A generally unexpected, therefore unpredictable, abnormal reaction to a drug. Sometimes, prior knowledge of a person's condition makes the risk clear; the abnormal sensitivity of patients with PORPHYRIAS to BARBITURATES is one example, and may prove fatal. Hereditary biochemical defects of red blood cells are responsible for many drug-induced haemolytic anaemias (see under ANAEMIA) and for FAVISM. It is likely that all those who have such unpredictable side-effects probably have a genetic disorder rendering them more liable to such problems.