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Growing antibiotic resistance is a major public health concern, but few studies have demonstrated a causal relation between antibiotic exposure and resistance in individual patients. In a double-blind, placebo-controlled study, researchers randomized 224 healthy adults to receive azithromycin (500 mg once daily for 3 days), clarithromycin (500 mg twice daily for 7 days), or placebo and measured macrolide resistance in oral streptococcal flora before and after treatment.
The proportion of macrolide-resistant streptococci increased significantly in both antibiotic groups compared with the placebo group, and this difference persisted for 6 months. Resistance was highest on day 8 (the day after treatment) in the clarithromycin group (mean increa…