Sildenafil prevents the destruction of an enzyme cGMP and allows cGMP to accumulate and persist longer.. The inhibition of Serotonin uptake inhibits the ejaculatory expulsion of reflex sildenafil citrate to the brain, thereby preventing premature ejaculation in men. For ejaculation to happen in men, a reflex has to be passed on to the brain via the sympathetic nervous system. Dapoxetine belongs to a class of medicines known as Selective Serotonin Reuptake Inhibitors (SSRI) which inhibits the uptake of serotonin in the neural cells. The longer cGMP persists, the more prolonged the engorgement of the penis The mechanism of action of Tadalafil is the same as Viagra (Sildenafil) and works by blocking an enzyme called PDE 5 which helps the smooth muscles in the penis to relax, thereby sildenafil citrate softabs increasing blood flow to the penis.