Selectivity

Selectivity refers to the preference that a ligand shows for binding to one target protein ahead of other target proteins. The protein for which selectivity is shown is the one for which the ligand has the lowest K_D. As the concentration of the ligand is increased, it will begin to lose its selectivity and will start to bind to other proteins, for which it has a higher K_D. This may result in off-target side-effects.

You may see some drugs referred to as being “specific” in their binding behaviour. This implies that the drug only binds to one target and will not bind to any other target under any circumstances. Since this is very unlikely to happen, selectivity is usually a more correct term to use.