Antibiotic therapy for acne is very common. Antibiotics are frequently used in acne, either systemically or topically. Systemic antibiotics are indicated as treatment of moderate and quite severe acne or if acne is considered as very serious by the patient for psychological or social reasons. Results are very often excellent, but failure is possible; in this case using another treatment, especially isotretinoin, is necessary. A few antibiotics are useful: tetracyclines (tetracycline, doxycycline, minocycline, lymecycline), erythromycin, co-trimoxazole and trimethoprim. Their side effects are reviewed. During pregnancy the best antibiotic is erythromycin. For the nursing mother it is generally said that tetracyclines are contraindicated but the risks if they exist are certainly slight. The mechanism of action of systemic antibiotics for acne is not perfectly clear as it is not only antimicrobial: they diminish chemotaxis of polymorphonuclear leukocytes, modify the complement pathways and inhibit the polymorphonuclear leukocyte chemotactic factor and the lipase production in Propionibacterium acnes.