The mechanism of action of  antibiotics is  primarily antimicrobial. Doses of 1 g tetracycline or erythromycin will suppress the number of P. acnes by 1 log cycle and reduce surface free  fatty acids  by up to 50 per cent. This reduction in P. acnes numbers is  not as large as that produced with  benzoyl peroxide; it is quite likely that an important mechanism of action of oral antibiotics is via their anti-inflammatory effects.

They probably help acne by reducing chemotaxis to P.  acnes and by  modifying the alternate and classical complement pathways. They are also potent scavengers of the superoxide radical. The dose of antibiotics required to decrease chemotaxis is  less than that which suppresses the number of P. acnes.