Precautions It is important that you tell all of your doctors that you take sildenafil. You’ll soon start receiving the latest Mayo Clinic health information you requested in your inbox. Children 1 year of age and older weighing 45 kilograms kg or more—20 mg 3 times a day. Sildenafil should not be used with any other medicine or device that causes erections. If you see air bubbles in the syringe, repeat the steps. Dosing The dose of this medicine will be different for different patients.Answers. Half an hour to an hour before intercourse. The maximum daily dose is of mg. every 24 hours. Avoid alcohol, it can increase potential side effects from the medication. Sildenafil tablets for erectile dysfunction come in different strengths ranging from 25mg to mg. The usual dose is 50mg, when you need it. Do not take it more than once a day. The dose can be increased to mg or decreased to 25mg depending on its effect. Take sildenafil up to 4 hours before you want to have sex. If you are taking sildenafil (Viagra) for ED, do not take it more than once a day. Take an hour before sexual stimulation (range 30 minutes to 4 hours). If you take it with a high-fat meal it may delay how quickly it starts working.