Tadalafil is a drug used to treat male erectile dysfunction (impotence). It was developed by the biotechnology firm ICOS and marketed worldwide by Eli Lilly and Company 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. It should be noted that the drug has not been formally studied in regard to multiple sexual attempts during a 36 hour period. Tadalafil is also currently undergoing Phase III clinical trials for the treatment of pulmonary hypertension.