Clarithromycin × Metoprolol

Metoprolol is mainly metabolised by CYP2D6; clarithromycin does not clinically significantly inhibit CYP2D6. The effect on metoprolol levels is minimal. Additive bradycardia is possible from clarithromycin's direct effect on the cardiac conduction system.

Bradycardia, dizziness, fatigue. Symptoms appear during clarithromycin courses in older patients and concurrent heart failure.

For short clarithromycin courses (7–14 days) with normal heart rate, no adjustment needed. In older patients and prior first-degree AV block, check ECG at one week. For long-term clarithromycin (e.g. Mycobacterium avium complex), antibiotic alternative: azithromycin (minimal HR effect).