As with sildenafil, a high-fat meal or alcohol keep the body from absorbing it as quickly. Generic versions of sildenafil, vardenafil and tadalafil are available. 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. Some examples of nitrates are nitroglycerin Nitro-Dur, Nitrostat, others , isosorbide mononitrate Monoket and isosorbide dinitrate Bidil. This medicine also works best when you take it an hour before sex on an empty stomach.Other prescription-only PDE5 inhibitors include: Vardenafil is available under the brand names Levitra and Staxyn and comes in regular and disintegrating tablet forms. A person takes it one hour. Sildenafil (Viagra). This medicine works best when you take it on an empty stomach an hour before sex. The body takes longer to absorb it after a high-fat meal or . Last updated: June 14, If you’re looking for a less costly Viagra, you might want to ask your doctor about sildenafil. It’s the active ingredient in both branded Viagra and the drug.