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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).[2][3]

Eli Lilly purchased ICOS Corporation for $2.1 billion dollars in 2006. As a result, Eli Lilly gained complete ownership of Cialis and promptly shut down ICOS operations, terminating the joint venture and employment of most (> 500) ICOS personnel. An exception to this mass layoff was the retention of 127 employees working at the ICOS biologics facility, which was subsequently acquired by CMC Biopharmaceuticals A/S(CMC).

Sildenafil (compound UK-92,480) was synthesized by a group of pharmaceutical chemists working at Pfizer’s Sandwich, Kent research facility in England. It was initially studied for use in hypertension (high blood pressure) and angina pectoris (a form of ischaemic cardiovascular disease). Phase I clinical trials under the direction of Ian Osterloh suggested that the drug had little effect on angina, but that it could induce marked penile erections.[1][2] Pfizer therefore decided to market it for erectile dysfunction, rather than for angina. The drug was patented in 1996, approved for use in erectile dysfunction by the Food and Drug Administration on March 27, 1998, becoming the first pill approved to treat erectile dysfunction in the United States, and offered for sale in the United States later that year.[3] It soon became a great success: annual sales of Viagra in the period 1999–2001 exceeded $1 billion.

The British press portrayed Peter Dunn and Albert Wood as the inventors of the drug, a claim which Pfizer disputes.[4] Their names are on the manufacturing patent application drug, but Pfizer claims this is only for convenience.

Even though sildenafil is available by prescription from a doctor, it was advertised directly to consumers on US TV (famously being endorsed by former United States Senator Bob Dole and football star Pelé). Numerous sites on the Internet offer Viagra for sale after an “online consultation”, a mere web questionnaire. The “Viagra” name has become so well known that many fake aphrodisiacs now call themselves “herbal Viagra” or are presented as blue tablets imitating the shape and colour of Pfizer’s product. Viagra is also informally known as “Vitamin V”, “the Blue Pill”, as well as various other nicknames.

In February 2007, it was announced that Boots the Chemist would trial over the counter sales of Viagra in stores in Manchester, England. Men aged between 30 and 65 would be eligible to buy four tablets after a consultation with a pharmacist.[5]

Pfizer’s worldwide patents on sildenafil citrate will expire in 2011–2013. The UK patent held by Pfizer on the use of PDE5 inhibitors (see below) as treatment of impotence was invalidated in 2000 because of obviousness; this decision was upheld on appeal in 2002.

Vardenafil was co-marketed by Bayer Pharmaceuticals and GSK under the trade name Levitra. As of 2005, the co-promotion rights of GSK on Levitra have been “transferred back” to Bayer in many markets outside of the United States. In Italy, Bayer markets the product as Levitra and GSK markets the product as Vivanza. Due to European Union trade rules, parallel imports may result in the Vivanza branded packs being seen alongside Levitra packs in pharmacies in other EU member states.