Aspirin blocks the production of chemicals known as prostaglandins. Certain types of prostaglandins cause the muscles inside of blood vessels to relax and also promote the elimination of sodium and water by the kidneys, both of which contribute to a decrease in blood pressure. Although it does not seem to directly alter the blood pressure-lowering ability of metoprolol, aspirin may lessen the overall benefit of using metoprolol because of its effect on the prostaglandins. Your doctor may want to monitor your blood pressure more frequently if aspirin and metoprolol are taken together on a regular basis. If you are experiencing problems, your doctor may consider adjusting the dose of aspirin. Discuss this potential interaction with your healthcare provider at your next appointment, or sooner if you think you are having problems.
This interaction is poorly documented and is considered moderate in severity.