Viagra is an oral drug for male impotence, also known as erectile dysfunction. Viagra has a great safety track record and proven effects that start acting in 30 minutes to 1 hour and last for about 4 hours.

Sildenafil citrate, sold as Viagra, Revatio and under various other trade names, is a drug used to treat erectile dysfunction and pulmonary arterial hypertension (PAH). It was developed and is being marketed by the pharmaceutical company Pfizer. It acts by inhibiting cGMP specific phosphodiesterase type 5, an enzyme that regulates blood flow in the penis. Since becoming available in 1998, sildenafil has been the prime treatment for erectile dysfunction; its primary competitors on the market are tadalafil (Cialis) and vardenafil (Levitra).

The mechanism of action of Sildenafil citrate involves the release of nitric oxide (NO) in the corpus cavernosum of the penis. NO binds to the receptors of the enzyme guanylate cyclase which results in increased levels of cyclic guanosine monophosphate (cGMP), leading to smooth muscle relaxation (vasodilation) of the intimal cushions of the helicine arteries, resulting in increased inflow of blood and an erection.

Sildenafil is a potent and selective inhibitor of cGMP specific phosphodiesterase type 5 (PDE5) which is responsible for degradation of cGMP in the corpus cavernosum. The molecular structure of sildenafil is similar to that of cGMP and acts as a competitive binding agent of PDE5 in the corpus cavernosum, resulting in more cGMP and better erections.[14] Without sexual stimulation, and therefore lack of activation of the NO/cGMP system, sildenafil should not cause an erection. Other drugs that operate by the same mechanism include tadalafil (Cialis) and vardenafil (Levitra).

Sildenafil is metabolized by liver enzymes and excreted by both the liver and kidneys. If taken with a high-fat meal, absorption is reduced; the time taken to reach the maximum plasma concentration

Dosage

Viagra pills are blue and diamond-shaped with the words “Pfizer” engraved on one side, and “VGR xx” (where xx stands for “25″, “50″ or “100″, the dose of that pill in milligrams) engraved on the other. The dose of sildenafil for erectile dysfunction is 25 mg to 100 mg taken not more than once per day between 30 minutes and 4 hours prior to sexual intercourse.

The dosage for pulmonary arterial hypertension (Revatio) is one 20 mg tablet three times a day. Revatio pills are white, round, film-coated tablets imprinted with “RVT 20″ embossed on one side

Viagra is one of the best-known drugs of all time:

Billions of spam e-mail messages advertise Viagra every day. There is so much Viagra spam, in fact, that Pfizer (the maker of Viagra) has a page addressing the problem, called Avoid Fake Viagra.

Pfizer has spent untold millions of dollars advertising Viagra, so you see advertisements for the drug constantly on TV.
Pfizer claims on its Web site that nine Viagra pills are dispensed every second — nearly 300 million tablets per year.
The name recognition of Viagra is so good that nearly every adult in America has heard of the drug and can tell you what it does.
What Viagra does is simple: When it works as intended, Viagra causes a man who is sexually stimulated to get an erection.

• More on Sexuality
• Viagra Quiz
• How Sex Works
• Female Orgasms
• DiscoveryHealth.com: Female Viagra

What should I tell my health care provider before I take this medicine?

They need to know if you have any of these conditions:

• anatomical deformity of the penis, Peyronie’s disease, or ever had an erection that lasted more than 4 hours
• benign prostatic hypertrophy (BPH)
• bleeding disorder
• cancer
• diabetes
• frequent heartburn or gastroesophageal reflux disease (GERD)
• heart disease, angina, high or low blood pressure, a history of heart attack, or other heart problems
• high cholesterol
• kidney disease
• liver disease
• sickle cell disease
• stomach or intestinal ulcer
• stroke
• eye or vision problems, including a rare inherited eye disease called retinitis pigmentosa
• an unusual or allergic reaction to sildenafil, other medicines, foods, dyes, or preservatives
• pregnant or trying to get pregnant
• Breast-feeding.

How should I take this medicine?

Take sildenafil tablets by mouth. Follow the directions on the prescription label. The dose is usually taken 1 hour before sexual activity. You should not take this dose more than once per day. Swallow the tablets with a drink of water. Do not take double or extra doses.

What drug(s) may interact with sildenafil?

Do not take sildenafil if you are taking the following medications:

• nitroglycerin-type drugs for the heart or chest pain such as amyl nitrite, isosorbide dinitrate, isosorbide mononitrate, nitroglycerin, even if these are only taken occasionally
• Sildenafil may also interact with the following medications:
• .alpha blockers, used for high blood pressure or an enlarged prostate. NOTE: Do not take doses of sildenafil higher than 25 mg within 4 hours of taking alpha blockers, such as alfuzosin (UroXatral®), doxazosin (Cardura®), prazosin (Minipress®), or terazosin (Hytrin®).
• bosentan
• certain drugs used for seizures such as carbamazepine, phenytoin, and phenobarbital
• certain drugs used for fungal or yeast infections, such as fluconazole, ketoconazole, and voriconazole
• certain drugs for the treatment of HIV infection or AIDS
• cimetidine
• cisapride
• clarithromycin
• diltiazem
• erythromycin
• grapefruit juice
• mibefradil
• monoamine oxidase inhibitors (Azilect®, Eldepryl®, Emsam®, Marplan®, Nardil®, Parnate®, Zelapar.)
• nitroprusside
• rifabutin
• rifampin
• quinidine
• some drugs for treating depression, anxiety or other mood problems (examples: fluoxetine, fluvoxamine, nefazodone)
• verapamil

Tell your prescriber or health care professional about all other medicines you are taking, including non-prescription medicines, nutritional supplements, or herbal products. Also tell your prescriber or health care professional if you are a frequent user of drinks with caffeine or alcohol, if you smoke, or if you use illegal drugs. These may affect the way your medicine works. Check with your health care professional before stopping or starting any of your medicines.