If you are a Mayo Clinic patient, this could include protected health information. Alternatively, the medicine may be taken 30 minutes to 4 hours before sexual intercourse. Your doctor will tell you not to take sildenafil if you are taking street drugs containing nitrates. If you are taking sildenafil for PAH, take the missed dose as soon as you remember it. Ask your pharmacist for a list of the ingredients.Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men. Sildenafil (Liqrev, Revatio) is used to improve the ability to exercise in .  · 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 it more . Cost. Overdose. Ask your doctor. Q&A. Drug Images. What is Viagra? If you have erectile dysfunction (ED), your doctor may recommend that you take Viagra. It’s a .