Sildenafil, sold under the brand name Viagra, among others, is a medication used to treat erectile dysfunction and pulmonary arterial hypertension. It is unclear if it is effective for treating sexual dysfunction in women. It is taken by mouth or by injection into a vein. Onset is typically within twenty minutes and lasts for about two hours.

Common side effects include headaches, heartburn, and flushed skin. Caution is advised in those with cardiovascular disease. Rare but serious side effects include a prolonged erection (priapism) that can lead to damage to the penis, vision problems, and hearing loss. Sildenafil should not be taken by people on nitrates such as nitroglycerin (glycerin trinitrate), as this may result in a serious drop in blood pressure. Sildenafil should not be taken within four hours of taking an alpha blocker.

Sildenafil acts by blocking phosphodiesterase 5 (PDE₅), an enzyme that promotes breakdown of cGMP, which regulates blood flow in the penis. It requires sexual arousal to work. It also results in dilation of the blood vessels in the lungs.

Pfizer originally discovered the medication in 1989 while looking for a treatment for heart-related chest pain. It was approved for medical use in the United States and in the European Union in 1998. In 2020, it was the 183rd most commonly prescribed medication in the United States, with more than 2 million prescriptions. It is available as a generic medication. In the United Kingdom, it is available over the counter. Most of the Active Pharmaceutical Ingredient (API) in Sildenafil, Sildenafil citrate, is produced in Pfizer's Ringaskiddy complex in County Cork, Ireland, which is Pfizer's largest production facility outside of the United States.