This medicine also works best when you take it an hour before sex on an empty stomach. Lastly, although sildenafil is considered interchangeable with brand Viagra, they do have slightly different inactive ingredients. This boosts blood flow and helps you get an erection from sexual activity. Medicines that you take by mouth are called oral medicines. Brian Staiger Pharm. Kidney disease that needs to be treated with dialysis.This means that 4 hours after you take a dose, your body has cleared half of the dose. In most cases, it takes about 5 half-lives for your body to completely remove a . Revatio is available as a generic under the name of sildenafil (20mg tablet). There is also a generic version of Viagra available, under the name sildenafil, in 25mg, . If you’re holding out for a generic version of branded Viagra, you won’t have to wait much longer. Teva Pharmaceuticals plans to release its generic of the “little blue .