A 55-year-old woman on long-term lithium for bipolar disorder develops a urinary tract infection. The physician avoids prescribing ciprofloxacin with the macrolide she is already taking for COPD exacerbation prophylaxis. Which macrolide is a strong CYP3A4 and P-glycoprotein inhibitor that would increase lithium toxicity risk?

• A Azithromycin
• B Telithromycin
• C Fidaxomicin
• D Clarithromycin ✓

Explanation

Clarithromycin is a potent CYP3A4 inhibitor (though less potent than erythromycin) and also inhibits P-glycoprotein. Azithromycin does not significantly inhibit CYP3A4 and is preferred when drug interactions are a concern. Telithromycin is a ketolide that also inhibits CYP3A4 but is rarely used. Fidaxomicin is a non-absorbed macrolide for C. difficile with no systemic interactions. The question tests the key distinction between clarithromycin and azithromycin metabolic profiles.

Reference: Katzung Basic and Clinical Pharmacology, 15th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.