Tadalafil is an orally administrated drug developed by the pharmaceutical company Eli Lily for treatment of male erectile dysfunction disease. It belongs to the second generation of phosphodiesterase type 5 inhibitors. Study has shown that compared with sildenafil, the onset rate of Tadalafil is very rapid with about 15 ~ 20 min for onset and with long-term effects up to 36h. Its T1/2 was 17.5h. In a study including 348 cases of patients with mild to severe erectile dysfunction, male patients were randomized for being subject to 20 mg of tadalafil or placebo. The results have showed that, compared with placebo, the intercourse success rate at 24 and 36 hours after taking the drug are both improved with most male patients can successfully have sexual intercourse for twice within 36 hours. There is also no difference in the incidence and severity of the adverse drug reactions compared with placebo. In the tadalafil group, more than 5% of men get headache and indigestion.