Adverse drug reactions are unwanted reactions to drugs that occur under normal conditions of use, and are probably responsible for 3-5% of all hospital admissions. They are usually classified as Type A (augmented) or Type B (bizarre) reactions. Type A reactions are responsible for 80-90% of all adverse responses to drugs. They are relatively common, well known, and predictable, and have a close temporal relation with drug administration. Although patients vary in their susceptibility, most Type A reactions occur in every patient who is given the drug in sufficient dose. Type B reactions are usually unrelated to the main effects of drugs; they are often dose-independent, uncommon and unpredictable. Although their cause may be obscure, they are sometimes related to genetic predisposition or drug hypersensitivity. In anaesthetic practice, at least three important Type B reactions are directly related to genetic predisposition (malignant hyperthermia, suxamethonium apnoea, hepatic porphyria). Hypersensitivity or allergic responses to drugs depend on immunological factors; there are four types, depending on the mechanism involved. Most severe allergic reactions to drugs are a result of Type I hypersensitivity (anaphylaxis). In anaesthesia, the main aetiological factors are muscle relaxants, antibiotics, latex rubber and chlorhexidine; reactions to these agents may cause hypotension, bronchospasm, facial and laryngeal oedema, or urticaria. Similar effects occur when histamine and other factors are directly released from mast cells by non-immunological mechanisms (anaphylactoid reactions). Finally, drugs that are given during pregnancy may cross the placental barrier and adversely affect the fetus. © 2005 Elsevier Ltd. All rights reserved.
