What should I do if I forget a dose? You need a prescription from a doctor to obtain sildenafil. If it is not treated right away, priapism can permanently damage your penis sudden vision loss in one or both eyes. 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. This medicine usually begins to work for erectile dysfunction within 30 minutes after taking it. Take sildenafil up to 4 hours before you want to have sex.SHOP NOW AT HIMS. Healthline ‘s review. Similar to Viagra and sildenafil, Cialis is a PDE5 inhibitor and works by improving blood flow to the penis and reducing ED symptoms. . Viagra (sildenafil) is a prescription drug that’s used to treat erectile dysfunction (ED). The drug comes as a tablet you’ll take by swallowing. It’s usually taken as needed about 1 hour. Similar to Viagra and sildenafil, Cialis is a PDE5 inhibitor and works by improving blood flow to the penis and reducing ED symptoms. Hims offers both brand name Cialis and generic Cialis.