Avanafil belongs to a class of medications called phosphodiesterase-5 (PDE5) inhibitors. When a man is sexually stimulated, his body releases nitric oxide, which increases the level of a substance called cGMP. This causes the muscles in the penis to relax and allows for increased blood flow, resulting in an erection. By inhibiting the PDE5 enzyme, avanafil prevents cGMP from being broken down too quickly, helping to sustain the erection.