Which opioid is hydrophilic and does not pass into the mother's milk?

• A. Morphine
• B. Fentanyl
• C. Remifentanil
• D. Hydromorphone

Answer: (A)

The key idea here is how a drug’s lipid solubility and ionization affect its passage into breast milk. Lipophilic (fat-loving) drugs diffuse into milk more readily, while hydrophilic (water-loving) drugs cross more slowly. Morphine is relatively hydrophilic, so it transfers into breast milk less readily than the other options listed, which are more lipophilic and cross into milk more easily. Hydromorphone is somewhere in between, while fentanyl and remifentanil are highly lipophilic and can be found in milk more readily. Therefore, morphine fits the description of being hydrophilic with limited milk transfer. Do keep in mind that even hydrophilic opioids can appear in milk, so clinical monitoring remains important.