When choosing one, tell your doctor or other health care professional what your preferences are, such as cost, ease of use, how long the drug’s effects last and side effects. If you experience a prolonged or painful erection for 4 hours or more, contact your doctor immediately. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy. Side Effects Along with its needed effects, a medicine may cause some unwanted effects. To use the oral liquid: Shake the bottle well for at least 10 seconds before each use. All medications must be declared upon arrival at the border. Precautions It is important that you tell all of your doctors that you take sildenafil.Updated on July 17, Medically reviewed by Mary Choy, PharmD Fact checked by Heather Mercer Table of Contents Prescriptions From Mexico When Is It Legal to Import . Avanafil (Stendra). Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. They enhance the effects of a chemical the body makes that . If you take sildenafil for pulmonary arterial hypertension, do not take Viagra® or other PDE5 inhibitors, including tadalafil (Cialis®) or vardenafil (Levitra®). Viagra® also .