This applies, for example, when you want to take Tadalafil brand name Cialis instead of Viagra. Registered prescribers — you can only buy prescription ED drugs online if you have a pre-existing prescription from your family physician. It lasts 4 to 5 hours, or longer if you have mild to moderate ED. Tadalafil is another licensed oral medication that treats ED, but it lasts for much longer — up to 36 hours Viagra only lasts around four hours. Its effects often last for 4 to 5 hours.Yes, penile pumps (also known as ED pumps) are over-the-counter devices that work to treat erectile dysfunction (ED), similar to Viagra. Viagra (sildenafil) is . Sildenafil Prices, Coupons and Patient Assistance Programs. Sildenafil is a member of the agents for pulmonary hypertension drug class and is commonly used for Erectile . Unless you live on the border with Mexico, you can NOT buy Viagra over the counter in stores like CVS, Walgreens, GNC, or Walmart. You need to go to your doctor, tell he or she you’re having problems .