S(-)-Metoprolol Dosing
S(-)-metoprolol (the S-enantiomer) is not available as a standalone therapeutic formulation—metoprolol is administered clinically only as a racemic mixture containing both R(+) and S(-) enantiomers in equal proportions. 1
Understanding Metoprolol Enantiomers
Metoprolol exists as two mirror-image molecules (enantiomers):
- S(-)-enantiomer: The pharmacologically active form responsible for beta-blocking effects
- R(+)-enantiomer: Less active form
When you take metoprolol (either immediate-release tartrate or extended-release succinate), you're receiving a 50:50 mixture of both enantiomers. The body metabolizes these differently through CYP2D6, with stereoselective metabolism that varies by genetic phenotype 1.
Clinical Dosing Regimens (Racemic Metoprolol)
Since only racemic metoprolol is available, here are the evidence-based dosing strategies:
For Heart Failure with Reduced Ejection Fraction (HFrEF)
Metoprolol succinate extended-release is the recommended formulation 2:
- Initial dose: 12.5-25 mg once daily
- Target dose: 200 mg once daily
- Titration: Increase every 2 weeks as tolerated
- Mean dose achieved in trials: 159 mg total daily 2
This regimen reduces all-cause mortality by 34% and provides consistent beta-blockade over 24 hours 3, 4.
For Acute Myocardial Infarction
Early treatment protocol 1:
- IV phase: 5 mg IV bolus every 2 minutes × 3 doses (15 mg total)
- Oral transition: Begin 50 mg orally every 6 hours, starting 15 minutes after last IV dose
- Continue: 48 hours at this dose
- Maintenance: 100 mg twice daily thereafter
For Hypertension
- Starting dose: 100 mg daily (can be given once or twice daily)
- Usual range: 100-450 mg daily 1
- Titration: Adjust at 2-week intervals based on blood pressure response
Important Pharmacokinetic Considerations
Gender Differences
Women achieve approximately 2-fold higher metoprolol exposure than men at equivalent doses 5. Pharmacokinetic modeling suggests that 50 mg in women provides similar drug exposure to 100 mg in men. However, current FDA labeling does not recommend gender-based dose adjustments 1.
CYP2D6 Metabolizer Status
- Poor metabolizers (8% of Caucasians, 2% of other populations): Experience several-fold higher plasma concentrations and decreased cardioselectivity 1
- Extensive metabolizers: Standard dosing applies
- Recent evidence shows that despite exposure differences, cardiovascular outcomes after MI were similar across CYP2D6 phenotypes 6
Hepatic Impairment
Initiate at low doses with cautious gradual titration 1. Elimination half-life can increase up to 7.2 hours (from normal 3-4 hours) depending on severity 1.
Renal Impairment
No dose adjustment required—renal failure does not significantly alter metoprolol pharmacokinetics 1.
Critical Clinical Pitfalls
Don't confuse formulations: Metoprolol tartrate (immediate-release) requires 2-4 times daily dosing; metoprolol succinate (extended-release) is once daily. They are NOT interchangeable mg-for-mg 3, 7.
Heart failure initiation: Always start low (12.5-25 mg) and go slow (titrate every 2 weeks). Patients are ineligible if they have hypotension, bradycardia, peripheral shock signs, or significant pulmonary congestion 2, 1.
S/R ratio variability: In long-term therapy, 30% of patients may show inverted plasma S/R ratios (<1) due to altered renal excretion or CYP2D6 saturation, particularly with renal impairment or polypharmacy 8.
Elderly patients: Use low initial doses due to decreased hepatic function and blood flow, though this increase in exposure is not usually clinically significant 1.