We compared the efficacy and safety of lymecycline 300 mg od vs lymecycline 150 mg bid or placebo in the treatment of moderate to severe acne. 271 patients received either oral lymecycline 300 mg od + placebo od, lymecycline 150 mg bid, or placebo bid, for 12 weeks. Reduction in inflammatory lesion counts at week 12 was the primary efficacy variable (global improvement was a primary efficacy parameter vs placebo) and safety was assessed by adverse events. Lymecycline 300 mg od was non-inferior to lymecycline 150 mg bid at all time points and superior to placebo throughout the study. Drug-related adverse events were similar for all treatment groups. Lymecycline 300 mg od is as effective and safe as lymecycline 150 mg bid in the treatment of moderate to severe acne. This new, once daily formulation could potentially contribute towards improved compliance rates with oral tetracyclines.