General Information

Viagra (sildenafil citrate) is a phosphodiesterase-5 (PDE5) inhibitor.

Viagra is specifically indicated for the treatment of erectile dysfunction.

Viagra is supplied as a tablet for oral administration. For most patients, the recommended dose is 50 mg taken, as needed, approximately 1 hour before sexual activity. However, Viagra may be taken anywhere from 30 minutes to 4 hours before sexual activity. The maximum recommended dosing frequency is once per day. Based on effectiveness and toleration, the dose may be increased to a maximum recommended dose of 100 mg or decreased to 25 mg.

Mechanism of Action

The physiologic mechanism of erection of the penis involves release of nitric oxide (NO) in the corpus cavernosum during sexual stimulation. NO then activates the enzyme guanylate cyclase, which results in increased levels of cyclic guanosine monophosphate (cGMP), producing smooth muscle relaxation in the corpus cavernosum and allowing inflow of blood.

Sildenafil enhances the effect of NO by inhibiting phosphodiesterase type 5 (PDE5), which is responsible for degradation of cGMP in the corpus cavernosum. Sildenafil has no direct relaxant effect on isolated human corpus cavernosum. When sexual stimulation causes local release of NO, inhibition of PDE5 by sildenafil causes increased levels of cGMP in the corpus cavernosum, resulting in smooth muscle relaxation and inflow of blood to the corpus cavernosum. Sildenafil at recommended doses has no effect in the absence of sexual stimulation.

Side Effects

Adverse effects associated with the use of Viagra may include, but are not limited to, the following:

• headache
• flushing
• dyspepsia
• abnormal vision
• nasal congestion
• back pain
• myalgia
• nausea
• dizziness
• rash

Clinical Trial Results

In clinical trials involving some 3,000 patients, 60 to 80% of those who took the drug reported improvements in the ability to achieve erections and sustain them after penetration, while about 20% of the patients who took a placebo reported improved erections.