selectivity

Selectivity is a drug's ability to pick out one target and largely ignore the rest, the way a well-aimed arrow hits one bullseye among many. The more sharply a drug discriminates between its intended target and similar molecules elsewhere in the body, the more selective it is — and, usually, the cleaner its side-effect profile.

Selectivity arises from differences in how tightly a drug binds different targets, and is often quoted as a ratio of affinities or potencies — for instance, a drug a hundred times more potent at one receptor subtype than another. It can be directed at a target class, an individual receptor, a receptor subtype, or even a single tissue where that target is concentrated.

Greater selectivity generally means fewer off-target effects, which is why much drug design chases it. But the honest caveat is that selectivity is relative and dose-dependent: a 'selective' drug pushed to high doses spills over onto secondary targets, and in some diseases a deliberately non-selective drug that hits several targets at once actually works better than a narrowly selective one.