Viagra (sildenafil) produces its physiological effects by blocking the enzyme that hydrolyzes the second messenger by which nitric oxide produces its physiological effects. The second messenger is

Viagra (sildenafil) is an effective and selective inhibitor of cGMP phosphodiesterase. Erections are produced by the release of NO, which inhibits the smooth muscle of the corpus caverosaallowing blood to fill the penis. NO increases the synthesis of cGMP, which produces smooth muscle relaxation. By blocking the breakdown of cGMP, Viagra prolongs the action of NO and erections.