Viagra is typically taken at a dose of 50mg about one hour before sex. John’s wort. Oral medicines for ED usually work well and cause few side effects. If you ever need emergency medical treatment for a heart problem, the healthcare providers who treat you will need to know when you last took sildenafil. People who are taking nitrates cannot, under any circumstances, use ED medication.VIAGRA is available by prescription only in 25 mg, 50 mg, and mg tablets. Patients should always ask their doctors for medical advice about adverse events. You are . Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. They enhance the effects of a chemical the body makes that relaxes muscles . Why is this medication prescribed? Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men.