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 20 minutes and lasts for about 2 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, however, 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 2017, it was the 217th most commonly prescribed medication in the United States, with more than two million prescriptions. In 2017, it became available as a generic medication after final patents by Pfizer expired. 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, County Cork, which is Pfizer's largest production facility outside of the United States.

The United States Food and Drug Administration (FDA or USFDA) is a federal agency of the Department of Health and Human Services. The FDA is responsible for protecting and promoting public health through the control and supervision of food safety, tobacco products, dietary supplements, prescription and over-the-counter pharmaceutical drugs (medications), vaccines, biopharmaceuticals, blood transfusions, medical devices, electromagnetic radiation emitting devices (ERED), cosmetics, animal foods & feed and veterinary products.

The FDA's primary focus is enforcement of the Federal Food, Drug, and Cosmetic Act (FD&C), but the agency also enforces other laws, notably Section 361 of the Public Health Service Act, as well as associated regulations. Much of this regulatory-enforcement work is not directly related to food or drugs, but involves such things as regulating lasers, cellular phones, and condoms, as well as control of disease in contexts varying from household pets to human sperm donated for use in assisted reproduction.

The FDA is led by the Commissioner of Food and Drugs, appointed by the President with the advice and consent of the Senate. The Commissioner reports to the Secretary of Health and Human Services. Robert Califf is the current commissioner, as of 17 February 2022.The FDA has its headquarters in unincorporated White Oak, Maryland. The agency also has 223 field offices and 13 laboratories located throughout the 50 states, the United States Virgin Islands, and Puerto Rico. In 2008, the FDA began to post employees to foreign countries, including China, India, Costa Rica, Chile, Belgium, and the United Kingdom.