Intolerance to analgesics is common in patients with bronchial asthma, nasal polyps and urticaria. Symptoms of intolerance resemble those of allergy, but the events precipitating them can rarely be traced to reactions between the drug and a specific antibody or sensitised T-lymphocytes. In 8 to 20% of adult asthmatics, aspirin and several other analgesics provoke asthmatic attacks, probably through inhibition of cyclo-oxygenase. This is a distinct and important clinical syndrome with a specific history, course and clinical presentation and a number of unique peculiarities which still require elucidation at the biochemical level. Up to 40% of patients with chronic urticaria develop an obvious increase in weals and swelling after taking aspirin. These reactions occur only when urticaria is active, and though the reason for them is not known, it appears that different mechanisms may be responsible in different patients. Skin reactions other than exacerbation of chronic urticaria are less common, but may create serious clinical problems. The clinical background of a patient often determines the type of adverse response to an analgesic. Thus, in certain individuals, analgesics can produce anaphylactic reactions and/or urticaria, probably through an immunological mechanism, while in some asthmatics they precipitate bronchoconstriction, probably through inhibition of bronchial cyclooxygenase. Study of untoward reactions to analgesics not only leads to safer pharmacotherapy, but it also offers a fascinating model for better understanding of some diseases.