What Is Viagra? What Is Sildenafil Citrate?
Viagra is the brand name for Sildenafil citrate, and is used for treating erectile dysfunction and pulmonary arterial hypertension. Originally developed by scientists in Great Britain, it was brought onto the market by Pfizer Inc., a US pharmaceutical company. Viagra is also sold under brand name Revatio.
Sildenafil citrate’s chemical formula is C22H30N6O4S.
Viagra works by inhibiting an enzyme called cGMP-specific phosphodiesterase type 5, that delays degradation of cGMP, which controls blood flow in the penis. It came onto the market in 1998 and has become the prime treatment for impotence (erectile dysfunction). Viagra’s main competitors are Cialis (tadalafil) and Levitra (vardenafil).
Dosage
Viagra, for erectile dysfunction, comes in blue, diamond-shaped pills, in doses of 25, 50, or 100 milligrams. The patient takes a maximum of one pill in a 24-hour period, between 30 minutes to 1 hour before sexual intercourse.
Revatio, for pulmonary arterial hypertension, comes in white, round, film-coated tablets. Patients take one 20 mg Revatio tablet three times a day.
Adverse events
According to clinical trial results, the most common side effects include headaches, nasal congestion, impaired vision, photophobia, and dyspepsia. Less commonly, some users experienced cyanopsia (everything had a tinted blue tinge). In very rare cases it can lead to vision impairment and nonarteritic anterior ischemic optic neuropathy. Postmarketing surveillance side effects have included (very rare) priapism, heart attack, sudden hearing loss, increased intraocular pressure, and ventricular arryhythmias. Since 2007, in the USA its labeling has included a warning of the potential risk of sudden hearing loss.
Viagra can decrease blood supply to the optic nerve, causing sudden vision loss. This very rare adverse event occurred mainly to patients who had heart disease, hypertension, diabetes, high cholesterol, or pre-existing eye problems. Nobody really knows whether the vision loss was caused by the Viagra.
Interactions – HIV patients on protease inhibitors should discuss using Viagra with their doctors – protease inhibitors increase the likelihood and severity of side effects. Experts say such patients should have no more than 25mg, and not more often than every 48 hours.
If the patient is taking alpha blockers, make sure they take that medication and Viagra at least four hours apart, to prevent low blood pressure.
Contraindications – the following individuals should not take Viagra (or check with their doctors first):
- Those on nitric oxide donors, nitrates and organic nitrites
- Men who are advised to refrain from sexual intercourse because of a cardiovascular risk factors
- Patients with severe hepatic impairment
- Patients with kidney disease
- Individuals with low blood pressure (hypotension)
- Those who had a recent heart attack
- Those who had a recent stroke
- Individuals with hereditary degenerative retinal disorders
Jet lag – Patricia V. Agostino, Santiago A. Plano, and Diego A. Golombek, from the Universidad Nacional de Quilmes, Argentina, were awarded the ig Nobel Prize in Aviation for discovering that Viagra helped hamsters recover from jet lag. (PNAS, 2007)
Exercise capacity – some athletes take Revatio to increase their exercise capacity.
Written by Christian Nordqvist
