If you are taking sildenafil to treat erectile dysfunction, you should know that it does not cure erectile dysfunction or increase sexual desire. If you have certain health conditions or are taking certain medications, your doctor may tell you to take sildenafil less often. These are commonly prescribed for an enlarged prostate or male pattern baldness. If you become pregnant while taking sildenafil, call your doctor. The following nonprescription or herbal products may interact with sildenafil: cimetidine Tagamet HB , St. But we get it — talking about ED is uncomfortable.Drug Images What is Viagra? If you have erectile dysfunction (ED), your doctor may recommend that you take Viagra. It’s a prescription drug that’s used in adult males* . Follow all directions on your prescription label and read all medication guides or instruction sheets. Revatio is usually taken three times each day, about 4 to 6 hours apart. Viagra is . You should not use Viagra if you are allergic to sildenafil, or: if you take other medicines to treat pulmonary arterial hypertension, such as riociguat (Adempas). if you take nitrates. .