Tell your doctor if any of these symptoms are severe or do not go away: headache. Tell your doctor or pharmacist if you are taking nitrates medications for chest pain such as isosorbide dinitrate Isordil , isosorbide mononitrate Monoket , and nitroglycerin Nitromist, Nitro-Dur, Nitrostat. Generic versions of sildenafil, vardenafil and tadalafil are available. Tadalafil Cialis. Sildenafil comes as a tablet and suspension liquid to take by mouth.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 Revatio, which is a lower dose. Sildenafil may be used to treat erectile dysfunction (ED or impotence) in men (Viagra brand) or pulmonary arterial hypertension in both men and women (Revatio brand), depending on the brand. Sildenafil blocks the action of an enzyme known as cGMP-specific phosphodiesterase type-5 (PDE-5). Your doctor won’t recommend your Viagra dosage by weight. But they may recommend a lower dose of the drug if you’re older than 65 years. This is because you may have an increased risk of side.