Side Effects Are Minimal. Clinical trials for MED found minimal side effects. These include a heart attack, stroke or a serious heart rhythm disorder in the past six months. Avanafil Stendra. Vardenafil usually works for 4 to 5 hours.FAQ Takeaway We did a deep dive into five over-the-counter alternatives to Viagra to determine whether they work and how to use them. Maskot/Getty Images . 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 .