Cialis (Tadalafil) is used to treat erectile dysfunction (ED) in men. Cialis does not cure ED or increase a man's sexual desire. The dose is usually taken 30 minutes to 1 hour before sexual activity this drug is fast acting (works in 40 minutes on empty stomach) but is also know to be effective for as long as 36 hours. You should not take this dose more than once per day.Contact your physician immediately if the erection lasts longer than 4 hours or if it becomes painful. This may be a sign of priapism and must be treated immediately to prevent permanent damage.

Do not take Cialis if you are using a nitrate drug  (Nitrostat, Nitrolingual, Nitro-Dur, Nitro-Bid), isosorbide dinitrate (Dilatrate-SR, Isordil, Sorbitrate), and isosorbide mononitrate (Imdur, ISMO) for chest pain or heart problems. Taking tadalafil with a nitrate medicine can cause a serious decrease in blood pressure, leading to fainting, stroke, or heart attack.