DOCTOR'S VIEW ARCHIVE
Sildenafil (Viagra) is approved for the treatment of impotence (also known as erectile dysfunction). It was the first oral medication for this purpose.
What are the causes of impotence? How does Viagra affect erection? Are there side effects and drug interactions? Read on...
Impotence is a consistent inability to sustain an erection sufficient for sexual intercourse. Impotence can be a total inability to achieve erection, an inconsistent ability to do so, or a tendency to sustain only brief erections. Incidence rises with age: about 5 percent of men at the age of 40 and between 15 and 25 percent of men at the age of 65 experience impotence. Yet, it is not an inevitable part of aging.
Medical professionals often use the term "erectile dysfunction" to describe impotence. Impotence and erectile dysfunction should be distinguished from other conditions that interfere with sexual intercourse, such as lack of sexual desire and problems with ejaculation and orgasm.
Damage to arteries, smooth muscles, and nerves of the penis can lead to impotence. One common cause of impotence is blood vessel and nerve damage due to diabetes mellitus. Other causes of impotence include low testosterone levels, kidney disease, chronic alcoholism, multiple sclerosis (a disease of the nervous system), atherosclerosis (leading to blood vessel hardening), surgical damage to nerves and blood vessels (for example, prostate surgery), and injury to the penis, bladder, pelvis and the spinal cord.
Some common medicines produce impotence as a side effect. These include certain high blood pressure drugs, antihistamines, antidepressants, tranquilizers, appetite suppressants, and cimetidine (an ulcer drug). Smoking affects blood flow in veins and arteries, and is also believed to lead to impotence.
Psychological factors such as stress, anxiety, guilt, depression, low self-esteem, and fear of sexual failure are associated with impotence.
If the underlying condition leading to impotence cannot be corrected, or if impotence problems persist, medications can help certain men achieve erection sufficient for sexual Intercourse as well as maintain erection after penetration.
The penis contains two chambers, called the corpora cavernosa, which run the length of the organ. Erection begins with sensory and mental stimulation. Impulses from the brain and local nerves cause the muscles of the corpora cavernosa to relax, allowing blood to flow in and fill the chambers. The inflow of blood creates pressure in the corpora cavernosa chambers, making the penis expand. Erection is reversed when muscles in the penis contract, stopping the inflow of blood and causing the blood in the chambers to flow out.
Cyclic guanosine monophosphate (cGMP), a signaling molecule within cells, tells the smooth muscle in the corpora cavernosa to relax and this lets blood flow into the chambers and be trapped for an erection.
The erection is lost when cGMP is degraded by an enzyme (PDE5), lowering the level of cGMP, so the muscle of the corpus cavernosa contracts again, the blood flows out of the chambers of the corpus cavernosa and the erection is no more.
Sildenafil (Viagra) is an oral agent recently approved by the United States FDA for the treatment of erectile dysfunction. Its major mode of action is inhibition of the enzyme PDE5. Inhibiting this enzyme allows the cyclic GMP to stay around longer, thus maintaining erection. Viagra is not an aphrodisiac, it is not effective without sexual stimulation. It mainly enhances the normal physiologic erection.
Clinical trials using Viagra have been conducted in patients with vascular diseases, diabetes mellitus, spinal cord injuries, psychogenic causes, and after radical prostrate surgery. Some 70%-80% of men reported improved quality of their erections. Over 50% of patients were able to have successful vaginal penetration using Viagra.
Viagra is administered orally one hour prior to the anticipated sexual activity, preferably on an empty stomach. The absorption of Viagra can be decreased by a fatty meal.
Viagra is eliminated mainly by the cytochrome p450 system in the liver. Elderly men and men with liver or kidney dysfunction have reduced ability to eliminate the drug. Therefore, these patients should start with the lowest dose (25 mg) to avoid problems with toxicity. Medications that interfere with cytochrome p 450 (such as erythromycin, cimetidine, and ketoconazole) can also decrease the elimination of Viagra. These other medications should preferably be stopped when using Viagra, or Viagra should be used cautiously starting with the lowest dose.
Viagra used alone can cause mild drop in blood pressure. But it should NOT be used together with nitrates (nitroglycerin, isordil, and other related preparations) because of significant low blood pressure (hypotension) that can develop.
Side effects are dose related. Headaches were noted in 16%, flushing 10%, dyspepsia 7%, and abnormal vision (disturbance in blue/green color discrimination) 3%.
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