Sometimes when men want to have sex, they have difficulty achieving or maintaining an erection—a condition called erectile dysfunction (ED). This is common in older men, or it may be caused by health issues such as depression, diabetes, or high blood pressure. In 1998, the Food and Drug Administration approved Viagra—the first oral medication to treat most types of ED.

Viagra (sildenafil), which was originally tested as a heart medication and whose true value was discovered accidentally, was a vast improvement over earlier injection treatments for ED: Instead of an immediate and uncontrollable erection, Viagra causes an erection only when a man is sexually aroused. Today, 9 Viagra pills are dispensed every second—that’s nearly 300 million tablets a year.

To understand how Viagra works, it’s important first to understand how an erection forms. When a man becomes sexually aroused, nitric oxide is released into the blood. The nitric oxide stimulates the production of a chemical called cyclic guanosine monophosphate (cGMP), which relaxes the smooth muscles that line the penis. Blood can then flow in freely, inflating it like a water balloon.

At the same time, another enzyme called phosphodiesterase (PDE) works to deactivate the cGMP. The body should produce enough nitric oxide to maintain levels of cGMP until ejaculation occurs, but when a person has ED, this doesn’t always happen. Viagra works by attaching to and disabling PDE in the penis so that the cGMP can build up. The larger the amount of cGMP, the greater the bloodflow and the greater the degree of erection.

Viagra targets one specific enzyme, called PDE-5, that is found primarily in the penis. However, the drug also has an effect on PDE-6—an enzyme in the retina. This can cause people who take Viagra to temporarily see with a bluish tinge. Viagra can also cause headaches and, infrequently, painful erections lasting as long as 24 hours (called priapism). There is also concern that younger men who take Viagra recreationally may become dependent and unable to have sex without it.

Drugs that contain nitrates, such as nitroglycerin for chest pain, can interact with Viagra. While Viagra is safe to take along with medications for heart disease, high blood pressure, and depression, there have been several cases of patients dying of heart attacks, sometimes during sex, after taking Viagra. One hypothesis is that these elderly patients are just not prepared for the physical exertion of sexual activity.

ADDITIONAL FACTS

  1. In 2008, a Google search for Viagra listed more than 17 million Web pages. In comparison, a search for aspirin listed only 3.3 million.
  2. Texas A&M University, in a state that is one of the nation’s top producers of watermelons, announced in 2008 that researchers had identified in watermelon rind a Viagra-like compound called citrulline, which helps relax the body’s blood vessels.