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Sildenafil (Viagra)

Sildenafil inhibits phosphodiesterasetype5 (PDE5), relaxes smooth muscle, and increases levels of cGMP in, and inflow of blood to, the corpus cavernosum. Sildenafil at recommended doses has no effect in the absence of sexual stimulation. The recommended dose is 50 mg taken, as needed, approximately 1 hour before sexual activity, but it may be taken anywhere from 0.5 hour to 4 hours before sexual activity. Sildenafil is known to enhance the hypotensive effects of nitrates. Thus, nitrates in any form are contraindicated with sildenafil use.


Table 21: Sildenafil (Viagra)

Author Year; Country
Research Design
Sample Size
Sheel et al. 2005; Canada




Population: 13 males, 8 subjects with cervical and 5 with thoracic injuries.

Treatment: oral dose of sildenafil citrate (25-100 mg) or no medication during penile vibratory stimulation.

Outcomes Measures: ECG, blood pressure.

1.    Sildenafil decreased base BP in cervical SCI.

2.    Men with cervical SCI had more pronounced AD during penile vibrostimulation than men with thoracic injuries.

3.    Administration of sildenafil had no effect on HR or BP during AD triggered by penile vibratory stimulation in men with SCI.

Effect Sizes: Forest plot of standardized mean differences (SMD ± 95%C.I.) as calculated from pre- and post-intervention data


The effect of sildenafil on AD was reported in one small RCT with 13 subjects (Sheel et al. 2005). Although sildenafil decreased resting BP, no effect on magnitude of AD resulting from penile vibrostimulation in men with SCI was observed.

[su_spoiler title=”Effect Size Forest Plots of RCTs with Available Data” style=”fancy”][su_row]Click on the image to enlarge[/su_row]
[su_lightbox type=”image” src=”/wp-content/uploads/Forest_AD_Sheel_2005.gif”][image_with_animation image_url=”/wp-content/uploads/Forest_AD_Sheel_2005.gif” alt=”Effect size SMD forest plot for Sheel et al. 2005, sildenafil citrate”][/su_lightbox]


There is level 2 evidence (from 1 RCT) (Sheel et al. 1995) that sildenafil citrate had no effect on changes in BP during episodes of AD initiated by vibrostimulation in men with SCI.

Sildenafil has no effect on AD responses in men with SCI during ejaculation.