The main component of Viagra is Sildenafil Citrate. Sildenafil belongs to a group of medications called phosphodiesterase type 5 inhibitors. Commonly the medicine is used to treat erectile dysfunction (male impotence). Sildenafil Citrate affects the response to sexual stimulation. Using a chemical called nitric oxide it enhances smooth muscle relaxation which allows increase blood flow into your penis to achieve and maintain erection. Sildenafil Citrate can be used for pulmonary arterial hypertension treating.