What should I do if I forget a dose? Cialis can last in the system for up to 36 hours. Take tadalafil exactly as directed. Mechanism of action[ edit ] Penile erection during sexual stimulation is caused by increased penile blood flow resulting from the relaxation of penile arteries and the smooth muscle of the corpus cavernosum. Consult Your Physician Before purchasing Cialis or any medication online, always consult with your doctor. Talk to your doctor about which dosing schedule is right for you. If you become pregnant while taking tadalafil, call your doctor.Cialis and generic Cialis, known as tadalafil, are available from online services, along with other ED treatment options for people looking to buy ED medication online.  · The typical dose for once-daily Cialis for ED is milligrams (mg) to 5 mg. When used as needed for ED, the usual Cialis dose is 10 mg (with a maximum daily dose of 20 mg). The typical dose of. Tadalafil (Cialis) is used to treat erectile dysfunction (ED, impotence; inability to get or keep an erection), and the symptoms of benign prostatic hyperplasia (BPH; an enlarged prostate) which include difficulty urinating (hesitation, dribbling, weak stream, and incomplete bladder emptying), painful urination, and urinary frequency and urgency in adult men.