Switching from Metoprolol ER 100 mg to Atenolol 50 mg with HCTZ 25 mg
Switch directly from metoprolol succinate ER 100 mg to atenolol 50 mg twice daily (or 100 mg once daily) while continuing hydrochlorothiazide 25 mg, with no tapering or overlap period required.
Rationale for Direct Substitution
Both metoprolol and atenolol are cardioselective (β1-selective) beta-blockers with similar mechanisms of action, making direct substitution safe and appropriate 1, 2. The key difference lies in their pharmacokinetics: metoprolol has a shorter half-life (3-7 hours) requiring extended-release formulation for once-daily dosing, while atenolol has a longer half-life (6-7 hours) allowing for once or twice-daily dosing 3, 4.
Dose Equivalency Considerations
Metoprolol ER 100 mg is approximately equivalent to atenolol 50-100 mg daily. While direct dose equivalency tables don't exist in guidelines, research suggests metoprolol and atenolol have similar antihypertensive efficacy at these doses 5. The FDA-approved dosing ranges support this: metoprolol tartrate 100-200 mg twice daily and atenolol 25-100 mg once daily for hypertension 6, 7, 3.
Specific Switching Protocol
Option 1: Atenolol 50 mg twice daily (preferred for consistent 24-hour control)
- Day 1: Stop metoprolol ER 100 mg; start atenolol 50 mg twice daily with HCTZ 25 mg once daily
- No washout period needed
- Monitor blood pressure and heart rate at 1 week and 4 weeks
Option 2: Atenolol 100 mg once daily (if adherence is a concern)
- Day 1: Stop metoprolol ER 100 mg; start atenolol 100 mg once daily with HCTZ 25 mg once daily
- This is the maximum recommended daily dose 3
- May provide less consistent 24-hour control than twice-daily dosing
Monitoring Parameters
Check blood pressure and heart rate 1 week after switching, then again at 4 weeks to ensure adequate control. Measure "trough" blood pressure (just before next dose) to confirm 24-hour efficacy 3. Target heart rate should remain 50-60 bpm at rest for most cardiovascular indications.
Important Caveats
Renal Function Assessment
Atenolol requires dose adjustment in renal impairment, unlike metoprolol 3. This is critical:
- CrCl 15-35 mL/min: Maximum atenolol 50 mg daily
- CrCl <15 mL/min: Maximum atenolol 25 mg daily
- Hemodialysis: Give 25-50 mg after each dialysis session
Metoprolol does not require renal dose adjustment 4, making this switch potentially problematic in patients with significant renal disease. Consider maintaining metoprolol if CrCl <35 mL/min.
Elderly Patients
Start with atenolol 25 mg daily in elderly patients (>65 years), especially those with any degree of renal impairment 3. Atenolol accumulates with age due to declining renal function, whereas metoprolol is hepatically metabolized.
Contraindications to Verify
Before switching, confirm absence of 1, 2:
- AV block greater than first degree (without pacemaker)
- Decompensated heart failure
- Severe reactive airway disease
- Cardiogenic shock
- Severe bradycardia (<50 bpm)
HCTZ Continuation
Continue hydrochlorothiazide 25 mg once daily without interruption 6, 7. The combination of beta-blocker plus thiazide diuretic is guideline-recommended for hypertension management and provides complementary mechanisms of blood pressure control 8, 9.
When NOT to Make This Switch
Do not switch to atenolol if:
- CrCl <35 mL/min – metoprolol is preferred due to hepatic metabolism 3, 4
- Heart failure with reduced ejection fraction – metoprolol succinate has proven mortality benefit; atenolol does not 10
- Recent myocardial infarction – metoprolol has Class I indication; atenolol has less robust evidence 10
In these scenarios, consider switching to a different metoprolol formulation or adjusting the dose rather than changing to atenolol.
Expected Outcomes
Blood pressure control should remain stable or improve with this switch, as both agents have similar antihypertensive efficacy when combined with HCTZ 11, 12. The combination provides additive blood pressure lowering through complementary mechanisms: beta-blockade reduces cardiac output and renin release, while HCTZ reduces plasma volume 6, 7.