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 it's effect last for 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.