Sub-antimicrobial-dose doxycycline for acne (RCT, 2003)

Fifty-one adults with moderate acne took either doxycycline 20 mg twice daily or a placebo for six months. The doxycycline group had significantly greater reductions in comedones and inflammatory lesions. The point of the low dose is that it is below the level that kills bacteria, and the study confirmed this: bacterial counts, antibiotic susceptibility and colonisation by other organisms were unchanged. That is the finding that matters — benefit without driving resistance. The trial was small and eleven people did not finish.