pharmacodynamics

Pharmacodynamics is the study of what a drug does to the body. A handy slogan: pharmacodynamics is the 'effect' side of the story. If you imagine a key turning in a lock, pharmacodynamics is about which lock the key fits, how tightly it turns, and what door swings open as a result.

Concretely, it covers how a drug binds to its target, how strongly it binds (affinity), how well that binding produces an effect (efficacy), and how the magnitude of effect rises with dose, captured by the dose–response curve. It explains why one drug is more potent than another, why some drugs activate a target while others block it, and why effects can plateau no matter how much more you give.

Pharmacodynamics also explains why a 'right' concentration of drug can still produce the wrong amount of effect in a given person. Receptor numbers, downstream signaling, disease state, and the presence of other drugs all shape the response. This is why pharmacodynamics is studied alongside pharmacokinetics: knowing how much drug reaches the site is only useful if you also know what that drug does once it arrives.