Your doctor or other health care professional can recommend one for you based on these factors, other health conditions you have and any other medicines you take. Anticonvulsants like phenytoin or carbamazepine and an antibiotic called rifampin can all cause Viagra to not do its job. What should I do if I forget a dose? These include a heart attack, stroke or a serious heart rhythm disorder in the past six months. Before taking sildenafil, tell your doctor and pharmacist if you are allergic to sildenafil, any other medications, or any of the ingredients in sildenafil tablet or suspension. Follow the directions on your prescription label carefully, and ask your doctor or pharmacist to explain any part you do not understand. You can take sildenafil with or without food.No, you should not take Viagra if you don’t have ED. Viagra is a prescription medication that can have serious side effects. And it’s approved only to treat ED, not to be used. 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. 50 mg mg Typical dosages The following information describes commonly recommended dosages for ED. However, be sure to take the dosage your doctor .