Basic information of  citrate:
(1)CAS No.: ;
(2)Appearance: golden brown or off-yellow crystal;
(3)ATC code G04BE03;
(4)PubChemL: CID 5281023;
(5)DrugBank: DB00203; (6)ChemSpider: 56586;
(7)UNII: 3M7OB98Y7H;
(8)KEGG: D08514;
(9)ChEMBL: CHEMBL1737;
(10)Formula: C₂₂H₃₀N₆O₄S;
(11)Mol. mass: base: 474.6.

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, an enzyme that regulates blood flow in the penis.

Sildenafil (compound UK-92,480) was synthesized by a group of pharmaceutical chemists working at Pfizer's Sandwich, Kent research facility in England. It was initially studied for use in hypertension and angina pectoris. The drug was patented in 1996, approved for use in erectile dysfunction by the US Food and Drug Administration on March 27, 1998, becoming the first oral treatment approved to treat erectile dysfunction in the United States, and offered for sale in the United States later that year. It soon became a great success: annual sales of Viagra in the period 1999–2001 exceeded $1 billion.

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. As well as erectile dysfunction, sildenafil citrate is also effective in the rare disease pulmonary arterial hypertension (PAH). It relaxes the arterial wall, leading to decreased pulmonary arterial resistance and pressure. Sildenafil has been shown to be useful for the prevention and treatment of high-altitude pulmonary edema associated with altitude sickness such as that suffered by mountain climbers. Professional athletes have been documented using sildenafil, believing the opening of their blood vessels will enrich their muscles. In turn, they believe that it will enhance their performance.

In clinical trials, the most common adverse effects of sildenafil use included headache, flushing, dyspepsia, nasal congestion and impaired vision, including photophobia and blurred vision.