What is the use of norethisterone tablets the main component of Viagra is Sildenafil Citrate. Sildenafil Citrate affects the response to sexual stimulation. It acts by enhancing smooth muscle relaxation using nitric oxide, a chemical that is normally released in response to sexual stimulation. This smooth muscle relaxation allows increased blood flow into certain areas of the penis, which leads to an erection. Sildenafil Citrate is applied for the treatment of erectile dysfunction (impotence) in men and pulmonary arterial hypertension. Sildenafil Citrate may also be used for other purposes not listed above. A serious allergic reaction to this drug is very rare, but seek immediate medical help if it occurs.