This medication is used to treat a certain type of arthritis (psoriatic arthritis). Apremilast is also used to treat a certain type of skin condition (moderate to severe plaque psoriasis). Apremilast belongs to a class of drugs known as phosphodiesterase 4 (PDE4) inhibitors. For the treatment of psoriatic arthritis, it decreases pain and swelling, and may help improve flexibility in the affected joints. For the treatment of plaque psoriasis, it may help to reduce the redness, thickening, and scaling of the skin that occurs with this condition. Apremilast is also used to treat mouth sores in people who have Behcet's disease. It helps to reduce the pain and improve the healing of these mouth sores.