These small differences affect the way each medication works, such as how quickly it takes effect and wears off, and the possible side effects. This medicine works best when you take it on an empty stomach an hour before sex. It might work longer if you have mild to moderate erectile dysfunction. Do not drink grapefruit juice when taking Viagra Sildenafil. Heart conditions: If you have or have had heart disease or a heart attack or stroke, you should speak to your doctor before taking any medication for erectile dysfunction. These medicines all work well for ED. Grapefruit juice: Grapefruit juice can increase the amount of Viagra Sildenafil in your bloodstream.Generic Viagra (sildenafil) is an alternative to the branded Viagra drug that can be used for treating erectile dysfunction (ED). It’s now sold by many manufacturers, including online. Is there an over-the-counter version of Viagra? Can I get Viagra online without seeing a doctor? Currently, there are no over the counter alternatives to Viagra available. Viagra Brand Name Viagra Common Name sildenafil In this drug factsheet: While in some cases the condition may improve over time, it can also be permanent. This rare .