Is it appropriate to switch a patient from metoprolol tartrate (metoprolol) 25mg twice daily (BID) to metoprolol succinate (metoprolol) 25mg BID?

Metoprolol Tartrate to Metoprolol Succinate Conversion

Metoprolol tartrate 25mg BID should not be converted to metoprolol succinate 25mg BID, but rather to metoprolol succinate 50mg once daily to maintain equivalent therapeutic effect. 1

Proper Conversion Guidelines

Metoprolol tartrate and metoprolol succinate have different pharmacokinetic properties that affect their dosing:

• Metoprolol tartrate is immediate-release with a shorter half-life requiring twice-daily dosing
• Metoprolol succinate is extended-release (CR/XL) designed for once-daily administration 2

Correct Conversion:

• Metoprolol tartrate 25mg BID (50mg total daily dose) → Metoprolol succinate 50mg once daily
• A 100mg metoprolol succinate tablet contains 95mg of metoprolol succinate and is equivalent to 100mg of metoprolol tartrate 3

Pharmacological Differences

The key differences between these formulations explain why a direct BID-to-BID conversion is inappropriate:

• Release Mechanism: Metoprolol succinate tablets disintegrate into individual pellets that act as diffusion cells, releasing the drug at a relatively constant rate over approximately 20 hours 3
• Plasma Concentration: Metoprolol succinate provides more consistent plasma concentrations with less fluctuation throughout the day 4
• Beta-Blockade Duration: Metoprolol succinate maintains relatively constant beta-blockade over 24 hours, allowing once-daily dosing 4

Evidence-Based Dosing

According to the ACC/AHA guidelines, the appropriate dosing for metoprolol in heart failure is:

• Metoprolol tartrate: 25-50mg twice daily (target dose)
• Metoprolol succinate: 12.5-25mg once daily (initial), 200mg once daily (target) 2

Clinical Implications

Inappropriate conversion (keeping BID dosing with succinate) could lead to:

1. Overdosing: Excessive beta-blockade due to the extended-release properties of succinate
2. Adverse Effects: Increased risk of bradycardia, hypotension, and other side effects
3. Reduced Efficacy: Disruption of the intended pharmacokinetic profile of the extended-release formulation

Monitoring After Conversion

After converting to metoprolol succinate:

• Monitor heart rate and blood pressure within 2 weeks
• Check for symptom improvement and side effects
• Titrate dose as needed toward target dose (200mg daily for heart failure patients) 1

Common Pitfalls to Avoid

1. Maintaining BID dosing with succinate: This negates the pharmacokinetic advantage of the extended-release formulation
2. Inadequate total daily dose conversion: Converting to an equivalent single daily dose is essential
3. Abrupt dose reduction: Could lead to rebound effects including tachycardia or exacerbation of underlying cardiac conditions 1

Remember that metoprolol succinate is specifically designed for once-daily dosing to maintain consistent beta-blockade over 24 hours, which is a key advantage over the immediate-release tartrate formulation 5, 6.