Tadalafil is a prescription medication which is available only under the brand name Cialis. Tadalafil is a member of the phosphodiesterase inhibitor family. Here's how tadalafil works: When a man becomes erect, the arteries in his penis carry blood through the spongy tissue inside. When a health problem keeps genital blood from flowing properly, tadalfil increases blood flow to the penis, resulting in an erection. This increased blood flow to the penis makes tadalafil effective for treating erectile dysfunction. Tadalafil has a thirty-six hour duration of action, and has thus become known as the "weekend pill. Cialis is available in five, ten and twenty milligram tablets. Tadalafil should be taken as needed about thirty to sixty minutes before sexual activity. This medication can be taken up to once a day.