Tadalafil does not prevent pregnancy or the spread of sexually transmitted diseases such as human immunodeficiency virus HIV. When the blood vessel muscle cells relax, this expands the vessel and increases blood flow; this effect is referred to as vasodilation. Tadalafil may cause side effects. Tadalafil comes as a tablet to take by mouth. Take tadalafil at around the same time every day.In fact, a recent review of studies found that some counterfeit . Tadalafil is a medicine used to treat erectile dysfunction (ED, also called impotence) and problems with urination caused by an enlarged prostate (benign prostatic hypertrophy or BPH). It is sometimes known by its generic name tadalafil, and sometimes known by its trade name, Cialis. There are also other brands of this medicine available.  · If you take too much tadalafil or take it together with these medicines, the chance for side effects will be higher. If you experience a prolonged erection for more than 4 hours or a painful erection for more than 6 hours, contact your doctor immediately. This includes prescription or nonprescription (over-the-counter [OTC]) medicines and.