Some examples of nitrates are nitroglycerin Nitro-Dur, Nitrostat, others , isosorbide mononitrate Monoket and isosorbide dinitrate Bidil. You can take it in a small dose daily or in a larger dose as needed. Its effects might last longer if you have mild to moderate ED. Oral medicine for ED also might not be safe or work well if you have: Very low blood pressure, called hypotension, or uncontrolled high blood pressure, called hypertension. Sildenafil Viagra.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. It’s a prescription . 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 . Viagra (sildenafil) is a prescription drug that’s used to treat erectile dysfunction (ED). The drug comes as a tablet you’ll take by swallowing. It’s usually taken as needed about 1 .