The following nonprescription or herbal products may interact with sildenafil: cimetidine Tagamet HB , St. Oral medicine for ED also might not be safe or work well if you have: Very low blood pressure, called hypotension, or uncontrolled high blood pressure, called hypertension. Take sildenafil at around the same times every day. This medicine works best when you take it on an empty stomach an hour before sex. Also tell your doctor if you have or have ever had pulmonary veno-occlusive disease PVOD; blockage of veins in the lungs ; a stomach ulcer; heart, kidney, or liver disease; a heart attack; heart failure; angina chest pain ; an irregular heartbeat; a stroke; chest pain; high or low blood pressure; high cholesterol; a bleeding disorder; blood circulation problems; blood cell problems such as sickle cell anemia a disease of the red blood cells , multiple myeloma cancer of the plasma cells , or leukemia cancer of the white blood cells ; conditions affecting the shape of the penis e.For treatment of erectile dysfunction: For oral dosage form (tablets): Adults up to 65 years of age—50 milligrams (mg) as a single dose no more than once a day, 1 hour before . 25 milligrams (mg) 50 mg mg Typical dosages The following information describes commonly recommended dosages for ED. However, be sure to take the dosage your . Optum Store Sildenafil If you are experiencing the symptoms of erectile dysfunction (ED), you may have considered using Viagra or a similar ED medication as a possible .