It works for up to 36 hours. Do not stop taking sildenafil without talking to your doctor. Side effects Side effects of oral ED medicines are often mild. Oral ED medicines might not work or might be dangerous if you also take any of the following: Nitrate medicines. This medicine works best when you take it on an empty stomach an hour before sex. Slowly pull back the plunger until the required dose. These include a heart attack, stroke or a serious heart rhythm disorder in the past six months.FAQs What to consider Cost Overdose Ask your doctor Q&A Drug Images What is Viagra? If you have erectile dysfunction (ED), your doctor may recommend that you take Viagra. . Drug Images What is Viagra? Viagra is a brand-name prescription drug that’s approved to treat erectile dysfunction (ED). With ED, you’re unable to have or maintain an erection. . you should know that sildenafil is available under the brand names Liqrev, Revatio, and Viagra. You should only be treated with one of these products at a time. The following .