The effect of long term tetracycline treatment for acne vulgaris on the occurrence of R factors in the intestinal flora of man.

R factors are known to be the most important mechanism of antimicrobial resistance of intestinal flora. Short courses with therapeutic doses (1000 mg/day) of tetracycline select for strains containing transferable resistance factors to more than one antimicrobial agent. In this report we show that long term treatment with very low doses (100 mg/day) of tetracycline for acne vulgaris has an equally strong effect favouring establishment of resistent strains and R factors in the intestinal flora of patients.