Erythromycin/tretinoin (trade name 'Isotrexin') is a topical gel with two active ingredients: erythromycin 2% w/w and tretinoin 0.05% w/w with a primary indication for the treatment of moderate acne vulgaris. Tretinoin is a pharmaceutical form of retinoic acid (a metabolite of vitamin A). Its mechanism of action is believed to involve reduction in the amount of sebum produced by sebaceous glands on the skin's surface.