International Board of Certified Lactation Consultant (IBCLC) Practice Exam

Which drug property leads to increased transfer of the drug into breast milk?

1. Molecular weight of >800 Daltons

2. High protein binding

3. High lipid solubility

4. Low maternal plasma concentration

The correct answer is: High lipid solubility

High lipid solubility is a drug property that significantly influences its transfer into breast milk. Lipid-soluble drugs can easily pass through biological membranes, including the mammary gland epithelial cells. This characteristic facilitates the drug's movement into breast milk, as breast milk itself has a high lipid content. When a drug is lipophilic, it tends to partition into milk because of the fat composition, leading to greater concentrations in the milk compared to the plasma. In comparison, molecular weight directly affects a drug’s ability to cross cellular barriers, but large molecules typically have a harder time moving into breast milk. High protein binding tends to restrict the amount of free (active) drug available in the plasma to cross into breast milk. Similarly, low maternal plasma concentration can result in lower concentrations in milk, as there is less of the drug circulating in the mother's system to be transferred. High lipid solubility stands out as the primary factor that enhances the transfer of the drug into breast milk, making it essential for understanding how various drugs behave during lactation.