Why is Viagra prescribed?
Viagra is an oral drug for male impotence, also known as erectile dysfunction (ED). It works by dilating blood vessels in the penis, allowing the inflow of blood needed for an erection.
Most important fact about Viagra
Viagra causes erections only during sexual excitement. It does not work in the absence of arousal.
How should you take Viagra?
Taking Viagra approximately 1 hour before sexual activity works best for most men. Depending on how and when the drug works for you, an interval of one-half hour to as much as 4 hours may prove ideal.
--If you miss a dose...
Viagra is not for regular use. Take it only before sexual activity.
--Storage instructions...
Store at room temperature.
What side effects may occur?
Side effects cannot be anticipated. If any develop or change in intensity, inform your doctor as soon as possible. Only your doctor can determine if it is safe for you to continue taking Viagra.
• More common side effects may include:
Abnormal vision (color tinge, blurring, sensitivity to light), acid indigestion, diarrhea, flushing, headache, nasal congestion, urinary tract infection
Heart attack, stroke, heart irregularities, dangerous surges in blood pressure, and sudden death have all been reported after use of Viagra, usually in men with existing cardiac risk factors, and typically during or shortly after sex.
Why should Viagra not be prescribed?
Do not take Viagra if you are taking any nitrate-based drug, including nitroglycerin patches (Nitro-Dur, Transderm-Nitro), nitroglycerin ointment (Nitro-Bid, Nitrol), nitroglycerin pills (Nitro-Bid, Nitrostat), and isosorbide pills (Dilatrate-SR, Isordil, Sorbitrate). Combining Viagra with these drugs can cause a severe drop in blood pressure.
If Viagra gives you an allergic reaction, do not use it again.
Special warnings about Viagra
If you have heart problems severe enough to make sexual activity a danger, you should avoid using Viagra. Use it cautiously--if at all--if you've had a heart attack, stroke, or life-threatening heart irregularities within the past 6 months. Be equally cautious if you have severe high or low blood pressure, heart failure, or unstable angina (crushing heart pain that occurs at any time).
If you take Viagra and develop cardiac symptoms (for example, dizziness, nausea, and chest pain) during sexual activity, do not continue. Alert your doctor to the problem as soon as possible.
If you have a condition that might result in long-lasting erections, such as sickle cell anemia, multiple myeloma (a disease of the bone marrow), or leukemia, use Viagra with caution. Also use cautiously if you have a genital problem or deformity such as Peyronie's disease. If an erection lasts more than 4 hours, seek treatment immediately. Permanent damage and impotence could result.
If you have a bleeding disorder, a stomach ulcer, or the inherited eye condition known as retinitis pigmentosa, use Viagra with caution. Its safety under these circumstances has not yet been studied.
To avoid low blood pressure, do not take the 50-milligram or 100-milligram dose of Viagra within 4 hours of taking an alpha-blocking drug such as Cardura.
Remember that Viagra offers no protection from transmission of sexually transmitted diseases, such as HIV, the virus that causes AIDS.
Possible food and drug interactions when taking Viagra
If Viagra is taken with certain other drugs, the effects of either could be increased, decreased, or altered. It is especially important to check with your doctor before combining Viagra with the following:

Other impotence remedies including Caverject and Muse
Alpha-blockers such as doxazosin (Cardura)
Amlodipine (Norvasc)
Cimetidine (Tagamet)
Erythromycin (E-Mycin, Ery-Tab, PCE)
Itraconazole (Sporanox)
Ketoconazole (Nizoral)
Nitrates such as Isordil, Nitro-Bid, and Nitro-Dur
Rifampin (Rifadin, Rimactane)
Ritonavir (Norvir)
Saquinavir (Fortovase, Invirase)
Special information if you are pregnant or breastfeeding
Viagra should not be used by women. Its affects during pregnancy and breastfeeding have not been studied.
Recommended dosage
Doses range from 25 milligrams to 100 milligrams, depending on the drug's effect. The usual dose is 50 milligrams. If you are over 65, have liver or kidney problems, or are taking erythromycin, ketoconazole, itraconazole, ritonavir, or saquinavir a dose of 25 milligrams may be sufficient. Your doctor will adjust the dosage if the drug is not working properly for you.
Take Viagra only before sexual activity. The manufacturer recommends a maximum of 1 dose per day (1 dose every 2 days for those taking ritonavir).
To avoid low blood pressure, do not take the 50-milligram or 100-milligram dose of Viagra within 4 hours of taking an alpha-blocking drug such as Cardura.
Overdosage
No overdose of Viagra has been reported. However, any medication taken in excess can have serious consequences. If you suspect an overdose, seek medical attention immediately.