Off-target side-effects

A result of a drug binding to a protein that is not the intended target of that drug, changing the behaviour of that protein and causing undesired effects. This may happen when concentrations of the drug in the body are higher than intended, through competition or inhibition of clearance or increased bioavailability or increased dosing rate. It may also happen when “normal” doses of a drug are administered to an individual who is an “outlier” in terms of clearance or bioavailability or plasma protein binding for that drug. These effects are sometimes predictable.

Some off-target side-effects are due to immunological responses to drugs or (more often) their reactive metabolites. Such effects are rarely dose-dependent, and they are less easy to predict. See Type B Adverse Drug Reactions.