The safety profile of grepafloxacin has been characterized in a number of preclinical and clinical studies. Preclinical investigations have shown that its toxicologic profile is similar to that of other fluoroquinolones, and phase I studies in humans have confirmed these data. A photosensitivity study demonstrated equivalence with ciprofloxacin, and a study in elderly patients taking the highest clinical dose of grepafloxacin indicated that prolongation of the QTc interval by grepafloxacin was less than 2 ms, 15 times less than that observed in a study of erythromycin. In phase II and III clinical investigations conducted in the United States and the United Kingdom, safety data have been gathered from more than 3000 patients with either community-acquired pneumonia or acute bacterial exacerbations of chronic bronchitis. The most common adverse events with grepafloxacin 400 or 600 mg were gastrointestinal, such as nausea, vomiting, and diarrhea. The frequency of these adverse events was similar to that seen with ciprofloxacin. Significantly more patients reported a mild, unpleasant metallic taste with grepafloxacin than with ciprofloxacin, but <1% of patients withdrew from therapy because of this. Headache was observed significantly more often in ciprofloxacin-treated patients than in grepafloxacin-treated patients. Recent postmarketing data confirm the good tolerability and safety profile of grepafloxacin. These data, from a case-report study of more than 9000 patients in Germany, demonstrated that only 2.3% of patients reported adverse events when grepafloxacin was used in routine clinical practice. The most frequently reported events were nausea (0.8%) and gastrointestinal symptoms (0.4%). Dizziness was reported by only 0.3% of patients, and only 4 patients (0.04%) reported photosensitization. Adverse events did not appear to be either dose dependent or related to diagnosis. More than 400,000 patients world-wide have received grepafloxacin treatment. No new safety issues have arisen from the spontaneous-report data and, with the exception of rare reports of an unpleasant taste, the most commonly reported events have been the same as those seen in clinical studies. These data support grepafloxacin as a well-tolerated fluoroquinolone suitable for the treatment of community-acquired respiratory tract infections.