The safety of antibacterials used to treat urinary tract infections is reviewed. The types of side-effects found relate to the gastrointestinal tract, the female genital tract, and the skin, with miscellaneous complaints such as headache also occurring. Gastrointestinal effects are by far the most common and may be induced by direct irritation of the bowel, alteration of gut flora, direct stimulation of smooth muscle or alteration of bacterial metabolism of bile acid. Rare toxic effects specific to individual agents are also observed. There is no perfect treatment for urinary tract infections but from the point of view of safety, beta-lactams, nitrofurantoin and for fomycin, while causing a significant incidence of minor side-effects, are only very rarely associated with serious toxicity.