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What is sildenafil, and how does it work (mechanism of action)?

Viagra (sildenafil) is an oral drug that is used for treating impotence (or erectile disfunction, ED, the inability to attain or maintain a penile erection). It is in a class of drugs called phosphodiesterase inhibitors (PDE-5 inhibitors) that also includes

It has been estimated that impotence affects 140 million men worldwide. Over half of all men with impotence are thought to have some physical (medical) cause. The remainder are believed to have psychogenic causes of impotence. Medical causes of impotence include diabetes and circulatory, neurological, or urological conditions.

Penile erection is caused by the engorgement of the penis with blood. This engorgement occurs when the blood vessels delivering blood to the penis increase the delivery of blood and the blood vessels carrying blood away from the penis decrease the removal of blood. Under normal conditions, sexual stimulation leads to the production and release of nitric oxide in the penis. Nitric oxide then activates the enzyme, guanylate cyclase, which causes the production of cyclic guanosine monophosphate (cGMP). It is the cGMP that is primarily responsible for the erection by affecting the amount of blood that the blood vessels deliver and remove from the penis.

Viagra inhibits an enzyme called phosphodiesterase-5 (PDE5) which destroys the cGMP. Thus, Viagra prevents the destruction of cGMP and allows cGMP to accumulate and persist longer. The longer cGMP persists, the more prolonged the engorgement of the penis. The FDA approved Viagra in March 1998.

What brand names are available for sildenafil?


Is sildenafil available as a generic drug?


Do I need a prescription for sildenafil?


What are the side effects of sildenafil?

Approximately 15% of persons taking Viagra experience side effects. The most common side effects are:

  • facial flushing (1 in 10),
  • headaches (1 in 6),
  • stomach pain,
  • nasal congestion,
  • nausea,
  • dizziness,
  • rash,
  • urinary tract infections (UTIs)
  • diarrhea, and
  • an inability to differentiate between the colors green and blue.
Erectile Dysfunction (ED) Causes and Treatment

What is the dosage for sildenafil?

  • The usual recommended dose is 25 to 100 mg 1 hour before sexual activity.
  • The maximum dose is 100 mg daily.
  • The elderly (over 65 years of age) should start at 25 mg before sexual activity.
  • Sildenafil is rapidly absorbed. Maximum observed plasma concentrations are reached within 30 to 120 minutes (median 60 minutes) of oral dosing in the fasted state. When sildenafil is taken with a high fat meal, the rate of absorption is reduced, with an average delay in the time to maximal concentration of 1 hour.

Which drugs or supplements interact with sildenafil?

Viagra increases the effects of the blood pressure lowering medications. It also increases the blood pressure lowering effects of nitrates, for example, isosorbide dinitrate (Isordil), isosorbide mononitrate (Imdur, Ismo, Monoket), nitroglycerin (Nitro-Dur, Transderm-Nitro) that are used primarily for treating angina. Patients taking nitrates should not receive Viagra.

Patients should not combine Viagra with other PDE5 inhibitors (for example, vardenafil [Levitra], tadalafil [Cialis]).

Cimetidine (Tagamet), erythromycin, ketoconazole (Nizoral), itraconazole (Sporanox), atazanavir (Reyataz), and mibefradil (Posicor) can cause marked increases in the amount of Viagra in the body. Patients taking these medications should be observed carefully if sildenafil is used.

It is expected that rifampin will decrease blood levels of Viagra and probably reduce its effectiveness.

Is sildenafil safe to take if I'm pregnant or breastfeeding?

Although extensive testing in animals has demonstrated no negative effects on the fetus, Viagra has not been studied in pregnant women. There is no effect on sperm count or motility of sperm in men.

It is not known whether sildenafil is excreted into breast milk.

What else should I know about sildenafil?

What preparations of sildenafil are available?

Sildenafil (Viagra) is a blue, film-coated, rounded, diamond-shaped tablet that comes in 25, 50, and 100 mg tablets for treating impotence.

How should I keep sildenafil stored?

Tablets should be kept at room temperature between 15 C and 30 C (59 F and 86 F).

Reference: FDA Prescribing Information.

Last Editorial Review: 8/6/2015 Erectile Dysfunction (ED) Causes and Treatment

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You are encouraged to report negative side effects of prescription drugs to the FDA. Visit the FDA MedWatch website or call 1-800-FDA-1088.

See more info: sildenafil on RxList

Reviewed on 8/6/2015


Reference: FDA Prescribing Information.

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Read more: Sex Tips for Men: How to Have a Better Sex Life

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