Side effects Side effects of oral ED medicines are often mild. Can sildenafil citrate cause erectile dysfunction? Tadalafil Cialis. Related stories:. Oral ED medicines might not work or might be dangerous if you also take any of the following: Nitrate medicines. As with sildenafil, a high-fat meal or alcohol keep the body from absorbing it as quickly.Avanafil (Stendra). Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. They enhance the effects of a chemical the body .  · Drugs including Viagra and Cialis lead to an average improvement of 4 to 8 points on the scale used by researchers and clinicians. The team behind the new study . Although Viagra is considered to be the most common prescription medication for ED, quite a few are on the market. They all work by improving blood flow .