Viagra (sildenafil) is used for the treatment of ED (erectile dysfunction). You will have to wait for at least 30 minutes before you can get an erection and feel the benefits of this medicine. For some people, however, it may take longer – but the period does not usually exceed one hour. You must not start taking this medicine if you have recently taken nitrates, such as isosorbide dinitrate, isosorbide mononitrate or nitroglycerin. Nitrates can come in a variety of forms – as sublingual tablets, patches, ointments, sprays and regular tablets, so it’s best to ask your pharmacist for additional information if you are purchasing any new drug you have been prescribed. Your doctor needs to know as much as possible about the medicines you are taking at the moment. Some of them can make the effects of Viagra stronger or weaker, while others will require additional tests to establish the safety or combining them you’re your medicine. Make sure your health care provider is aware of the fact you are currently taking any other erectile dysfunction treatment drugs, medicines for seizures, high blood pressure medications, HIV protease inhibitors, antifungals (itraconazole, ketoconazole), beta blockers, alpha blockers (doxazosin, prazosin, rifampin, alfuzosin). Such medicines as erythromycin, efavirenz, barbiturates, amlodipine, phenobarbital, rifabutin, bosentan, anticoagulants, cimetidine, carbamazepine, nevirapine, tamsulosin, or phenytoin must also be reported to your health care provider.