Forzest 20 mg works by inhibiting the PDE5 enzyme, which is responsible for breaking down cyclic GMP (cGMP), a substance that helps relax the smooth muscles in the blood vessels of the penis. When PDE5 is inhibited, cGMP levels increase, leading to the dilation of blood vessels and increased blood flow to the penis. This allows men to achieve and maintain an erection when they are sexually aroused.