Metoprolol slows down nerve impulses travelling from the brain to the heart, which can, in turn, slow your heart rate. Bradycardia (abnormally slow heart beat): This is defined as a resting heart rate of 60 beats per minute or less.Some of the more common side effects that may be experienced when taking metoprolol (sold as Lopressor, Toprol XL, Betaloc, Lopresor) include: NOTE: If you experience symptoms of an allergic reaction which often involve a combination of itching, the development of a rash or hives, difficulty breathing and noticeable swelling of the face, lips, throat and / or tongue visit your doctor or an emergency room immediately.The side effects listed below pertain to the orally administered form of this medication. There are a number of possible metoprolol side effects, it is best to speak to your doctor about these and consult with him/her should any of these persist and become bothersome. Metoprolol (Lopressor / Toprol XL) outlook.Can Metoprolol (Lopressor / Toprol XL) interfere with other medications?.What are the side effects of Metoprolol (Lopressor / Toprol XL)?.Warnings when taking Metoprolol (Lopressor / Toprol XL)?.