Antianxiety drugs, like other drugs used in psychiatry, can cause a wide range of adverse effects. Many physiological systems may be affected, but, as the main action of antianxiety drugs is on the central nervous system, this system is particularly vulnerable. All antianxiety drugs have the potential to produce untoward effects on higher cerebral functions, although the effect seen is also influenced by psychological and social factors. The most common effects is oversedation, which is a particular problem for the very young and the very old. It is also a serious problem for those who drive motor vehicles and may be a hazard when working in dangerous situations. Subjects are especially vulnerable when (a) antianxiety drugs are first introduced; (b) the dose is increased; and (c) these agents are taken in combination with alcohol and other drugs. Dependence on antianxiety drugs is well known, but only recently has it been recognised that dependence on benzodiazepines is a larger problem than previously realised. Other adverse effects are reviewed and summarised according to the system they predominantly affect. A review of this kind can easily give a biased impression of the dangers of antianxiety drugs; it should be made clear at the outset that many effects are rare, and in some instances a causal connection with the drug has not been established with certainty. Overall, benzodiazepines are the most widely used of all drugs and are remarkably safe-even when taken in massive overdoses. Some unwanted effects are readily preventable if antianxiety drugs are used with caution or avoided altogether in conditions where pathological disturbances of tissue sensitivity or drug disposition lead to exaggerated reactions. Particular care should be taken when prescribing these drugs for children and the elderly, and drugs that are not clearly essential for the well-being of the mother should be avoided during pregnancy and breast feeding. Antianxiety agents are grossly overprescribed. The frequency of occurrence of some adverse effects is therefore not so much a manifestation of the intrinsic toxicity of antianxiety drugs, but a reflection of their widespread use. Overprescribing and irrational prescribing also contribute to self-poisoning with these and other agents and to the cost of health services. The reasons for overprescribing are complex, but one contributing factor is the ready availability of effective antianxiety drugs.