Grapefruit may interact with tadalafil and lead to unwanted side effects. Call your doctor for medical advice about side effects. Do not take two doses at one time. Your doctor will probably tell you not to take tadalafil if you are taking street drugs containing nitrates. Ask your pharmacist or doctor for a copy of the manufacturer’s information for the patient.What is Cialis? Cialis (tadalafil) is a prescription drug used to treat erectile dysfunction (ED) and benign prostatic hyperplasia (BPH). It belongs to a class of drugs called. 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. Reviewed: March 5, The Cialis brand of tadalafil is used in men to treat erectile dysfunction (impotence) and symptoms of benign prostatic hypertrophy (enlarged prostate). Adcirca and Alyq.