Cialis, the brand name for tadalafil, is a prescription medication used to treat erectile dysfunction (ED) and symptoms of benign prostatic hyperplasia (BPH). It belongs to a class of drugs called phosphodiesterase type 5 (PDE5) inhibitors, which work by increasing blood flow to the penis during sexual stimulation, enabling an erection. Cialis is known for its long duration of action, up to 36 hours.