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Several drugs in the fluoroquinolone class of antibiotics are known to interfere with the metabolism of theophylline. This report, based on data from the Food and Drug Administration's Spontaneous Reporting System, suggests that this drug interaction can have serious clinical consequences. The authors reviewed 48 reports of adverse events that occurred when ciprofloxacin or norfloxacin was prescribed in conjunction with theophylline. About a third of the patients had seizures, and many others had central nervous system side effects such as agitation, confusion, nausea, and vomiting. Most events occurred within several days of starting the antibiotic; on average, patients' blood theophylline concentrations had doubled. Many of the patients w…