idiosyncratic reaction

An idiosyncratic reaction is a strange, individual response to a drug that you simply cannot predict from how the drug normally works or from the dose taken. It is the body of one particular person reacting in a way most people's bodies do not — a quirk of that individual's biology rather than a property of the drug everyone shares.

What sets it apart is unpredictability and independence from dose. Unlike a Type A reaction, which is an exaggeration of expected pharmacology and scales with the amount taken, an idiosyncratic (Type B) reaction can appear at a perfectly normal — even tiny — dose, in a person with no warning signs, and often surprises both patient and prescriber.

The roots are usually hidden in the individual: an inherited enzyme variant (a pharmacogenetic difference), an unusual immune predisposition, or a metabolic quirk that turns a normally harmless drug into a reactive, toxic intermediate in that person. A classic example is haemolysis (red-cell destruction) triggered by certain drugs in people with G6PD deficiency, an inherited enzyme defect.

Because these reactions are rare and unforeseeable, they are a major reason serious problems can slip through clinical trials and only emerge once a drug is in wide use — and they are a core target of post-marketing pharmacovigilance and, increasingly, of pharmacogenomic screening.