Do not take more or less of it or take it more often than prescribed by your doctor. It works to treat erectile dysfunction by increasing blood flow to the penis during sexual stimulation. Tadalafil is in a class of medications called phosphodiesterase PDE inhibitors. You may attempt sexual activity at any time between doses. Talk to your doctor about eating grapefruit and drinking grapefruit juice while taking this medicine. One advantage Cialis has over Viagra and Levitra is its You need a prescription from a doctor to buy tadalafil.Tadalafil (Cialis) 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). Tadalafil works by helping to relax the blood vessels in the penis, allowing blood to flow into the penis, and causing an erection. Tadalafil (Cialis) is taken by mouth either as needed before sexual activity or once daily for erectile dysfunction (ED).It’s also taken once daily to treat symptoms from an enlarged prostate (benign prostatic hyperplasia, or BPH).Tadalafil (Cialis) is available both as brand-name and generic tablets. Common side effects of this medication include . Cialis lasts up to 36 hours in the body, while Viagra lasts 4–6 hours in the body. Both are taken 30 minutes before sex. Cialis can cause limb pain, while Viagra can cause vision change, rash.