Oral medicines for ED usually work well and cause few side effects. Oral ED medicines might not work or might be dangerous if you also take any of the following: Nitrate medicines. Common ones include: Flushing of the skin. Generic versions of sildenafil, vardenafil and tadalafil are available. This medicine works best when you take it on an empty stomach an hour before sex. These include a heart attack, stroke or a serious heart rhythm disorder in the past six months.Is there an over-the-counter Viagra? Viagra is only available with a prescription in the United States, Canada, and most other countries. Viagra is sold under the generic name. Drug Images What is Viagra? If you have erectile dysfunction (ED), your doctor may recommend that you take Viagra. It’s a prescription drug that’s used in adult males* with . According to price-comparison site , generic Revatio will cost you as little as $ per pill, depending on where you fill your prescription. That’s compared with .