The safety of ciprofloxacin was established on a data base (compiled through the end of 1988) of 9,473 well-documented treatment courses world-wide. The daily dosages ranged between 200 and 2,000 mg orally. Thirty-eight percent of the patients received doses between 1 and 10 mg/kg body weight, 46 percent between 11 and 20 mg/kg, and the remaining 16 percent more than 20 mg/kg body weight daily. Ciprofloxacin was administered to 4,214 women (45 percent) and 5,252 men (55 percent). The duration of treatment ranged from less than two days to more than 90 days. A 600-mg daily dose of ciprofloxacin was used mostly in Japan (2,341 patients). The daily dose of 1,000 mg was administered chiefly in the United States and Europe (2,288 patients). The age of the patients ranged from less than one year to 99 years (mean, 50.6 years). More than 38 percent were older than 60 years. According to COSTART terminology, the following drug-related side effects were observed in the different organ systems: digestive, 4.9 percent; metabolic-nutritional, 4.4 percent; central nervous system, 1.5 percent, skin, 1.1 percent; hemic and lymphatic, 0.9 percent; urogenital, 0.8 percent; body as a whole, 0.5 percent; cardiovascular, 0.2 percent; special senses, 0.2 percent; musculoskeletal, 0.1 percent; and respiratory, 0.1 percent. Several patients had more than one reaction. The total incidence of side effects for the treated patients was 9.3 percent. The vast majority of adverse reactions were mild or moderate (94 percent). Serious side effects were reported for 55 patients (6 percent). Based on the 9,473 courses, the incidence of severe reactions was 0.6 percent. Ciprofloxacin treatment was discontinued due to side effects in 146 patients (1.5 percent), mostly due to gastrointestinal reactions (80 patients). The worldwide data from clinical trials with oral ciprofloxacin clearly demonstrate that the drug is relatively safe, and the side effects are usually mild or moderate in intensity and are reversible.