Cialis drug blister pack. Cialis (tadalafil) is an anti-impotence drug that works in the same way as Viagra. It blocks the enzyme phosphodiesterase-5 (PDE-5) in the body. This increases the effects of nitric oxide, which is usually released in response to sexual stimulation. It relaxes the smooth muscle, allowing blood to enter the penis, causing an erection. The drug takes around quarter of an hour to show its effects, and sexual response can be enhanced for up to 24 hours after a single dose. Cialis is manufactured by Eli Lilly & Co. and Icos Corporation. It became available in Britain in February 2003.