Three medications work synergistically in Trimix: Papaverine increases cyclic adenosine monophosphate (cAMP) production, which relaxes the smooth muscle in the walls of blood vessels. Its mechanism is not completely understood because it works on many pathways. Ultimately, it increases blood flow into the penis and inhibits outflow resulting in an erection. Phentolamine is an alpha-adrenoceptor antagonist that relaxes and dilates blood vessels entering the penis, therefore, increasing blood flow. Alprostadil is a synthetic version of prostaglandin E1. It also dilates blood vessels and increases blood flow into the penis.