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Idiosyncratic drug reactions, also known as type B reactions, are drug reactions which occur rarely and unpredictably amongst the population. This is not to be mistaken with idiopathic which entails that the cause is not known.

Idiosyncratic drug reactions appear to not be concentration dependent. A minimal amount of drug will cause an immune response, but it is suspected that at a low enough concentration, a drug will be less likely to initiate an immune response.

In adverse drug reactions involving overdoses, the toxic effect is simply an extension of the pharmacological effect. On the other hand, clinical symptoms of Idiosyncratic drug reactions are different than the pharmacological effect of the drug. The proposed mechanism of most idiosyncratic drug reactions is immune mediated toxicity. To create an immune response, you must have a foreign molecule that antibodies can bind to (ie. the antigen) and you must have cellular damage. Very often, drugs will not be immunogenic because they are too small to bind antibodies. However, a drug can cause an immune response if the drug binds a larger molecule. Some unaltered drugs such as penicillin will bind avidly to proteins. Others must be bioactivated into a toxic compound that will in turn bind to proteins. The second criteria of cellular damage can come either from a toxic drug/drug metabolite or it may come from an injury or infection. These will sensitize the immune system to the drug and cause a response.

See idiosyncrasy.