This medicine usually begins to work for erectile dysfunction within 30 minutes after taking it. Precautions It is important that you tell all of your doctors that you take sildenafil. Using this medicine with any of the following may cause an increased risk of certain side effects but may be unavoidable in some cases. Sildenafil Viagra. If your dose is different, do not change it unless your doctor tells you to do so.Viagra (sildenafil) is a prescription drug that’s used to treat erectile dysfunction (ED). Viagra can cause side effects that range from mild to serious. Examples include low blood pressure and.  · This includes prescription or nonprescription (over-the-counter [OTC]) medicines and herbal or vitamin supplements. Side Effects. Along with its needed effects, a medicine may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention. AdHeute bestellen, morgen erhalten: Online-Rezept und gratis Lieferung innert 24 Stunden. Unser Online-Ärzteteam verspricht professionelle Beratung und gratis Express-Lieferung.