affinity

Affinity describes how tightly a drug grips its target — how much it wants to stay bound rather than drift away. A high-affinity drug clings to its receptor even at very low concentrations, like a strong magnet snapping onto metal.

Quantitatively, affinity is captured by the dissociation constant Kd, the concentration at which half the receptors are occupied: a smaller Kd means higher affinity. Affinity governs how readily binding happens, but it says nothing about what happens after binding — that is the job of efficacy. Both an agonist and an antagonist can have high affinity for the same receptor.

Affinity is shaped by the fit between drug and binding site: shape complementarity, hydrogen bonds, electrostatic and hydrophobic interactions all contribute. Tighter, more selective binding can mean a drug works at lower doses and is less likely to stray onto unintended targets, though high affinity alone never guarantees a useful therapeutic effect.