nurse recognizes the principle; the prescriber and references set the specifics.
Pharmacodynamics
Pharmacodynamics describes the drug–receptor interaction. An agonist binds a receptor and activates it; an antagonist binds and blocks it. Two properties distinguish drugs: affinity (how tightly the drug binds) and Efficacy Maximum effect a drug can produce Full entry → (how strongly it produces a response once bound). The dose-response relationship introduces two often-confused ideas: Potency Dose needed for a given effect Full entry → (dose needed for a given effect) and efficacy (the maximum effect a drug can produce) — efficacy is usually the more clinically important. The Therapeutic index Margin between effective and toxic doses Full entry → compares effective and toxic doses: a narrow index means a small margin, demanding close monitoring of the person and, where indicated, drug levels. These are frameworks; verify drug-specific parameters against current references and prescriber orders.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Pharmacokinetics | Pharmacodynamics | PK = body acts on the drug; PD = drug acts on the body |
| Absorption | Distribution | Absorption = into the blood; distribution = from blood into tissues |
| Potency | Efficacy | Potency = dose needed; efficacy = maximum effect possible |
| Agonist | Antagonist | Agonist activates; antagonist blocks |
| Half-life | Duration of action | Half-life = plasma level fall; duration = how long the effect lasts |
| Bound drug | Free drug | Bound = inactive reservoir; free = active |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Pharmacokinetics is like tracking a package: it gets picked up (absorption), driven around town (distribution), sorted at the depot (metabolism), and delivered out of town (excretion). Pharmacodynamics is what happens when the package arrives — it turns a key that starts or stops a machine in the body. To know how fast a medicine works and how long it lasts, you watch both the package and the machine.
Worked example
Three people take the same oral medication, which is heavily metabolized in the liver (this scenario teaches the framework; the drug, doses, and adjustments come from references and prescriber orders). Person A takes only this drug: each dose passes through the liver, where much is inactivated before reaching general circulation — the first-pass effect — so a larger oral dose is needed, and the effect builds slowly over several half-lives. Person B adds a second medication that inhibits the liver enzyme that breaks down the first drug. Now less is metabolized, so more survives into the bloodstream: levels rise, effects strengthen, and toxicity risk grows. A nurse who understands PK predicts this, monitors for both effect and adverse effects, and raises it with the prescriber, who may adjust the dose — per current references and orders. Person C receives the same drug by IV: absorption is bypassed, onset is faster, and a smaller dose may suffice. One drug, three stories — PK/PD makes sense of all three.
Key takeaways
- PK = what the body does to the drug (ADME); PD = what the drug does to the body.
- The first-pass effect lowers oral bioavailability; IV, sublingual, and buccal routes avoid it.
- Only free (unbound) drug is active; bound drug is a reservoir.
- Enzyme induction lowers levels; enzyme inhibition raises them — watch when drugs are added or stopped.
- Half-life drives dosing intervals; steady state arrives after roughly 4–5 half-lives.
- Agonists activate receptors; antagonists block them.
- Potency ≠ efficacy; a narrow therapeutic index means close monitoring.
- Verify drug specifics against current references, the formulary, and prescriber orders.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
List the four components of pharmacokinetics (ADME).
Show answer
Absorption, distribution, metabolism, and excretion.
What is the first-pass effect, and which route is most affected?
Show answer
Liver enzymes inactivate part of an oral dose before it reaches general circulation; the oral (enteral) route is most affected.
If a new drug inhibits the enzyme metabolizing a current drug, what happens to the current drug's level?
Show answer
Levels would be expected to rise, with stronger effects and greater toxicity risk — verify and notify the prescriber.
What is the difference between potency and efficacy?
Show answer
Potency is the dose needed for a given effect; efficacy is the maximum effect the drug can produce.
What does a narrow therapeutic index imply about monitoring?
Show answer
A small margin between effective and toxic doses — monitoring must be close, per references and orders.
After roughly how many half-lives is steady state reached?
Show answer
Roughly four to five half-lives.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Pharmacokinetics
- What the body does to a drug: ADME
- Pharmacodynamics
- What the drug does to the body
- Bioavailability
- Fraction of a dose reaching general circulation
- First-pass effect
- Liver inactivates part of an oral dose
- Half-life
- Time for plasma level to fall by 50%
- Agonist / antagonist
- Activates / blocks a receptor
- Potency
- Dose needed for a given effect
- Efficacy
- Maximum effect a drug can produce
- Therapeutic index
- Margin between effective and toxic doses
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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