Your doctor may need to change the doses of your medications or monitor you carefully for side effects. Use the medicine exactly as directed. Your doctor will probably tell you not to take tadalafil if you are taking street drugs containing nitrates. Tell your doctor if any of these symptoms are severe or do not go away: headache. It works to treat erectile dysfunction by increasing blood flow to the penis during sexual stimulation. Stop and get medical help at once if you have nausea, chest pain, or dizziness during sex. Grapefruit may interact with tadalafil and lead to unwanted side effects.Cialis (tadalafil) increase increases blood flow to a certain area of the body and is used to treat erectile dysfunction (impotence). Includes Cialis side effects, interactions and . Tadalafil oral tablet is available as a generic drug and as brand-name drugs. Brand names: Cialis, Adcirca. Tadalafil comes only as a tablet you take by mouth. Tadalafil is used to . 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 .