But the side effects of sildenafil citrate, even with a prescription, can be intense. Its effects often last for 4 to 5 hours. Generic versions of sildenafil, vardenafil and tadalafil are available. The danger comes from taking it with other drugs or getting it from someone without a medical license. Oral ED medicines might not work or might be dangerous if you also take any of the following: Nitrate medicines. Kidney disease that needs to be treated with dialysis. This medicine works best when you take it on an empty stomach an hour before sex.Still, if your doctor says it’s okay for you to use and you end up paying out-of-pocket, you can still get the generic for a lot less than Viagra. According to price-comparison site. Can I buy generic Viagra without a prescription? No. In most countries, including the United States, Viagra is not available without a prescription. Never order medicines from an online pharmacy that doesn’t list its phone number and physical address, has prices that seem too good to be true, or tells you that no .