Sildenafil, sold as Viagra and other trade names, is a medication used to treat erectile dysfunction and pulmonary arterial hypertension.Its use for sexual dysfunction in women is not clear.

Common side effects include headaches and heart burn, as well as flushed skin. Caution is advised in those who have cardiovascular disease. Rare but serious side effects include prolonged erections, which can lead to damage to the penis, and sudden-onset hearing loss. Sildenafil should not be taken by people who take nitrates such as nitroglycerin, as this may result in a severe and potentially fatal drop in blood pressure.

It acts by inhibiting cGMP-specific phosphodiesterase type 5 (PDE5), an enzyme that promotes degradation of cGMP, which regulates blood flow in the penis.

It was originally discovered by Pfizer scientists Andrew Bell, David Brown, and Nicholas Terrett.Since becoming available in 1998, sildenafil has been a common treatment for erectile dysfunction; its primary competitors are tadalafil (Cialis) and vardenafil (Levitra).