One of the most common questions we get as lactation consultants is, “Is it safe to take this medication while breastfeeding?” Having a solid understanding of pharmacokinetics can help us answer confidently and support families in making informed decisions. Here’s a practical guide to help you assess medications and breastfeeding safety.

Relative Infant Dose (RID)

Relative Infant Dose (RID) is one of the best tools to estimate the safety of a medication. It compares the infant’s dose (through breast milk) to the mother’s dose, expressed as a percentage.

• Rule of Thumb: Medications with an RID <10% are considered relatively safe for breastfeeding.
• Examples:
  • Ampicillin: RID 0.17%-0.51% (safe)
  • Gentamicin: RID 2.1% (safe)
  • Magnesium sulfate: RID 0.2% (safe)

You don’t need to memorize the calculation, but understanding how to interpret RID can give you confidence in guiding families.

Oral Bioavailability

Even if a medication transfers into breast milk, it must be absorbed by the baby’s gut to have any effect. Many drugs are poorly absorbed, destroyed in the stomach, or metabolized in the liver, which makes them less concerning.

• Examples:
  • Gentamicin: Given intravenously because its oral bioavailability is <1%. The baby won’t absorb much, if any, from breast milk.
  • Insulin: Oral bioavailability is negligible, which is why it’s always injected.

Milk/Plasma Ratio (M/P)

The milk/plasma ratio shows how much of a drug is in breast milk compared to the mother’s blood.

• Guidelines:
  • M/P > 1-5: This can be concerning because the drug may concentrate in breast milk.
  • M/P < 1: Ideal, as minimal amounts transfer to breast milk.

Keep in mind that M/P ratios can vary with maternal plasma levels, which fluctuate throughout the day.

pH and pKa

Here’s the short version: medications with a high pKa (greater than 7.2, the pH of breast milk) can get “trapped” in milk due to ionization. Drugs with a lower pKa are less likely to accumulate in milk.

• Takeaway: Focus on medications with a lower pKa when possible, especially if they are weakly basic drugs.

Key Factors for Safe Medications

When evaluating a medication, consider these properties:

• High molecular weight (>800 Daltons): Larger drugs are less likely to transfer into milk.
  • Examples: Insulin (6,000 Daltons), heparin (40,000 Daltons).
• High protein binding (>90%): Drugs that bind to proteins like albumin stay in maternal plasma and don’t enter milk easily.
  • Examples: Ibuprofen (99%), warfarin (99%).
• Low lipid solubility: Medications with high lipid solubility transfer into milk more easily. If a drug acts on the central nervous system (CNS), it’s likely highly lipid-soluble.
  • Examples: Antidepressants and anti-anxiety medications.
• Short half-life: Drugs with shorter half-lives clear from the mother’s bloodstream faster, reducing exposure to the baby.

Special Considerations

• Preterm infants: Their immature systems make them more susceptible to medications. Always use extra caution.
• First days postpartum: Medications may transfer more easily due to gaps in the alveolar cells during colostrum production. However, the low volume of milk during this time minimizes infant exposure.
• Topical and ophthalmic medications: These typically don’t reach significant levels in breast milk.

Antibiotics and Other Medications

Most antibiotics are safe, but some require extra caution.

• Chloramphenicol: Classified as L4 (potentially hazardous) in Hale’s Medications and Mothers’ Milk. This antibiotic can be toxic to neonates and is contraindicated unless absolutely necessary.
• Radioactive iodine: Avoid entirely during breastfeeding. It concentrates in breast milk and is dangerous for infants.

How to Use This Information

When helping families, always weigh the benefits of breastfeeding against the potential risks of the medication. Use reliable resources like Hale’s Medications and Mothers’ Milk to assess specific drugs.

Conclusion

Medications are less likely to transfer into breast milk if they have:

• A low maternal plasma level.
• A short half-life.
• High molecular weight (>800 Daltons).
• High protein binding (>90%).
• Low lipid solubility.

Remember, in most cases, less than 1% of a mother’s medication dose ends up in her milk. By understanding these key factors, you can confidently guide families and advocate for the safety of both mother and baby.

References

Hale, T. W. (2021). Hale’s medications & mothers’ milk 2021: A manual of lactational pharmacology (19th ed.). Springer Publishing Company.

Lawrence, R. A., & Lawrence, R. M. (2021). Breastfeeding: A guide for the medical profession (9th ed.). Elsevier.

Mannel, R., Martins, P. J., & Walker, M. (Eds.). (2013). Core curriculum for lactation consultant practice (3rd ed.). Chapter 25: Lactational pharmacology (T. W. Hale & F. J. Nice). Jones and Bartlett Learning.