Isotretinoin is a drug used in treating severe cystic acne- also known as nodular acne- that does not respond to other treatments including clindamycin or benzoyl peroxide that is applied to the skin. It belongs to the class referred to as retinoids. Isotretinoin works by decreasing the production of facial oil (sebum). Very high amounts of sebum can lead to severe acne. If left untreated, severe acne may result in permanent scarring. Isotretinoin works by slowing down the production of various substances that result acne.