Sildenafil citrate is a selective inhibitor of type V phosphodiesterase (PDE5) that enhances the effect of nitric oxide (NO) which is released in the corpora in response to sexual stimulation.
The release of NO stimulates the enzyme guanylate cyclase to produce cyclic GMP, which in turn relaxes arterial smooth muscle.
Cyclic GMP is then broken down by PDE5. Sildenafil as a PDE5 inhibitor prevents the breakdown of cyclic GMP and hence prolongs arterial relaxation and inflow of arterial blood and thereby enhances erection. Virecta therefore causes erection only in the context of sexual stimulation.