Tadalafil anti-impotence drug molecule, computer model. This is the active ingredient in the drug Cialis. It increases the penile response to sexual stimulation, making it easier to attain and maintain erection. Atoms are shown as tubes, and are colour-coded: carbon (green), oxygen (red), nitrogen (blue) and hydrogen (white). The drug works by blocking the action of the enzyme phosphodiesterase 5 (PDE-5), increasing the body's response to nitric oxide. This relaxes smooth muscle, allowing blood to enter the penis, leading to an erection. Its effects start about 15 minutes after a dose, and can last for a day. Cialis is made by Eli Lily & Co and Icos Corporation.