Tadalafil is sometimes taken as needed, usually at least 30 minutes before sexual activity and not more often than once every 24 hours. Talk to your doctor about eating grapefruit and drinking grapefruit juice while taking this medicine. Do not take more or less of it or take it more often than prescribed by your doctor. It belongs to the drug class known as PDE5 inhibitors. Tadalafil may cause side effects. This increased blood flow can cause an erection.Note: Some pharmacies do not allow the savings card to be used for opioid drugs. How . The four main medicines taken by mouth for erectile dysfunction are: Avanafil (Stendra). Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. They enhance the effects of a chemical the body makes that relaxes muscles in the penis, called nitric oxide. AdDon’t go to the pharmacy without checking prices at GoodRx® first. No commitment or fees. Find the lowest price in your neighborhood & use our free coupons to save up to 80%Service catalog: Prescription Coupons, Prescription Discounts, Online Doctor Visits.