Sildenafil, also known as Viagra, works by inhibiting an enzyme called phosphodiesterase that causes blood vessels to dilate. This can cause it to decrease blood pressure, but its effects work better at treating erectile dysfunction by causing arteries to dilate in the penis, with a resulting erection.
For Treatment of Erectile Dysfunction
A normal dose for adults is 50 mg PO, approximately 1 hour before sexual activity, up to once daily.
For Those With Risk of Increased Plasma Levels
Patients with liver impairment, severe kidney impairment, on other medications (ketoconazole, erythromycin, saquinavir), and who are older than 65 have risk of higher levels of sildenafil citrate in the blood after ingestion. They should take 25 mg PO before sexual activity.
Contraindications
Sildenafil citrate should not be used at the same time as nitrate-based drugs. This can result in dangerous drops in blood pressure, also known as hypotension.
For Patients Also on Ritonavir
Ritonavir is a medication used to treat HIV infection and drastically increases blood levels of sildenafil citrate. As a result, it is recommended to not exceed more than a 25-mg PO dosage per 48 hours.
For Patients on Alpha-Blocker Therapy
Patients that are on alpha-blocker therapy, which are a class of medications used to lower blood pressure, must also use caution with sildenafil citrate usage. Patients may take a 25-mg PO dose of sildenafil citrate at any time with alpha-blockers; however, dosages of 50 to 100 mg PO should not be taken within four hours of alpha-blocker adminstration.
References
- Revatio (sildenafil citrate) package insert. New York, NY: Pfizer; Aug 2005
- "Conns Current Therapy 2009, 1st Edition;" Rakel & Bope; 2008
