Sildenafil citrate (Viagra) is a vasodilator and it is extensively metabolized by CYP3A4 and 2C9 liver enzymes. Therefore, combining Sildenafil citrate with other vasodilators will increase the incidence of hypotension, and CYP3A4 or 2C9 inhibitors will increase the blood levels of Viagra.

Viagra is the brand name for Sildenafil citrate, and is used for treating erectile dysfunction and pulmonary arterial hypertension. Originally developed by scientists in Great Britain, it was brought onto the market by Pfizer Inc., a US pharmaceutical company. Viagra is also sold under brand name Revatio.

 

Sildenafil citrate’s chemical formula is C22H30N6O4S.

 

Sildenafil citrate works by inhibiting an enzyme called cGMP-specific phosphodiesterase type 5, that delays degradation of cGMP, which controls blood flow in the penis. It came onto the market in 1998 and has become the prime treatment for impotence (erectile dysfunction). Sildenafil citrate’s main competitors are Cialis (tadalafil) and Levitra (vardenafil).

 

Care should be exercised by patients that are also taking protease inhibitors for the treatment of HIV. Protease inhibitors inhibit the metabolism of Sildenafil citrate, effectively multiplying the plasma levels of Sildenafil citrate, increasing the incidence and severity of side effects. It is recommended that patients using protease inhibitors limit their use of Sildenafil citrate to no more than one 25 mg dose every 48 hours.Other drugs that interfere with the metabolism of sildenafil include erythromycin and cimetidine, both of which can also lead to prolonged plasma half life levels.

Concomitant use of Sildenafil citrate and an alpha blocker may lead to low blood pressure, but this effect does not occur if they are taken at least four hours apart.

 

The recommended dose of Sildenafil citrate is 25-100 mg taken 30 minutes to 4 hours before sexual activity. Sildenafil citrate increases the effects of blood pressure lowering medications. It also increases the blood pressure lowering effects of nitrates (for example, isosorbide dinitrate) that are used primarily for treating angina. Patients taking nitrates should not receive Sildenafil citrate. Sildenafil citrate should not be combined with Revatio or other PDE5 inhibitors (for example, Levitra [vardenafil], Cialis [tadalafil]). Tagamet (cimetidine), erythromycin, ketoconazole, Sporanox (itraconazole), and Posicor (mibefradil) can cause marked increases in the amount of Sildenafil citrate in the body. Patients taking these medications should be observed carefully if Sildenafil citrate is used.