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Fluoroquinolone antibiotics inhibit bacterial DNA gyrase, which is necessary for the replication of DNA as well as of plasmids involved in certain types of bacterial resistance. Nalidixic acid, a nonfluorinated quinolone, has been used since the 1960s to treat urinary tract infections, but its value is limited by poor systemic distribution, lack of action against gram-positive organisms, and rapid development of bacterial resistance. In contrast, the fluoroquinolones, which contain a fluorine atom, quickly achieve therapeutic concentration in plasma, tissues, and urine after oral administration and are active against some gram-positive bacteria. Although the fluorine atom also retards the development of bacterial resistance, widespread use of fluoroquinolones during the 1990s led to a significant and continuing rise in resistance among aerobic gram-negative bacilli. Second- and third-generation fluoroquinolones are useful in community-acquired pneumonia, acute exacerbation of chronic bronchitis, bacterial rhinosinusitis, and urinary tract infections. Several of these agents are approved for single-dose oral treatment of uncomplicated gonorrhea. They are generally inactive against anaerobes and beta-hemolytic streptococci. Trovafloxacin, the only fourth-generation fluoroquinolone currently available, has the broadest spectrum, including anaerobic organisms that cause intraabdominal and pelvic infections. Its usefulness is limited, however, by a high risk of hepatotoxicity. The fluoroquinolones are generally well tolerated. The most frequent side effects are nausea, abdominal distress, dizziness, and insomnia. Elimination is primarily renal, and dosage must be adjusted for patients with renal failure. The drugs accumulate in articular cartilage and can can cause severe damage during rapid growth of that tissue; hence, they are contraindicated in patients younger than 18. Blood dyscrasias and renal toxicity have occasionally been observed. Use during strenuous exercise may be hazardous to joints and can cause tendon rupture. These drugs can interfere with the hepatic biotransformation of theophylline and warfarin.