Avanafil relaxes muscles of the blood vessels and increases blood flow to particular areas of the body. Avanafil helps men with erectile dysfunction get and maintain an erection long enough for sexual intercourse. Do not take avanafil if you are taking or have recently taken nitrates or nitroglycerin. Avanafil belongs to a group of drugs called phosphodiesterase (PDE) inhibitors. This medication comes in tablet form and is taken typically once a day, on an as needed basis, approximately 30 minutes before sexual activity. Common side effects of avanafil include headache, flushing, and back pain.