Tadalafil is an orally administered drug used to treat male erectile dysfunction (impotence). It was initially developed by the biotechnology firm ICOS and subsequently developed and marketed worldwide by a joint venture of ICOS Corporation and Eli Lilly and Company (Lilly ICOS LLC) under the brand name Cialis.

In the United States, tadalafil has Food and Drug Administration approval and became available in December, 2003 as the third impotence pill after sildenafil (Viagra) and vardenafil (Levitra). Due to its 36-hour effect it is also known as the weekend pill. As with sildenafil and vardenafil, it is recommended that tadalafil be used no more than once daily. Tadalafil is also currently undergoing Phase 3 clinical trials for the treatment of pulmonary arterial hypertension.

The history of Cialis cannot be discussed without mentioning Pfizer's drug, Viagra (sildenafil). The FDA's approval of Viagra on March 27, 1998, was a groundbreaking event for the treatment of erectile dysfunction and sales eventually reached over a billion dollars. The FDA subsequently approved Levitra (vardenafil) on August 19, 2003, and Cialis (tadalafil) on November 21, 2003.

In 1993, the Bothell, Washington-based biotechnology company Icos began studying IC351, which is a phosphodiesterase type 5 (PDE5) enzyme inhibitor. In 1994, Pfizer scientists discovered that sildenafil citrate, which also inhibits the PDE5 enzyme, caused patients that were participating in a clinical study of a heart medicine to have erections. Although the ICOS scientists were not testing the chemical compound IC351 for erectile dysfunction, it was recognized that the compound could have potential usefulness for the treatment of this disorder. Soon Icos received a patent in 1994 for IC351, which is structurally different from sildenafil (and vardenafil), and Phase 1 clinical trials began in 1995. In 1997, Phase 2 clinical studies were initiated in patients with erectile dysfunction and led to pivotal Phase 3 trials that supported approval.

In 1998, ICOS Corporation and Eli Lilly and Company formed a joint venture (Lilly ICOS LLC) to further develop and commercialize the drug for erectile dysfunction, and two years later they filed a New Drug Application with the U.S. Food and Drug Administration for IC351 (under the generic name tadalafil and the brand name Cialis). In May of 2002, Lilly ICOS reported to the American Urological Association that clinical trial testing in men with erectile dysfunction showed that tadalafil works for up to 36 hours, and one year later tadalafil was approved. One advantage that Cialis has over Viagra and Levitra is that tadalafil has a half-life of 17.5 hours[1] (and thus Cialis is advertised to work for up to 36 hours, after which time there is still about one quarter of the absorbed dose in the body) as compared to 4 hours half-life for sildenafil (Viagra).

Eli Lilly purchased ICOS Corporation for $2.1 billion dollars in 2006.