If you have an erection that lasts more than 4 hours, get medical help right away. The products discussed in this site may have different product labeling in different countries. The contents herein are for informational purposes only. Terms and conditions of use. Prolonged erection: If an erection lasts longer than 4 hours a rare occurrence , seek immediate medical assistance. These factors may affect how you should use Viagra Sildenafil. 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.7 Over The Counter Medications for ED. Depending on the cause of erectile dysfunction, the most effective treatment can vary. Typically, the best way to treat erectile dysfunction is to address the underlying cause. Second is by using oral medications like Viagra® (sildenafil), Cialis® (tadalafil), or Levitra® (vardenafil), all of which are. Can I buy generic Viagra over the counter or do I need a prescription? In the U.S. and most other countries, generic Viagra is only available with a prescription. You can get Viagra for free in Canada if your health insurance provider covers it fully. Contact your provider or us to help you find out if you qualify to get Viagra for free. Tell your doctor or prescriber about all prescription, over-the-counter (non-prescription), and herbal medications that you are taking. Also tell them about any.