A nursing mother who is an ultrarapid CYP2D6 metabolizer takes codeine for post-episiotomy pain. Her breastfed infant becomes excessively sedated with respiratory depression. The mechanism is:

• A Codeine itself crosses into milk in toxic amounts in ultrarapid metabolizers
• B The infant has immature glucuronidation and cannot clear codeine
• C Excessive conversion of codeine to morphine raises morphine levels in maternal plasma and milk ✓
• D Codeine inhibits the infant's respiratory centre via placental transfer

Explanation

Codeine is a prodrug converted by CYP2D6 to morphine. Ultrarapid metabolizers generate supratherapeutic morphine concentrations, which pass into breast milk and can cause fatal infant respiratory depression. This is the textbook reason codeine is avoided in breastfeeding. The issue is maternal overproduction of morphine, not transfer of codeine itself, and the drug reaches the infant through milk rather than the placenta after birth.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.