Patients should always ask their doctors for medical advice about adverse events. The information provided is for educational purposes only and is not intended to replace discussions with a healthcare provider. An inherited eye disease that affects the retina. Oral ED medicines might not work or might be dangerous if you also take any of the following: Nitrate medicines. In some patients, the use of VIAGRA with alpha-blockers can lead to a drop in blood pressure or to fainting medicines called HIV protease inhibitors, such as ritonavir Norvir , indinavir sulfate Crixivan , saquinavir Fortovase or Invirase , or atazanavir sulfate Reyataz some types of oral antifungal medicines, such as ketoconazole Nizoral and itraconazole Sporanox some types of antibiotics, such as clarithromycin Biaxin , telithromycin Ketek , or erythromycin other medicines that treat high blood pressure other medicines or treatments for ED VIAGRA contains sildenafil, which is the same medicine found in another drug called REVATIO.VIAGRA is available by prescription only in 25 mg, 50 mg, and mg tablets. Patients should always ask their doctors for medical advice about adverse events. You are . While some of what’s sold as generic Viagra online is fake, several reputable companies sell sildenafil online. Each of the companies below has been vetted by our team for . By some estimates, more than half of all ED drugs sold online are fakes. Some of these drugs include ingredients you wouldn’t want to put in your body — things like talcum .