Sildenafil, better known by its trade name, Viagra, is a drug which acts to relax muscles that control the flow of blood, allowing blood to flow more easily through many parts of the body, particularly the vessels of the penis. Although it is often used to treat high blood pressure in the pulmonary artery (and House did use it on one occasion for that purpose), its primary use is to treat male erectile dysfunction.
The drug does have some side effects, such as headache, flushing, dyspepsia, nasal congestion, impaired vision and photophobia. Rarer side effects are priapism, low blood pressure, heart attack, arrhythmia, stroke, increased pressure within the eyes and sudden deafness.
It should not be given to certain patients, such as those taking protease inhibitors for AIDS, nitrate based drugs such as nitroglycerine, those with poor liver function, people who already have low blood pressure, those who have recently suffered a stroke or heart attack and people who have disorders of the retina.
The drug's Canadian patent set precedent in Canadian law when the Supreme Court of Canada ruled that the patent holder's application did not properly disclose that sildenafil was the active ingredient. As a result, the drug is off-patent in Canada while it remains on-patent in the United States.