The fact that aminopyrine is readily nitrosated and that the nitrosation product, dimethylnitrosamine, is potentially carcinogenic has led to the drug being withdrawn from therapeutic use. In the case of the other analgesics, nitrosation is not of any importance, neither in toxicological nor quantitative terms. The characteristic features of the metabolic pathways of phenacetin, paracetamol, and aspirin can occasionally entail toxicological consequences, such as methaemoglobin formation, liver damage, and long presence in the body, respectively. The prolonged and continuous administration of any of the three classes of drugs is not without risk. Adverse reactions of the allergic type cannot be detected by animal experiments.