If you are taking sildenafil to treat erectile dysfunction, follow your doctor’s directions and the guidelines in this paragraph. Take sildenafil as needed before sexual activity. If you experience chest pain, dizziness, or nausea during sexual activity, call your doctor immediately and avoid sexual activity until your doctor tells you otherwise. How should this medicine be used? Kidney disease that needs to be treated with dialysis. However, if you take sildenafil with a high-fat meal, it will take longer for the medication to start to work.Viagra is only available with a prescription in the United States, Canada, and most other countries. Viagra is sold under the generic name sildenafil, which is also only available . If you are taking sildenafil (Viagra) for erectile dysfunction, do not take any more doses of sildenafil (Viagra) or similar medications such as tadalafil (Cialis) or vardenafil (Levitra) . Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. They enhance the effects of a chemical the body makes that relaxes muscles .