Pharmacokinetics Cheat Sheet for Nursing Students

Pharmacokinetics describes how the body handles a drug — how it gets in, moves around, is broken down, and leaves. Mastering these four steps (ADME) is essential for safe medication administration and a common source of NCLEX-style questions.

ADME: The Four Core Steps

• A — Absorption: Drug enters the bloodstream.
• D — Distribution: Drug moves through the body to tissues.
• M — Metabolism: Drug is broken down, primarily by the liver.
• E — Excretion: Drug leaves the body, primarily through the kidneys.

Absorption

Factors that influence how quickly and completely a drug reaches the bloodstream:

• Route: IV is the fastest.
• Blood flow: Increased blood flow increases absorption.
• GI motility: Diarrhea reduces absorption.
• Food: Can delay absorption.

Route speed (fastest → slowest): IV → IM → SubQ → PO

Distribution and Protein Binding

Many drugs travel bound to plasma proteins (especially albumin). Only the free (unbound) drug is active.

• High protein binding: Less free drug, longer duration, lower toxicity risk.
• Low protein binding: More free drug, faster effect, higher toxicity risk.

Exam rule: Low albumin → more free drug → increased toxicity risk.

Metabolism (Liver)

• Normal liver: Normal drug breakdown; standard dosing.
• Impaired liver: Drug accumulates; lower dose required.

Key rule: Liver disease typically requires a lower dose at longer intervals.

Excretion (Kidneys)

• Normal kidneys: Normal elimination; standard dosing.
• Renal impairment: Drug buildup; dose reduction needed.

Monitor: BUN, creatinine, and GFR.

Half-Life and Steady State

• Half-life: Time required to reduce drug concentration by 50%.
• Steady state: Reached in 4–5 half-lives.
• Long half-life: Longer duration of drug action.

Therapeutic Index (TI)

The TI is the safety margin between an effective dose and a toxic dose.

• Narrow TI: Small safety margin; requires close monitoring.
• Wide TI: Large safety margin; minimal monitoring.

Narrow TI drugs to know: Digoxin, Lithium, Warfarin.

Common Exam Traps

1. Confusing pharmacokinetics (what the body does to the drug) with pharmacodynamics (what the drug does to the body).
2. Ignoring liver or kidney function when evaluating dosing.
3. Missing the impact of low albumin on free drug levels.
4. Assuming oral (PO) drugs act quickly.

Key Takeaways

• Remember ADME: Absorption, Distribution, Metabolism, Excretion.
• IV is the fastest route of administration.
• Low albumin = more free drug = higher toxicity risk.
• Liver metabolizes; kidneys excrete — impairment of either requires dose adjustment.
• Steady state = 4–5 half-lives.
• Monitor narrow TI drugs closely: digoxin, lithium, warfarin.