Two questions, one mirror
Pharmacology has a beautifully simple skeleton. Everything fits under one of two questions, and they are mirror images of each other. Pharmacodynamics (PD) asks what the drug does to the body: how it binds its target, how strongly, and what effect that produces. Pharmacokinetics (PK) asks what the body does to the drug: how it gets in, where it travels, how it is broken down, and how it leaves. A short mnemonic many students use: PD is 'dynamic' effect, PK is 'kinetic' movement.
PHARMACODYNAMICS (PD) PHARMACOKINETICS (PK) what drug does to body what body does to drug ---------------------- --------------------- binds target Absorption produces effect Distribution dose -> response Metabolism potency, efficacy Excretion (ADME) DRUG <----- both halves describe the SAME pill ----->
PD: the effect side
On the PD side we ask how a drug, once it reaches its target, produces an effect. We describe it with a dose–response curve: as the dose rises, the effect usually rises too, up to a maximum. From that curve we read how much drug is needed (potency) and how big an effect it can produce (efficacy). This is the side that explains *why* a drug helps — it lowers blood pressure, relieves pain, kills bacteria — through its mechanism of action.
PK: the journey side
On the PK side we follow the drug's journey, summarised by the four letters of ADME: Absorption, Distribution, Metabolism, Excretion. PK answers practical questions: how much of a swallowed dose actually reaches the bloodstream (bioavailability), and how long the drug lingers before half of it is gone (half-life). PK is the side that explains *when* and *how often* to dose.