Using this medicine with any of the following may cause an increased risk of certain side effects but may be unavoidable in some cases. Key facts It usually takes 30 to 60 minutes for sildenafil to work for erectile dysfunction. It is important to tell your doctor about any heart problems you have now or may have had in the past. Children 1 year of age and older weighing 45 kilograms kg or more—20 mg 3 times a day. The following interactions have been selected on the basis of their potential significance and are not necessarily all-inclusive. It is not known if the medicine is safe for use in these patients.AdGet a Prescription for ED treatment, discreetly deliverED to your door. Typical dosing for Sildenafil (Viagra) The typical dose is to take 50 mg of sildenafil (Viagra) by mouth, about 1 hour before sex, as needed. The maximum recommended dose is mg. Don’t take sildenafil (Viagra) more than once daily (at any dose). Written by Brian Leonard, PharmD, BCACP, BCGP. Before you start taking Viagra, tell your doctor and pharmacist about any prescription, over-the-counter, or other drugs you take. Sharing this information with them may help prevent possible.