Drug allergy.

Drug allergy is an important complication in the use of agents such as penicillins, cephalosporins, sulphonamides, insulin and streptokinase. The allergenic properties of drugs are a function of molecular size and chemical reactivity. Factors determining an individual's risk of an allergic response are not fully understood but include genetic predisposition, prior exposure, route of administration, drug dosage, age and concomitant disease. The most dangerous but least common form of drug allergy is generalised anaphylaxis. The majority of reactions are non-anaphylactic and involve the skin, with a lesser incidence of haematological, renal, musculoskeletal, cardiorespiratory and other systemic manifestations. The only definitive test for allergy in a patient with a history of previous allergic reaction is rechallenge, a dangerous and seldom indicated procedure. An alternative is skin testing, but this requires an experienced practitioner and has intrinsic risk. In vitro testing may be of value in predicting the risk of rechallenge. Safe use of a suspect drug requires a careful assessment of risk and a cautious approach. Use of an offending drug in a high-risk patient is rarely indicated, but if it is considered essential, initial therapy or desensitisation in an intensive care environment is recommended.