Metoprolol is a beta-1-adrenergic receptor blocker that acts specifically on cells in the heart. The effect on beta-2 receptors is negligible. This inhibition decreases cardiac output through negative chronotropic and inotropic effects (heart rate and beating force are decreased).
When administered orally, metoprolol is almost completely absorbed in the gastrointestinal tract. Maximum serum concentration is reached 20 min after intravenous administration and 1-2 hours after oral administration. The bioavailability of metoprolol is 100% with intravenous administration and approximately 50% with oral administration. Plasma protein binding is approximately 11%. Metoprolol undergoes significant hepatic first-pass metabolism, accounting for approximately 50% of the administered dose. Metoprolol metabolism is mainly determined by the activity of CYP2D63 and to a lesser extent by the activity of CYP3A4. Metoprolol is excreted primarily through the kidneys. The immediate-release formulations of metoprolol have a half-life of approximately 3-7 hours
taken concomitantly with other cardiac drugs, such as Ca antagonists (verapamil and diltiazem) or antiarrhythmic drugs such as disopyramide. Combination may cause life-threatening side effects.