Sildenafil (Viagra) is now prescribed in over 90% of erectile dysfunction cases. Studies indicate that it helps 70% of patients achieve sexual function. In one 1999 study, overall male satisfaction was 65%. Not surprisingly, the best results occurred in men who had the fewest sexual problems before treatment, but even men with severe erectile dysfunction had a 41% satisfaction rate. A 2000 study of men who had responded well initially to sildenafil found that 96% of them were satisfied with the treatment after two to three years.
Administration and Effect. Sildenafil is effective within 20 to 40 minutes. The drug works only when the man experiences some sexual arousal. Patients usually take 50 mg, although lower doses (eg, 25 mg) may be appropriate in some groups, such as elderly patients. Sildenafil should not be used more than once a day and the dose should not exceed 100 mg. Used alone or in combination with penile injections, it may help men who did not respond to initial therapy with penile injections alone, but side effects can be quite intense when the combination is used.
Mechanism of Actions. Sildenafil was originally developed for heart disease, but was found to have a unique mechanism of action that targeted only factors in the penis. The drug blocks the enzyme phosphodiesterase-5 (PDE5). This action maintains persistent levels of cyclic GMP, a chemical that is produced in the penis during sexual arousal and which is the primary chemical that relaxes smooth muscles and increases blood flow.
Common Side Effects. Common side effects include the following:
- Muscle aches.
- Gastrointestinal distress.
- Nasal congestion.
Other effects of Viagra have raised somewhat greater concern:
- Visual Effects. About 2.5% of men experience abnormal visual effects that include seeing a blue haze and temporary increased brightness. Experts believe that visual disturbances are related to the inhibition of phosphodiesterase enzymes, but the effect appears to be temporary and insignificant. Still, men at risk for eye problems and who take sildenafil regularly should have frequent eye examinations with an ophthalmologist.
- Dangerous Interaction with Nitrate Drugs. Taking sildenafil along with nitrates can cause a sudden and dangerous drop in blood pressure, and the effects have been fatal in some men. No one taking nitrates, including nitroglycerin or the street drug amyl nitrate, should take sildenafil. In men with normal blood pressure and in those taking other antihypertensive medication, sildenafil causes modest decreases in blood pressure and very small changes in heart rate, but does not appear to pose a significant risk.
- Interactions with Other Drugs. Other drugs that may have interactions with sildenafil include certain antibiotics, such as erythromycin, and acid blockers, such as cimetidine (Tagamet).
- Risk of Priapism. The drug poses a very low risk for priapism in most men. (Priapism is sustained, painful, and unwanted erection.) Exceptions are young men with normal erectile function who take Viagra.
- Risk of Urinary Tract Infections in Female Sexual Partners. One medical center reported an unusually high incidence of urinary tract infections in female sexual partners of men who were taking sildenafil. Older women who experience vaginal dryness may be at higher risk for this effect if they are experiencing a sudden increase in sexual activity.
Good Candidates for Viagra. Viagra is a good choice for any man in good health who does not have conditions that preclude taking it. Studies are indicating that sildenafil is also safe and effective for many men who have the following medical conditions:
- Type 1 or 2 diabetes.
- Controlled hypertension.
- Stable heart disease, with symptoms responsive to drug therapy, but not taking nitrates.
- Conditions requiring chronic dialysis.
One study reported that sildenafil was successful in achieving erections in almost two thirds of patients with severe erectile dysfunction due to medical conditions that included high blood pressure and diabetes.
Viagra may also help restore erectile dysfunction in some men with the following conditions:
- Spinal bifida.
- Radiation therapy for local prostate cancer.
- Spinal cord injury with some erectile response.
- Nerve-sparing radical prostatectomy. Viagra may help restore potency in an average of 30% of patients who have had radical prostatectomy. In one study it had an 80% success rate in younger men who were potent before surgery and who had bilateral nerve sparing procedures. (The rate was 40% with only unilateral procedure.) It may take nine months or longer to respond to the drug, so men might benefit from alprostadil injections starting right after surgery to preserve elasticity and help prevent scarring. It is unlikely to be effective for men over 55 who had unilateral or no nerve sparing procedures.
Higher-Risk Candidates. Studies have not yet been conducted using sildenafil in men with high-risk medical conditions. Men with the following conditions should not take Viagra without the recommendation of their physicians and even then should use it with caution:
- Severe heart disease, such as unstable angina, a history of heart attack, or arryhthmias. Viagra increases nerve activity associated with cardiovascular function, especially during physical and mental stress. Men with heart disease may benefit from an exercise test to determine whether resuming sexual activity increases their risk of a heart attack.
- Recent history of stroke.
- Hypotension (very low blood pressure).
- Uncontrolled diabetes.
- Uncontrolled hypertension.
- Taking anticoagulant therapy.
- Heart failure.
- Retinitis pigmentosa. (With this genetic disease, people do not produce phosphodiesterase-5 and do not respond to Viagra.)
SOURCE: American Journal of Obstetrics & Gynecology
Revision date: July 3, 2011
Last revised: by David A. Scott, M.D.