Erythromycin is an antibiotic that acts against the bacteria P.acne.the bacteria responsible for acne inflammation.It also has antiinflammatory action and reduces the swelling and redness.In recent times the bacteria P.acne has become resistant to commonly used antibiotics and only antibiotic treatment is not recommended for acne by most experts in the field.Antibiotics should be used under medical supervision in combination with topical treatment with retinoids or benzoyl peroxide for optimum result.The half life of erythromycin is less so you need to take it  at 6 to 8 hourly intervals