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Focus on Sildenafil

logoBen Green MB, ChB, MRCPsych

Halton Hospital,
Runcorn, Cheshire, UK.


Sildenafil is manufactured by Pfizer under the trade name Viagra. Tablets are available in 25 mg, 50 mg or 100 mg formats. The prevalence of male erectile dysfuntion is about 10% across all age ranges. Only 5% of men seek treatment, and before sildenafil the usual treatments included penile implants, vasodilators given by injection and vacuum devices. The relatively increased acceptability of oral sildenafil may stimulate demand for treatment.


Male erectile dysfunction.


Relaxation of the smooth muscle cells of the corpus cavernosum leads to penile erection. This relaxation may be mediated by nitric oxide via cyclic guanosine monophosphate (cGMP). Sildenafil is an inhibitor of type 5 -cGMP phosphodiesterase.   Sidenafil inhibits the breakdown of cGMP in erectile tissues and therefore aids the action of the nitric oxide/cGMP induced erection.

Sildenafil is well absorbed and reaches its maximum plasma levels an hour after ingestion. Food delays absorption. It has a terminal half-life of 4-5 hours. There should be no accumulation of the drug if used once-daily.



Penile tumescence (in association with sexual stimulation) has been found to occur 30-40 minutes after dosing. Higher doses lead to a longer duration of rigidity (Boolell M et al, 1996) . Rigidity may last more than 20 minutes in about half of patients. (Boolell, M, 1997).

Gingell et al (1996)  showed that a 50 mg dose  given daily for 28 consecutive days  improved erections in 89% of patients. Dose-response relationships were shown for erection frequency, rigidity, duration, number of satisfactory intercourses and quality of sex life. 317/351 patients elected to continue on open-label sildenafil for a further 12 months. 87.1% were continuing to take the drug at the end of this period.

There is some evidence of its efficacy in patients with impotence secondary to diabetes mellitus and  traumatic spinal injury.

Cautions and Contra-Indications

Retinitis pigmentosa. Concomitant use of amyl nitrate and other nitrates e.g. for angina. Recent stroke or heart attacks preclude sildenafil use. Other cautions include hypotension, heart of liver disease, sickle cell anaemia, leukaemia, multiple myeloma, bleeding disorders, active, active peptic ulceration, Peyronie's disease or any other deformation of the penis. Should not be used along with other treatments for erectile dysfunction.

Side Effects

Headache (19%), dyspepsia (14%) , flushing (9%), diarrhoea, muscle pain and altered vision (Abel et al, 1997). Discontinuation rates of about 3-5% occur because of side effects.


Concomitant use of nitrates.


Abel PV et al (1997) Sildenafil (Viagra) an oral treatment for erectile dysfunction: a 12 week, double blind, placebo-controlled study. Eur. Soc. Impot. Res. 50, 159.

Boolell M et al (1996)  Sildenafil: a novel effective oral therapy for male erectile dysfunction. Br J Urol, 78: 257-261.

Boolell M. (1997)  Sildenafil: a novel effective oral therapy for male erectile dysfunction. Br J Urol, 79: 664.

Gingell C et al (1996) UK-92,480 A new oral treatment for erectile dysfunction: a double blind placebo-controlled, once daily dose response study. Proc.Am.Urol.Assoc. 155 (Suppl):(abstract A70)


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Last Amended: 23 March 1999

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