Cialis is a prescription drug for the treatment of erectile dysfunction (ED). Cialis is the brand name of the drug tadalafil. This medication is known technically as a "reversible phosphodiesterase type 5 (PDE5) inhibitor". PDE5 inhibitors increase blood flow to the genitals. More information on how Cialis works is available here. Cialis functions the same way as other PDE5 inhibitors in the treatment of ED. Viagra, Levitra and Cialis are all PDE5 inhibitors. As with other ED drugs, Cialis is only effective during sexual stimulation. The mere act of taking Cialis will not cause an erection in men. Perhaps Cialis can best be defined by comparing and contrasting it with its competitors, Viagra and Levitra: Cialis is the first joint product of Lilly, an Indiana based pharmaceutical company and ICOS, a biotechnical firm located in Washington state. The search for an effective erectile dysfunction drug began in 1993 when ICOS began researching IC351, a compound which inhibits the PDE5 enzyme. The following year ICOSS receives the first patent on IC351 and in 1995 clinical trials start. Two years later in 1997, ICOS conducts its first study in patients with impotency problems and by 1998, a partnership between Lilly and ICOS is formed. The goal: to commercialize drugs for the treatment of erectile dysfunction. The following year, 1999, saw the initiation of the Phase III clinical studies on the patented compound. The newly formed company did not file a new drug application with the Food and Drug Administration until 2001. At this time, the manufacturer names the drug Cialis. In April of 2002 the FDA reviews the application, but declines to approve it. The government agency wants to see more clinical studies, the resolution of some manufacturing issues and the complete labeling for the product. In May of that year, Lilly ICOS reports that Phase III test results for Cialis are positive. In fact, data demonstrate that Cialis stays in the male’s system for up to 36 hours, providing a larger window for sexual activity than with Viagra. These findings are presented in Orlando, Florida to the 97th meeting of the American Urological Association. Meanwhile, in July 2002, the European Committee Proprietary Medicinal Products recommends the approval of the drug in Europe. Approval comes in November of 2002 for European sales, with marketing of the product following in February 2003. The FDA approves the sales of Cialis in the United States in November 2003.