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A summary of agents used to viagra shop online treat ED is shown in Table 1.

The choice of drug therapy for ED depends on the etiology.

Phosphodiesterase Inhibitors Sildenafil citrate, the only phospho-diesterase inhibitor currently approved by the FDA for the treatment of ED, is commonly prescribed. Sildenafil was the 46th most dispensed drug in 2001.(4)Sildenafil acts by inhibiting the cGMP-specific phos-phodiesterase type 5 (PDE5) enzyme in the corpora cavernosa, which prevents the breakdown of cGMP to an inactive compound.

The resulting increased availability of cGMP enhances the erectile response, because cGMP is able to relax the penile smooth muscle and vasodilate the penile arteries. Sildenafil does not directly induce erection of the penis but augments the response to sexual stimulation.

Results vary, depending on the cause of the ED, but studies have shown good efficacy rates for silde-nafil.

A meta-analysis of 27 trials including a total of 6659 patients was 18 recently published.(5)Overall, sildenafil was more efficacious than placebo in improving erectile function. The primary efficacy parameter, successful sexual intercourse attempts, was 57% for men given sildenafil and 21% for men given placebo. The usual dosage range for sildenafil is 25 to 100 mg daily.

Sildenafil should be taken as needed 30 to 60 minutes before sexual intercourse, no more than once daily.

Patients should be informed that silde-nafil has no effect in the absence of sexual stimulation, and it should not be taken with a fatty meal because this may decrease the onset of action.

Dose adjustment is necessary for men who are over 65 years old, men with hepatic impairment, men with renal impairment (creatinine clearance Alprostadil Alprostadil is a synthetic prostaglandin E1 analogue online doctor viagra with alpha-adren-ergic blocking activity. Alprostadil stimulates cyclic adenosine mono-phosphate (cAMP) production in the corpora cavernosa, and increased levels of cAMP relax smooth muscle and dilate the penile arteries, inducing an erection.

The intracavernosal form of alprostadil is administered with a short needle injected through the side of the penis, directly into the corpus cavernosa. The intraurethral form of alprostadil is administered with a hand-held delivery device. An alprostadil pellet is inserted through the penile opening into the urethra and is absorbed through the urethral mucosa into the surrounding erectile tissue. Alprostadil online doctor viagra is highly effective in treating ED, with success rates of approximately 80% with intracaver-nosal injection(8)and 65% with urethral administration.(9) To find the optimal dose for a given patient, alprostadil dose titration is performed in the physician?s office, starting with the lowest dose and increased until erection occurs. The usual dosage range for the intracaver-nosal form of alprostadil is 1.25 to 2.5 mcg. The maximum dose is 60 mcg up to 3 times a week, allowing at least 24 hours between each dose. Some adverse effects include penile pain, bruising at the injection site, priapism, hematoma/ecchymosis, hypotension, headache, and dizziness.

The usual dosage range for the intraurethral form of alprostadil is 125 to 250 mcg up to a dose of 1000 mcg.

It is recommended that this system be administered after urination to facilitate absorption. Adverse effects include penile pain, urethral burning, dizziness, hypotension, minor urethral bleeding/spotting, and female partner burning/itching.

Using alprostadil is contraindicated in men with a predisposition to pri-apism, men in whom sexual activity is inadvisable or contraindicated, and men with abnormal penile anatomy/penile implants. Men using the intraurethral form of alprostadil to treat their ED should avoid sexual intercourse with pregnant women because the alprostadil may cause harm to the unborn fetus.

Other Therapies Trazodone, which is an atypical antidepressant with serotonergic and alpha-adrenolytic activity, is thought to help erection by indirectly stimulating corporeal smooth muscle relaxation.(10,11)Reports of prolonged erection and improved sexual function have prompted use of trazodone to treat ED. Although preliminary data showed efficacy, double-blind, placebo-controlled trials have shown marginal efficacy of trazodone in treating organic ED.(12)Trazodone also may be considered for patients with psy-chogenic ED. Yohimbine, an indole alkaloid obtained from the bark of the yohimbine tree, is a centrally acting alpha(2) adrenergic-receptor antagonist. It acts at the adrenergic receptors in brain centers associated with libido and erection, and it may facilitate stimulated erections.(10)A meta-analysis of 7 randomized, placebo-controlled studies of men with erectile dysfunction showed that yohimbine was better than placebo in treating ED. This benefit was most significant, however, in patients with psychogenic ED, and efficacy was only marginal in patients with organic ED.(13)Yohimbine is generally recommended for those with psy-chogenic ED.(14) Papaverine is an opium alkaloid that inhibits phosphodiesterase at the postreceptor level and induces smooth muscle relaxation in the sinusoids and dilation of helicine arteries.(10)It increases cAMP and cGMP concentrations in penile erectile tissue, thus facilitating erection.

It is effective and low in cost but can cause significant priapism and fibrosis when used alone at higher dosages.

This adverse-effect profile limits routine use of papaverine; however, lower dosages can be utilized if used in combination with other vasoactive injection therapy.

(Phentolamine, a competitive alpha(1) and alpha(2)adrenergic-receptor blocking agent, dilates arteries and abolishes sympathetic inhibition of erections. Although it does not cause erections alone, it can be used in combination with other vasoactive injection therapy.(10)Significant adverse effects with phentolamine include orthostatic hypotension and tachycardia.

Oral phentolamine has been reported to be effective in treating ED and is available in several South American countries but not in the United States.