All PDE5 inhibitors, including Viagra and Cialis, require a valid prescription regardless of the pharmacy or provider. These medicines are called PDE5 inhibitors. According to the FDA, generic medications contain the same active ingredients as brand names, meaning there’s no difference in effectiveness or safety. Common ones include: Flushing of the skin. You take one 2. Its effects might last longer if you have mild to moderate ED.Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. They enhance the effects of a chemical the body makes that relaxes muscles . Con: Occasionally unbelievable almost migraine-like headaches. Extremely painful erection (if you take viagra, but don’t get it used) Difficulty achieving erection (this only applies . It doesn’t cause you to get an erection just makes it easier to get one and maintain. It still requires you to have sexual stimulation to get an erection. For me it takes affect within .