A medication taken to treat erectile dysfunction in men, better known by its trade name, Viagra. Sildenafil is a nitric oxide compound that enhances the sequence of biochemical events that increases blood flow to the penis and relaxes smooth muscle tissue, allowing an erection to develop in response to sexual stimulation. Generally a man should take a dose of sildenafil 30 minutes to an hour before intended sexual intercourse, although the drug can produce its effects for as long as four hours. If there is no sexual stimulation, erection does not occur. Sildenafil often is effective in men with Parkinson’s disease as it acts directly on smooth muscle tissue rather than through neuromuscular communication from the brain. It also is effective when erectile dysfunction is related to age or is a side effect of medications such as antidepressants and adrener-gic antagonists.
Men who are taking nitrate-based medications for heart conditions such as ischemic heart disease, angina, arrhythmias, or congestive heart failure cannot use sildenafil; the combination of nitrates can cause fatal heart attacks. Sildenafil does not interact with dopaminErgic medications (such as lEvodopa) or dopaminE agonist medications (such as bromocriptine and ropinirolE); therefore, it is generally safe for men with Parkinson’s as long as they are not taking other medications that can interact with it. Although there has been open-label use of sildenafil for treatment of sexual dysfunction (difficulty with physical arousal and orgasm) in women, at present this use is not U.S. Food and Drug Administration-(FDA)-approved, and evidence of its effectiveness is inconclusive.
Sildenafil’s common side effects include flushing, headache, sweating, hypotension (low blood pressure), and the perception of looking through a blue filter (which affects blue-green color differentiation). This color misperception occurs because an enzyme in the retina, phosphodiesterase type 6 (PDE6), is chemically similar to the enzyme in the erection sequence, phosphodiesterase type 5 (PDE5), that sildenafil suppresses. The coincidental suppression of PDE6 reduces the retina’s ability to perceive green, allowing blue to become dominant. This is a short-term effect that has no health consequences, but men who experience it sometimes find it disconcerting. A rare but serious side effect is priapism, a persistent erection that lasts longer than four hours. This is often painful and can require medical attention to prevent permanent damage to the penis.