Generic versions of sildenafil, vardenafil and tadalafil are available. This leads to more blood flow to the penis, leading to stronger erections. Is 20mg of Cialis better than mg of Viagra? Nitric oxide is a substance that causes blood vessels to relax. It works for up to 36 hours. Viagra can be taken as long as four hours or as little as 30 minutes before sex.Cons can cause headaches and muscle aches must be swallowed whole can cause indigestion can make you appear flushed not available over-the-counter (OTC) . Both Viagra and Cialis can be taken 30 minutes before sexual activity. However, Cialis lasts much longer and is noteworthy for the amount of time it remains in . Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. They enhance the effects of a chemical the body makes that relaxes .