Viagra (sildenafil) is a phosphodiesterase-5 (PDE5) inhibitor used for the treatment of erectile dysfunction. What Are Side Effects of Viagra? Side effects of Viagra include: .Dosing The dose of this medicine will be different for different patients. Medicines that block an enzyme called CYP3A4. Although not all of these side effects may occur, if they do occur they may need medical attention. Children 1 year of age and older weighing 20 to 45 kg—20 mg 3 times a day. Use only the brand of this medicine that your doctor prescribed.Viagra (sildenafil) is a prescription drug that’s used to treat erectile dysfunction (ED). The drug comes as a tablet you’ll take by swallowing. It’s usually taken as needed about 1 . Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. They enhance the effects of a chemical the body makes that relaxes muscles . Viagra is classified as a phosphodiesterase type 5 (PDE5) inhibitor. For information on the dosage of Viagra, including its pill form, strengths, and how to take the drug, keep .Oral medicines for ED usually work well and cause few side effects. Be sure to let your doctor and pharmacist know that you are taking these medications before you start taking sildenafil. Age greater than 50 years or Coronary artery disease or Crowded disc or low cup to disc ratio in the eye an eye disorder or Diabetes or Hyperlipidemia high fats in the blood or Hypertension high blood pressure or Non-arteritic anterior ischemic optic neuropathy or NAION serious eye condition , history of or Smoking—May increase the chance for a serious side effect in the eye called NAION. Do not use this medicine if you are also using a nitrate medicine, often used to treat angina or high blood pressure. Other uses for this medicine This medication is sometimes prescribed for other uses; ask your doctor or pharmacist for more information. This medicine works best when you take it on an empty stomach an hour before sex. Talk to your doctor about eating grapefruit and drinking grapefruit juice while taking this medicine.