How does Viagra work?

Viagra promotes erections by relaxing the smooth muscle of the blood vessels thus increasing blood flow in the penis in response to sexual stimulation. It does this by specifically blocking a particular enzyme (protein that assists chemical reactions) called phosphodiesterase type 5 (PDE 5). This is the enzyme that normally breaks down chemicals causing the erectile response. Therefore, by blocking the breakdown of erectile chemicals, the drug promotes a harder and more prolonged erection. Viagra promotes penile erection only in response to sexual stimulation and has no effect in the absence of sexual stimulation.

Therefore, if a healthy man took Viagra and occupied himself with usual daily activities, he would not be expected to develop an erection within the five or six hours in which the drug is active unless sexual stimulation took place.

How well does Viagra work?

Many studies have compared Viagra to other therapies in men with erectile dysfunction from many causes.

Viagra improves erections in 78 per cent of men with erectile dysfunction regardless of the cause.

Specifically, it can improve erections in 70 per cent of patients treated for hypertension and in 57 per cent of patients with diabetes.

Single doses of Viagra enable more men to achieve an erection and to maintain this erection for longer than with placebo (dummy tablets).

Single doses of Viagra seem to produce dose-related rigidity at the base and the tip of the penis in response to looking at sexually stimulating images.

Viagra has also been linked with an increased rate of erections.