A patient receiving warfarin develops a marked rise in INR after starting a macrolide for Mycoplasma pneumonia. Which macrolide was most likely prescribed, based on cytochrome interactions?

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

Explanation

Clarithromycin is a potent inhibitor of CYP3A4 and increases levels of warfarin, statins, carbamazepine and other 3A4 substrates. Azithromycin has negligible CYP450 inhibition, which is why it is preferred when a macrolide must be combined with narrow-index drugs. Roxithromycin has some interaction potential but far less than clarithromycin. Fidaxomicin is a gut-selective Clostridioides difficile agent without systemic CYP effects and is not used for pneumonia.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.