About Sildenafil Citrate Phosphodiesterase-5 Enzyme Inhibitor, Anti-erectile dysfunction agent. Mechanism of Action of Sildenafil Citrate Sildenafil is a phosphodiesterase enzyme (PDE5) inhibitor. This enzyme is responsible for the degradation cyclic guanosine monophosphate (cGMP) produced in reaction to sexual stimulation. The more cGMP is available, the more durable the erection. Sildenafil inhibits the PDE5 enzyme, preserving cGMP levels, therefore aiding erection viability and durability Erection of the penis is caused by the filling of the penis with blood. Filling occurs because the blood vessels that bring blood to the penis increase in size and deliver more blood to the penis and at the same time, the blood vessels that take blood away from the penis decrease in size and remove less blood from the penis. Sexual stimulation that leads to an erection causes the production and release of nitric oxide in the corpora cavernosa of the penis.