Sildenafil citrate, sold as Viagra, Revatio and under various other trade names, is a drug used to treat erectile dysfunction and pulmonary arterial hypertension (PAH).It acts by inhibiting cGMP-specific phosphodiesterase type 5 (PDE5), an enzyme that promotes degradation of cGMP, which regulates blood flow in the penis. The primary indication of sildenafil is treatment of erectile dysfunction (inability to sustain a satisfactory erection to complete intercourse). Its use is now standard treatment for erectile dysfunction in all settings, including diabetes. Medical Uses 1)Sexual dysfunction 2)Pulmonary hypertension 3)Antidepressant-associated sexual dysfunction 4)Altitude sickness Sildenafil Dosage Viagra:for erectile dysfunction, comes in blue, diamond-shaped pills, in doses of 25, 50, or 100 milligrams. The patient takes a maximum of one pill in a 24-hour period, between 30 minutes to 1 hour before sexual intercourse. Revatio:for pulmonary arterial hypertension, comes in white, round, film-coated tablets. Patients take one 20 mg Revatio tablet three times a day.