In 1998, Viagra became the first oral medication approved by the United States Food and Drug Administration to treat male impotence, also known as erectile dysfunction. Since that time, the drug has been prescribed to approximately 16 million men worldwide.

Viagra is the trade name of sildenafil citrate, a synthetic heterocyclic piperazine derivative marketed by Pfizer, Incorporated. Within the body, the drug acts by selective inhibition of cyclic guanosine monophosphate (cGMP) phosphodiesterase type 5, which facilitates the relaxation of smooth muscles and an influx of blood to the corpus cavernosum. The production of these conditions typically results in erection during sexual stimulation. In fact, studies indicate that as many as four out of five men suffering from impotence are restored sexual function when Viagra is taken, no matter what originally caused the dysfunction. Moreover, taken in pill form approximately an hour before sexual activity, Viagra is much less invasive than many earlier forms of treatment, such as pump devices, surgical implants, and suppositories.

Though safe and effective for most users, Viagra may produce a number of undesirable side effects. The most common of these effects include acid indigestion, headache, blurred vision, sensitivity to light, nasal congestion, diarrhea, and urinary tract infection. A large number of less common side effects have also been reported by some users, but men with previously existing cardiac problems seemingly incur the greatest danger of all Viagra users. In fact, the manufacturer of the drug cautions doctors against prescribing Viagra to anyone who has recently experienced a heart attack or stroke, or those with serious arrhythmias, severe high or low blood pressure, or history of heart failure. The reason for such an advisory is that heart attack, stroke, and other dangerous cardiac conditions have all been reported after use of Viagra, chiefly in persons previously exhibiting cardiac risk factors and sometimes resulting in death.