Medication Transfer Into Breast Milk

Sandy Sundquist Beauman, MSN, RNC-NIC / December 2018

While most neonatal and infant healthcare providers recognize the benefits of breastfeeding, there is often concern about the transfer of maternal medications into breast milk. That may lead to an inclination to discourage the use of human milk in the presence of at least some medications.

We instruct mothers to record medications they may be taking while pumping their milk for babies in the NICU with the idea that we may or may not feed it to the infant. There are so many variables to passage of drugs into human milk.  Some of those include:

  • Molecular weight
  • Timing of the medication and infant feeding
  • Make up of milk, e.g. colostrum versus mature milk
  • Maternal conditions
  • Half-life of the drug
  • Infant characteristics
  • Frequency and volume of feedings

Characteristics of the medication, including molecular weight, degree of ionization, solubility characteristics, and protein binding all matter when considering how much of a drug may transfer into the milk.

Molecular weight

Furthermore, there are many barriers to passage into the alveoli where milk is made. Higher molecular weights are less likely to pass into the milk, and very high molecular weight medications cannot pass. Heparin and insulin are two medications with molecular weights that are too high.1, 2 The molecular weight of heroin is 369.4 and methadone is 309.4, both of which are quite low. Methadone is more likely to lead to withdrawal in the infant as it also passes through the placenta. However, Methadone, and other drugs, even though they pass into human milk, may not be a contraindication to breastfeeding.3

Other factors mentioned earlier such as degree of ionization, solubility, and protein binding may prevent lower molecular weight medications passing into the milk, or such small amounts are passed that there is no concern for the infant. Drugs that are bound to protein cannot pass into the milk, nor do non-ionized drugs pass across the membranes. Additionally, drugs that bind to proteins in plasma may not bind with proteins in milk.

Timing related to infant feeding

Timing of the dose related to infant feeding may also affect the amount of passage. Since milk is manufactured as the infant suckles or mom is pumping, a drug that is peaking at that time would result in more transfer to the milk. Taking the medication after pumping or breastfeeding would result in less transfer than may otherwise be present in the milk. However, since infants often breastfeed every 1-3 hours, it may be difficult to avoid the peak of a medication.

If the drug binds with fat, higher amounts may be transferred into colostrum, which is higher in fat than mature milk.1

Maternal conditions

Maternal conditions that may lead to compromised clearance of the medication, such as poor liver or kidney function, may increase levels which could result in higher peak levels of the medication and/or a longer time at a high peak level. That may or may not lead to more transfer into the milk depending on medication characteristics as mentioned above.

Half-life of the drug

Half-life of the drug also will determine how the drug is cleared. Drugs with a long half-life maintain a therapeutic level for a longer period of time, often making it a more attractive way to treat something, but also increases the accumulation of the drug in the mother and therefore, the potential for transfer into the milk.

Infant characteristics

While infant characteristics do not affect the amount of medication transferred into the milk, they can influence whether the transfer of the medication should be a concern. Preterm infants have slower gastrointestinal motility, delayed gastric emptying, and increased permeability of the GI tract. For those reasons, medication transferred into maternal milk may have an increased effect in infants. Therefore, we often ask mothers to pump and freeze their milk when taking medications of concern. This milk can be fed to the infant later when he is more mature without any effect.

Increased frequency and higher volume of feedings will allow for more of the medication to be transferred to the infant if it is present in the milk.

Medication exposure

The next question to be answered is whether it matters if the infant is exposed to medication that could potentially be transferred into human milk. Only about 1 – 2% of maternal medication is transferred into milk, and potentially to the infant. The risk of the small amount of medication that may be transferred should be weighed against the benefits of the mother’s milk. There are very few absolute contraindications to breastfeeding. Considerations of alternative medications may be helpful as well, as is avoiding certain medications while breastfeeding whenever possible. In some cases, it may be a drug that we would treat the infant with anyway. Again, levels passed through the human milk are quite low in many cases.

Furthermore, there are measures that may be taken if there is indeed concern about the effect of a drug that may be transferred to the infant. An awareness of the drug properties, and whether there may be another drug that may treat the condition with less potential for transfer may allay fears of the potential for effect on the infant. Also, a drug with a short half-life may be taken after breastfeeding, rather than before. Medications that may present concerns in the neonatal period may be safe to feed as the infant matures and therefore, milk may be pumped and frozen for later consumption.

There are a lot of moving parts to consider in making decisions about breastfeeding and maternal medication use. With few exceptions, the benefits of breastfeeding outweigh the risks when it comes to medication use. A good source to consult for information about drug transfer in breast milk is Medications and Mothers’ Milk by Hale & Rowe.4


  1. Banta-Wright, S. (1997). Minimizing infant exposure to and risks from medications while breastfeeding.
  2. Gardner, D. K. (1987). Drug passage into breast milk: Principles and concerns. Journal of pediatric and perinatal nutrition (USA).
  3. Hudak ML, Tan RC. The Committee on Drugs and the Committee on Fetus and Newborn Neonatal drug withdrawal. Pediatrics 2012; 129: e540–e560.
  4. Hale, T. W., & Rowe, H. E. (2016). Medications and Mothers’ Milk 2017. Springer Publishing Company.

About the Author

Sandy Sundquist Beauman has over 30 years of experience in neonatal nursing. In addition to her clinical work, she is very active in the National Association of Neonatal Nurses, has authored or edited several journal articles and book chapters, and speaks nationally on a variety of neonatal topics. She currently works in a research capacity to improve healthcare for neonates. Sandy is also a clinical consultant with Medela LLC. You can find more information about Sandy and her work and interests on LinkedIn.