Metoprolol Tartrate is the medication with the imprint M18. It’s strength is 25mg. Metoprolol is in a group of drugs called beta-blockers. Beta-blockers affect the heart and circulation (blood flow through arteries and veins). Metoprolol is used to treat angina (chest pain) and hypertension (high blood pressure). It is also used to treat or prevent heart attack. ! Any comments?