The drug combination erythromycin/tretinoin (trade name Isotrexin) is a topical gel with active ingredients erythromycin 2% w/w and tretinoin 0.05% w/w for the treatment of acne.
Tretinoin is a type of vitamin A. It reduces the production of sebum. (Sebum is a sticky oil-like substance which is produced from the sebaceous gland.)
Erythromycin is an antibiotic.
Acne is caused by blockage of the pores. This leads to an accumulation of the skins natural oils and bacteria (Propionibacterium acnes) in the skin (which feed on the oils and keratin) which further inflame the gland causing acne. The active ingredients reduce the changes the bacteria causes to try to stop the acne.