Isotroin 20 Capsule belongs to a class of drugs called retinoids (a synthetic form of vitamin A) primarily used to treat a severe form of acne known as nodular acne that no other treatments, including antibiotics, can clear. Nodular acne is severe acne that occurs due to excessive oil production in the skin cells, causing swollen, red and tender lumps in the skin. Acne nodules are larger in size than typical pimples.