Metoprolol Pharmacology

Metoprolol exists in two primary oral forms: the immediate-release tartrate (Lopressor), typically dosed twice daily, and the extended-release succinate (Toprol XL), designed for once-daily administration due to its 24-hour duration of action. While both treat hypertension, angina, and atrial fibrillation, only the succinate formulation carries specific evidence for use in heart failure with reduced ejection fraction (HFrEF). A critical differentiator for metoprolol is its primary metabolism via the CYP2D6 enzyme; consequently, inhibitors like paroxetine, bupropion, and fluoxetine can significantly elevate drug concentrations, increasing the risk of bradycardia and toxicity. Clinical management requires caution when switching formulations due to slight bioavailability differences and a strict avoidance of crushing extended-release tablets to maintain proper pharmacokinetics. As a beta-1 selective agent, it offers a safer profile for patients with asthma compared to non-selective beta blockers by minimizing respiratory interference.