Patients with moderate acne took either a sub-antimicrobial dose of doxycycline (20 mg twice daily) or placebo. The low dose reduced acne lesions more than placebo and was well tolerated. Importantly, tests showed no detectable increase in antibiotic resistance among skin bacteria, supporting the idea that doxycycline can help acne partly through anti-inflammatory effects rather than by acting as a strong antibiotic.