Never take an extra dose to make up for a forgotten one. Precautions It is important that you tell all of your doctors that you take sildenafil. And look at the site or last author on the articles that appear. Do not use this medicine if you are also using a nitrate medicine, often used to treat angina or high blood pressure. If you see air bubbles in the syringe, repeat the steps. This medicine usually begins to work for erectile dysfunction within 30 minutes after taking it. Children 1 year of age and older weighing 20 to 45 kg—20 mg 3 times a day.Taking sildenafil with a nitrate medicine can cause a sudden and serious decrease in blood pressure. Contact your doctor or seek emergency medical attention if your erection is painful or lasts longer than 4 hours. A prolonged erection (priapism) can damage the penis. Viagra mg (Pfizer VGR ) View all images. Price Drugs. Find the Nearest Pharmacy & Save. Search by Pharmacy Name. Enter an Address or Zip Code. Search. Back. KROGER CO in Marietta, Georgia. Get discounts Missing: sildenafil. Sildenafil is used to treat erectile dysfunction (ED) and to reduce blood pressure in pulmonary arterial hypertension (PAH). It is taken when needed for ED and daily for PAH. A headache, stomach upsets, and flushing are the main side effects, and it should not be taken within 24 hours of a nitrate. 5. Tips.