Because they target bacteria and not our own tissues, antibiotics are often not scrutinized for side effects by the Federal Drug Administration or manufacturers as carefully as are, say, psychiatric drugs. But in the bodies of people, cats, rats and mice, fluoroquinolones not only kill bacteria but also appear to attach to certain brain and nerve receptors, kill tendon cells, and cause other kinds of havoc.

Clinical trials conducted over three decades in the process of gaining FDA approval for fluoroquinolones—which encompass dozens of antibiotics with "flox" or "ox" in the generic name—showed that psychiatric and central-nervous-system problems occurred in more than 10 percent of patients. Such trials, as well as "adverse drug reaction" (ADR) reports that began to be filed by U.S. doctors and patients once the drugs were being marketed, indicate serious reactions in about 1 to 2 percent of cases in which the drugs are administered.

A study of ADRs in Italy, published in 2005, found that among more than 50 types of drugs, fluoroquinolones accounted for 11 percent of all adverse events and were involved in the largest number of serious problems, edging out antidepressants