Sildenafil (Viagra)

Download as a PDF

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: Sildanefil (Viagra) 


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.

Effect Size Forest Plots of RCTs with Available Data
Click on the image to enlarge
Effect size SMD forest plot for Sheel et al. 2005, sildenafil citrate


  • 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.