ED occurs more frequently in older men. You need a prescription from a doctor to buy tadalafil. Do not take more or less of it or take it more often than prescribed by your doctor. Your doctor will probably tell you not to take tadalafil if you are taking street drugs containing nitrates. Vardenafil usually works for 4 to 5 hours. Sexual activity may be a strain on your heart, especially if you have heart disease. Generic versions of sildenafil, vardenafil and tadalafil are available.Valid at all major chains including Walgreens, CVS Pharmacy, Target, WalMart Pharmacy, Duane Reade and 65, pharmacies nationwide. No manufacturer promotions could be .  · But on average, its effects start to lessen 2 to 3 hours after you take a dose. The typical dose for once-daily Cialis for ED is milligrams (mg) to 5 mg. When used as needed for ED, the usual. stockcam / Getty Images What is generic Cialis? Generic Cialis is called tadalafil, which is the active ingredient in Cialis. Cialis is Food and Drug Administration (FDA) approved to treat.