Which drug property allows a drug to readily enter breast milk?

• A. Large molecular weight
• B. Lipophilic
• C. Protein-bound
• D. Hydrophilic

Answer: (B)

Lipophilicity enables a drug to cross the lipid-rich barriers into milk via passive diffusion. Breast milk contains fats, so lipid-soluble drugs readily partition into milk and diffuse from plasma into milk more easily than hydrophilic or large, highly protein-bound molecules. In contrast, large molecular weight or high protein binding limits transfer, and hydrophilic drugs cross poorly. Thus, being fat-soluble (lipophilic) is the property that best explains why a drug readily enters breast milk.