Sildenafil- Works by relaxing and dilating blood vessels. Enhances the effects of nitric oxide, a natural chemical your body produces that relaxes muscles in the penis. Works within 1 hour after taking.
Tadalafil- Works the same as Sildenafil, lasts up to 36 hours. Starts working within 30 minutes after taking.
Phentolamine- a smooth muscle relaxant. It works by causing blood vessels in the penis to expand, which increases blood flow and ultimately causes an erection. More specifically, it works by blocking the nerves that signal arterial walls to contract. Arterial contraction constricts blood flow. Without adequate blood flow to the penis, an erection is not possible. When Phentolamine is injected into a corpora cavernosa of the penis, the muscle cells in the arterial wall relax, and the arteries dilate. In turn, blood is able to flow freely into the corpacavernosa. At the same time, veins should remain constricted, which helps to keep the blood there long enough for an erection to occur.