Take sildenafil exactly as directed. These medicines all work well for ED. If you take too much sildenafil or take it together with these medicines, the chance for side effects will be higher. Sildenafil may cause side effects. If you experience chest pain, dizziness, or nausea during sexual activity, call your doctor immediately and avoid sexual activity until your doctor tells you otherwise. This can happen if you are sick with fever, diarrhea, or vomiting; sweat a lot; or do not drink enough liquids.In most males,* Viagra works within 1 hour after it’s taken. It’s possible that Viagra can begin working within 30 minutes of being taken. But other times, it may take up to 4 hours to start. What is the closest thing to Viagra over-the-counter? There are no over-the-counter alternatives to Viagra. However, people may wish to try natural supplements that people can purchase without a. The four main medicines taken by mouth for erectile dysfunction are: Avanafil (Stendra). Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. They enhance the effects of a chemical the body makes that relaxes muscles in the penis, called nitric oxide.