Pharmacokinetics vs Pharmacodynamics
These two terms confuse many students, but the distinction is simple: pharmacokinetics is the journey of the drug through the body; pharmacodynamics is the effect the drug has on the body. Both are essential for understanding dosing and interactions.
Pharmacokinetics: ADME
Pharmacokinetics is summarised as ADME — Absorption, Distribution, Metabolism, Excretion. It answers: how much drug gets in, where it goes, how it's broken down, and how it leaves. A drug taken orally may be poorly absorbed; a drug metabolised by the liver may interact with other liver-metabolised drugs.
The half-life determines dosing frequency. A drug with a short half-life needs frequent dosing; one with a long half-life can be given once daily.
Pharmacodynamics: dose-response
Pharmacodynamics describes the relationship between drug concentration and effect. A higher dose usually means more effect — up to a maximum. Beyond that, more drug adds toxicity without benefit. This is why doubling a dose is rarely the answer when a drug 'isn't working enough'.
The therapeutic window is the range between the minimum effective dose and the toxic dose. Drugs with a narrow window (like warfarin) need careful monitoring.
Practical example
Warfarin: narrow window
Warfarin has a narrow therapeutic window: too little and the patient clots, too much and the patient bleeds. Its pharmacokinetics (metabolism by CYP enzymes) means other drugs can raise or lower its levels. Its pharmacodynamics (anticoagulant effect) means the dose-response is steep. Both PK and PD explain why warfarin requires INR monitoring.
Key takeaways
- Pharmacokinetics = what the body does to the drug (ADME).
- Pharmacodynamics = what the drug does to the body (dose-response).
- Half-life determines dosing frequency.
- Narrow therapeutic windows require monitoring.
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