Sildenafil Citrate

Paula Anne Ford-Martin & Rebecca J Frey. The Gale Encyclopedia of Medicine. 3rd edition. Volume. 4. Detroit: Gale, 2006.


Sildenafil citrate (Viagra) is a medication used to treat erectile dysfunction (ED), or impotence, in men.


Labeled Use

Viagra treats erectile dysfunction, the inability to achieve and/or maintain an erection of the penis that is adequate for sexual intercourse. Ten to fifteen million men in the United States suffer from ED, and by age 65, up to 25% of men have experienced impotence problems. Erectile dysfunction can be caused by a number of physical and psychological conditions, including diabetes, depression, prostate cancer, spinal cord injury, multiple sclerosis, artherosclerosis, and heart disease. Injuries to the penis that cause nerve, tissue, or vascular damage can trigger impotence. It is also a common side effect of some prescription medications, including antihistamines,antidepressants, antihypertensives, antipsychotics, beta blockers, diuretics, tranquilizers, appetite suppressants, cimetidine (Tagamet), and finasteride (Propecia).

A study of African American and Hispanic men published in 2002 reported that Viagra appears to be equally safe and equally effective across different racial and ethnic groups in the United States.

Investigational Uses

Although not approved for use in women, clinical studies have shown that sildenafil citrate may be effective in relieving female sexual dysfunction for some women. In one study, both female and male study participants who suffered from sexual dysfunction related to their use of such psychotropic medications asbenzodiazepines reported an increase in arousal and overall sexual satisfaction when they began taking Viagra. Several studies have also indicated the drug may be effective in improving libido and arousal in women taking selective serotonin uptake inhibitors (SSRIs).

Another possible use of sildenafil in women is the treatment of infertility. Women who have had repeated failures within vitro fertilization (IVF) due to poor development of the tissue that lines the uterus may benefit from treatment with vaginal suppositories containing sildenafil. One study reported that 70% of patients had a significant thickening of the uterine lining, with 29% having a successful implantation of a new embryo, and 45% achieving ongoing pregnancies.

Another investigational study conducted by researchers at Johns Hopkins University School of Medicine in Baltimore, and published in the August 2000 issue of the Journal of Clinical Investigation found that Viagra may have additional clinical promise for people with diabetes beyond treating ED. In animal studies, Viagra was effective in relaxing the pyloric muscle of stomach, improving digestion and relieving the symptoms of gastroparesis. Up to 75% of people with diabetes suffer from gastroparesis, which causes bloating, nausea, loss of appetite, and vomiting. Further human studies are needed to evaluate Viagra’s effectiveness in treating this common diabetic complication.

Because of its capacity to enhance nitric oxide production, sildenafil has been investigated as a possible treatment for other disorders that are caused by impaired nitric oxide production. One such disorder is esophageal motility dysfunction (achalasia), in which the smooth muscles of the esophagus and the cardiac sphincter remain constricted, causing difficulty in swallowing, regurgitation of food, and chest pain when eating. A study published in 2000 in the journal Gastroenterology found that sildenafil temporarily improved the condition in some patients by relaxing the lower esophageal muscles. An Italian study reported in 2002 that sildenafil shows genuine promise as a treatment for spastic esophageal disorders.


Viagra is not labeled or approved for use by women or children, or by men without erectile dysfunction. The medication may also be contraindicated (not recommended for use) in patients with certain medical conditions.

Because sexual activity can stress the heart, men who have heart problems should check with their physician to see if sexual activity is recommended. Viagra may trigger temporary hypotension (low blood pressure) and is known to increase cardiovascular nerve activity, so it is prescribed with caution in men with a history of heart attack, artherosclerosis, angina, arrhythmia, and chronic low blood pressure problems. However, a study published in the March 15, 2001, British Medical Journal found no evidence that the drug causes a higher incidence of heart attack. A four-year update on the safety of Viagra published in September 2002 corroborated the findings of the British report, and stated that the only absolute contraindication for the use of sildenafil is the concurrent use of nitrates.

Anyone experiencing cardiovascular symptoms such as dizziness, chest or arm pain, and nausea when participating in sexual activity after taking Viagra should stop the encounter. They should also not take Viagra again until they have discussed the episode with their healthcare provider.

It is recommended that men with kidney or liver impairments, and men over age 65, start at the lowest possible dosage of Viagra (25 mg). Clinical studies have shown that the drug builds up in the plasma of these patients to a concentration that is three to eight times higher than normal. Caution is also recommended in prescribing the drug to individuals with retinitis pigmentosa, a rare genetic eye disorder. Viagra should not be taken more than once per day by anyone.

Viagra has not been studied for use on patients with stomach ulcers and bleeding disorders, and its safety in these individuals is unknown. Men who have either of these conditions should let their physician know before taking Viagra. It should also be used with caution in men with misshapen or deformed penises, such as those with Peyronie’s disease, cavernosal fibrosis, or with angulation of the penis.

Men who take medications containing nitrates (e.g., nitroglycerin, isosorbide mononitrate, isosorbide dinitrate) should never take Viagra, as the interaction between the two drugs may cause a dramatic drop in blood pressure, and possibly trigger a heart attack or stroke. This includes illegal recreational drugs such as amyl nitrates (also known as poppers).

Viagra may also interact with other prescription and over-the-counter (OTC) medications, either magnifying or diluting the intended therapeutic effects of one or both drugs. Some drugs that have a known interaction with Viagra include the protease inhibitor ritonavir and the antibiotic erythromycin. For this reason, it is critical that men who are prescribed Viagra let their healthcare providers know all the medications they are taking.

Other medications and therapies for erectile dysfunction, including vacuum or pump devices, drug injections (Caverject), and urethral suppositories (MUSE), should never be used in conjunction with Viagra.


Sildenafil citrate was originally developed in 1991 as a treatment for angina, or chest pain. The drug, marketed under the name Viagra, received FDA market clearance as a treatment for impotence in March 1998, and since that time it has been prescribed for over 10 million men worldwide. It was the first oral medication approved for ED treatment. A newer drug, tadalafil, has been developed to treat men who do not respond to sildenafil. Tadalafil has gained preliminary approval in the European Union (EU), and is in the final stages of regulatory approval in Canada as of November 2002.

Viagra is a vasodilator, a drug that has the effect of dilating the blood vessels. It works by improving blood circulation to the penis, and by enhancing the effects of nitric oxide, the agent that relaxes the smooth muscle of the penis and regulates blood vessels during sexual stimulation, allowing the penis to become engorged and achieve an erection.

The average recommended dose of Viagra is 50 mg. For men that do not respond adequately to this amount, the dosage may be increased up to 100 mg or decreased to 25 mg. The medication is taken approximately one hour before sexual activity is planned, and may remain effective for up to four hours.

Viagra does not increase sexual desire. Sexual stimulation and arousal are required for the medication to be effective. Despite its widespread use as a recreational drug, it is not an aphrodisiac and there is no clinical evidence that it improves sexual performance in men who are not suffering from ED.

Many insurance plans provide coverage or reimbursement for sildenafil citrate provided it is prescribed to treat erectile dysfunction. A 1999 report issued by a health insurance consulting group indicated that almost half of the men taking Viagra at least once weekly receive insurance reimbursement for the drug. The pills cost approximately $10 each, and insurers may limit coverage to a specific number of pills each month.


Viagra requires time to be absorbed by the body and become effective. The average recommended time frame for taking the drug is one hour before initiating sexual activity, although depending on an individual’s response to the drug, this time can vary from four hours to 30 minutes.

Men should always consult with their physician before beginning treatment with sildenafil citrate. The medication is not for everyone, and a healthcare professional needs to evaluate medical history and perform a thorough medical examination before prescribing the drug. In addition, erectile dysfunction may be a symptom of an undiagnosed condition (i.e., diabetes) for which treatment is critical, and may actually reverse the impotence problem.


The most commonly reported side effects of Viagra are headache, flushing of the face, upset stomach, and nasal congestion.

Other less common side effects include, but are not limited to:

  • vision problems, including sensitivity to light, blurred vision, and a color tinge to vision
  • urinary tract infection
  • diarrhea
  • dizziness
  • rash

Side effects may be reduced or eliminated through adjustments to dosage. Men who experience these symptoms should consult their physician.

Priapism, a painful and prolonged erection that lasts for two to six hours, is a rare but potentially serious side effect of Viagra. Because prolonged erection can permanently damage the tissues of the penis, anyone who experiences an erection lasting over four hours should call a healthcare professional immediately.

Men who are taking Viagra and inadvertently or intentionally take a medication containing nitrates may suffer from life-threatening hypotension—a severe drop in blood pressure.

The cardiovascular risks of sildenafil citrate are still under investigation. The drug is known to cause dips in blood pressure and to boost cardiovascular nerve activity. Some cardiovascular-related deaths have been reported in men who use Viagra, but it is unclear whether the fatalities were due to the drug itself or to the underlying heart disease. Further complicating the picture is the fact that the stress of sexual activity may have triggered the fatal cardiac event with or without the use of Viagra. The BMJ study, and a report published in the April 18, 2001 issue of the Journal of the American Medical Association (JAMA) suggest that the drug does not increase the risk of heart attack. However, JAMA also notes that further studies are necessary to confirm this finding.

Although it is a prescription drug, as of early 2001 there was still a thriving illicit market for Viagra via the Internet. Aside from the health risks recreational use of the drug poses to individuals with heart conditions and other contraindicated disorders, any adverse effects caused by Viagra cannot be tracked by regulatory authorities if it has been illegally obtained. In addition, the drug appears to be toxic in large doses. In November 2002, a group of French toxicologists reported the case of a 56-year-old male who took a fatal overdose of Viagra.

Normal Results

When used as directed, Viagra allows men with erectile dysfunction to achieve and maintain a penile erection when aroused during sexual activity. Double-blind, randomized clinical trials of sildenafil citrate have shown that the drug has an 63-82% efficacy rate in improving erectile activity among men with ED, depending on the dose administered (between 25 and 100 mg), compared to a 24% improvement in men receiving a placebo.