Sildenafil (Viagra)

Sildenafil (Viagra) is a phosphodiesterase-5 inhibitor that is used to treat impotence. It is produced by Pfizer.

One 50mg tablet of sildenafil is usually taken from half an hour to four hours before sexual activity. The dose can be varied from 25 to 100mg depending on efficacy, side-effects, and potential drug interactions.

Two large placebo-controlled clinical studies have found that sildenafil is effective at improving erectile function and sexual satisfaction among men with erectile dysfunction. It does not improve erectile function in men with normal sexual functioning. Experts have suggested that causes of impotence (diabetes, heart trouble, alcohol intake, smoking and anti-depressants) should be treated in conjunction with the use of sildenafil. Sildenafil has not been studied in women.

Sildenafil can also be used to treat pulmonary hypertension, a dangerous condition caused by high blood pressure in the artery that supplies the lungs, which can be a complication of HIV infection.^{1 2} It is marketed as Revatio for this purpose.

The most common side-effects of sildenafil are headaches, facial flushing, indigestion, nasal congestion, transient disturbance of colour vision and raised blood pressure. In clinical trials, 30% of participants reported a side-effect but only 2% stopped taking the drug due to side-effects.

In the United States, 69 men taking sildenafil died between March and July 1998, during which time 3.6 million prescriptions were dispensed.

Sildenafil should not be taken in conjunction with nitrates, including amyl nitrite (poppers) or drugs used to treat angina. If nitrates and sildenafil are taken together, blood pressure may fall to dangerously low levels.

Protease inhibitors and non-nucleoside reverse transcriptase inhibitors (NNRTIs) are metabolised by the CYP3A4 enzyme in the liver, which also processes sildenafil. Consequently, anti-HIV drugs and sildenafil may produce high levels of sildenafil in the blood and worsen side-effects.^{3 4} This has led to the death of at least one man.⁵

Pfizer recommends that people taking protease inhibitors or NNRTIs take sildenafil at the reduced dose of 25mg. Patients taking ritonavir (Norvir) should not take more than 25mg sildenafil every 48 hours.

Other drugs metabolised by the CYP3A4 pathway, such as ketoconazole (Nizoral), itraconazole (Sporanox) and erythromycin (Erymax / Erythrocin / Erythroped / Erythroped A) should be taken with the reduced dose of sildenafil. Furthermore, people on medications metabolised by CYP3A4 should not take two doses of sildenafil within one 48-hour period.