Apremilast is used to treat psoriatic arthritis (a condition that causes joint pain and swelling and scales on the skin). It is also used to treat moderate to severe plaque psoriasis (skin disease in which red, scaly patches form on some areas of the body) in people who may benefit from medications or phototherapy (a treatment that involves exposing the skin to ultraviolet light). Apremilast is used to treat ulcers in the mouth in people with Behcet's syndrome (a disorder that causes blood vessel swelling in the body). Apremilast is in a class of medications called phosphodiesterase inhibitors. It works by blocking the action of certain natural substances in the body that cause inflammation.