Metoprolol vs Propranolol
## Overview
Metoprolol is a beta-1 selective (cardioselective) beta-blocker, while propranolol is a non-selective beta-1 and beta-2 antagonist — the prototype beta-blocker. This selectivity difference is clinically significant in patients with asthma, COPD, or diabetes.
## Mechanism of Action
Metoprolol blocks beta-1 receptors selectively. Propranolol blocks both beta-1 and beta-2 receptors with equal affinity. Beta-2 blockade in the lungs can cause bronchoconstriction; in the pancreas, it can mask tachycardia (a warning sign of hypoglycemia) and impair glycogenolysis.
## Pharmacokinetics
Propranolol has high lipophilicity, ~90% first-pass metabolism by CYP1A2 and CYP2D6, bioavailability only ~25–30%, and a short half-life of 3–6 h (requiring dosing 2–3× daily for IR formulation). An LA formulation extends coverage. Propranolol readily crosses the CNS.
Metoprolol IR has bioavailability ~50%, half-life 3–7 h (CYP2D6-dependent). Metoprolol succinate XL provides 24-h coverage. Both are lipophilic.
## Pharmacodynamics and Indications
Propranolol has uniquely broad indications: hypertension, angina, arrhythmias, post-MI, hypertrophic cardiomyopathy, essential tremor, thyrotoxicosis (peri-operative), migraine prophylaxis, portal hypertension, infantile hemangiomas (off-label). Its non-selective beta-blockade is especially useful in conditions driven by excess sympathomimetic tone across all tissues.
Metoprolol's cardioselectivity makes it preferred in cardiac patients who also have reactive airway disease.
## Safety in Asthma/COPD
Propranolol is contraindicated in asthma and COPD due to beta-2 blockade causing bronchospasm. Metoprolol at standard doses can be cautiously used in mild COPD but should be monitored. Even cardioselective beta-blockers lose selectivity at higher doses.
## Key Takeaways
- Propranolol: non-selective, broad indications, first-line for tremor/thyrotoxicosis/portal HTN/migraine
- Metoprolol: cardioselective, preferred in asthma/COPD, HFrEF (XL formulation)
- Both lipophilic with CNS side effects; propranolol has higher first-pass extraction
บทสรุป
Propranolol is indicated when non-selective beta-blockade is needed (essential tremor, thyrotoxicosis, portal hypertension, certain arrhythmias). Metoprolol is preferred for cardiac indications (HFrEF, post-MI rate control, hypertension) especially in patients with pulmonary disease.