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Prior research has suggested an association between antimicrobial use and resistance, but none has shown a causal effect. To compare the effects on resistance of two macrolides commonly used for treating respiratory infections, investigators in Belgium randomized 224 healthy adult volunteers to receive azithromycin (500 mg once daily for 3 days), clarithromycin (500 mg twice daily for 7 days), or placebo. Macrolide resistance in oral streptococcal flora was assessed at baseline, the day after the conclusion of therapy, and periodically through day 180.
The proportion of macrolide-resistant streptococci increased significantly in both treatment groups compared with the placebo group. No significant increase in macrolide resistance was seen in…