Medications and Breast Milk

By Krista Gray, IBCLC. Last updated March 20, 2016.

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Medications and Breast MilkIntroduction

Most breastfeeding mothers will use some sort of medication while breastfeeding whether it is prescribed from a doctor or herbal in nature.  Many doctors will suggest a mother wean when taking a drug.  However, in actual fact, this is almost always not necessary.  Though most everything which a mother ingests, including medications, will enter her milk, the amount is usually small and the impact on her baby minimal.

How does a drug pass into breast milk?

There are several avenues (passive diffusion, lipid solubility, molecular weight, ion trapping, protein binding, and/or cellular transport system) by which a drug may be able to pass into breast milk.

Passive Diffusion

The most common is passive diffusion – which means that the concentration of a drug in maternal plasma is the same as that in breast milk.  Alcohol is a prime example.  As a mother’s blood plasma levels increase, so does the amount in her milk.  As her levels decrease, the quantity in her milk does as well.  This is why “pumping and dumping” breast milk is not effective to remove alcohol.  Knowing that most drugs pass into breast milk by passive diffusion is immensely helpful in choosing a medication.  Drugs with a short half-life (rather than “long-lasting”) will clear from breast milk quicker.  In fact, many times it is possible to time feedings so that the baby will nurse when levels of a drug in breast milk are already very low for example, right before taking a dose.

Lipid Solubility

The lipid solubility of the medication is another means by which it may transfer to breast milk.  Breast milk has more lipids (fat) than plasma. Therefore, the more lipid soluble a drug, the more likely it is to be found in breast milk.  In fact, a highly lipid soluble drug may have higher concentrations in breast milk than maternal plasma. 1

Molecular Weight

A drug’s molecular weight can be a factor impacting transfer into breast milk.  Medications with a smaller weight (300 daltons or less) can pass into milk more readily that those with weights exceeding 500-800 daltons.  Thus, drugs with high molecular weights (such as insulin) rarely enter breast milk. 2

Ion Trapping

While blood plasma maintains a pH of 7.4, breast milk is slightly more acidic with a pH of 7.2.  A highly alkaline drug may change its ionic state in breast milk and then be unable to pass back out to maternal serum. Therefore it is best to select medications with a lower pKa – ie. drugs that are more acidic. 3

Protein Binding

The ability of a medication to bind to protein (albumin) in the mother’s plasma is yet another factor.  Only the portion of a drug that is unbound can transfer into breast milk.  Ibuprofen is an example of a drug that is highly bound; therefore, little transfers to breast milk. 4

Cellular Transport System

Finally, there are few drugs that seem to have their own cellular pumping system.  Some medications are transported in, and others transported out.  Iodine (especially radioactive) is the most relevant to breastfeeding as this drug is actively pumped into breast milk.  5

Is it true that there are greater concentrations of a drug in breast milk during the first few days following birth?

Yes, this is typically the case.  However, it does not mean that your baby is getting greater doses of the medication through your milk.  The reason for greater drug transfer during the first few days postpartum is due to hormones surrounding lactation.  But since a baby’s total colostrum/milk intake is small, the total amount of drug he receives is still usually negligible.

What should I consider about taking a medication while breastfeeding?

Though any medication has the potential to enter breast milk, it is almost always in small amounts that are not considered harmful to your baby.  Formula has known risks associated with it.  Breast milk is a living substance that is perfectly suited for your baby each and every day.  It is resilient enough to withstand the times a mother has a need for medication and still make milk of excellent quality.  In fact, typically when a drug does enter breast milk, it is in a dose much lower than if the baby were prescribed the medication outright!  So, here are a few things to consider about taking a medication while breastfeeding:

  • Can I delay treatment?
  • Is there an alternative medication that is still effective and would be safer for my baby than what is prescribed?
  • Can I time feeds around the medication so that my baby gets less – ie. nurse then take the medication; not taking a “long-lasting” drug; etc.
  • What is the Relative Infant Dose (RID)?  If it is less than 10% most drugs are considered safe. The RID for most drugs is less than 1%! 6
  • Talk with your doctor about choosing medications that have high protein binding, high molecular weight, poor penetration to central nervous system, short half-life and low oral bioavailability.
  • Select drugs that are commonly considered safe to use with pediatric patients.
  • Watch your baby for any side effects to see how medications affect your baby.

Show 6 footnotes

  1. Hale, T. (2012) Medications and Mother’s Milk 15th ed. Amarillo, TX: Hale Publishing, L.P., p.9.
  2. Riordan, J. & Wambach, K. (2010) Breastfeeding and Human Lactation 4th ed. Sudbury, MA: Jones & Bartlett Publishers, LLC p. 167.
  3. Hale, T. (2012) Medications and Mother’s Milk 15th ed. Amarillo, TX: Hale Publishing, L.P., p.9.
  4. Hale, T. (2012) Medications and Mother’s Milk 15th ed. Amarillo, TX: Hale Publishing, L.P., p.9.
  5. Hale, T. (2012) Medications and Mother’s Milk 15th ed. Amarillo, TX: Hale Publishing, L.P., p.9.
  6. Hale, T. (2012) Medications and Mother’s Milk 15th ed. Amarillo, TX: Hale Publishing, L.P., p.10.
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