Long-term antibiotic therapy for acne vulgaris: effects on the bowel flora of patients and their relatives.

We have studied the effects of long-term tetracycline and erythromycin administration on the fecal flora of patients and their relatives. Tetracycline administration selects for multiply antibiotic-resistant organisms in both patients and relatives. Erythromycin exerts no such selection pressure. The mechanisms by which this may occur are discussed.