Products and services Proper Use Drug information provided by: Merative, Micromedex ® Use sildenafil exactly as directed by your doctor. Do not use more of it and do not use .Alternatively, the medicine may be taken 30 minutes to 4 hours before sexual intercourse. Patients should always ask their doctors for medical advice about adverse events. Different brands may not work the same way. What special precautions should I follow? 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. Children 1 year of age and older weighing 20 kg or less—10 mg 3 times a day.Take sildenafil exactly as it was prescribed for you. Follow all directions on your prescription label and read all medication guides or instruction sheets. Revatio is . Take Viagra exactly as it was prescribed for you. Follow all directions on your prescription label. Do not take this medicine in larger or smaller amounts or for longer than . Sildenafil (Viagra). This medicine works best when you take it on an empty stomach an hour before sex. The body takes longer to absorb it after a high-fat meal or alcohol. Its effects .Alpha-blockers are sometimes prescribed for prostate problems or high blood pressure. Your doctor may adjust your dose if needed. How oral medicines differ Each oral medicine for ED has a slightly different chemical makeup. These medicines are called PDE5 inhibitors. If you are taking sildenafil for PAH, take the missed dose as soon as you remember it. Recent heart or blood vessel problems. If you are taking sildenafil to treat erectile dysfunction, you should know that it does not cure erectile dysfunction or increase sexual desire.