It is widely documented that breastfeeding provides babies with optimal nutrition. The formulation of breastmilk includes immune system components which provide the baby with a degree of immune protection, as well as all the baby’s nutritional needs for the first six months of life. Other benefits for the baby include eyesight and cognitive development.

Breastfeeding is also very important in establishing the bond between mother and baby, and can aid maternal recovery in the post-partum period. Medicine use in the breastfeeding mother presents a unique dilemma to healthcare providers. The risks and benefits of the proposed medicine, to both mother and baby, must be carefully weighed in the context of the importance of continuing to breastfeed.

Factors Influencing Drug Transfer

The majority of drugs will inevitably pass into breastmilk to some extent, although the final concentration will vary widely between drugs. The mechanism of transfer between maternal serum and breastmilk is primarily through passive diffusion, that is, the passage of drug from a higher concentration in the maternal serum to a lower concentration in the breastmilk.

It is a common misconception that once present in breastmilk, drugs are ‘caught’ and any milk produced during the period of treatment must be discarded. This is not necessarily the case. In the same way drugs may passively pass into the breastmilk, they may pass back into the maternal serum for elimination when the maternal serum level lowers after a period of time.

There are several factors affecting how much drug will transfer into the breastmilk. Foremost of these is the concentration of the drug in the maternal serum, which itself is dictated by maternal drug absorption and elimination. Drugs which are poorly absorbed (or not absorbed at all) from the mother’s gut will not pass into breastmilk (e.g. nystatin, psyllium, simethicone or sucralfate). Short half-life drugs will also pose a smaller risk to the baby as the drug will be eliminated quickly from maternal serum and expose the breastmilk to lower drug levels (e.g. lignocaine or permethrin).

The physical characteristics of each drug will also impact on its passive diffusion into breastmilk. Larger molecules (e.g. heparin or insulin), and drugs which are highly protein bound (e.g. phenytoin), are not physically able to pass into breastmilk. Low molecular weight drugs are much more likely to enter breastmilk. It is important to note there is increased permeability between the mammary alveolar cells in the first 72 hours post-partum. During this period, higher molecular weight drugs may pass through that may not be able to enter at a later stage.

The pH of each drug also will impact its concentration, as breastmilk is slightly acidic in comparison to blood. Ionisation of a weakly basic drug (e.g. erythromycin) occurs in breastmilk, which increases the drug’s water solubility and inhibits its ability to pass back into the maternal serum.

Neonatal Considerations

Once a drug has passed into breastmilk and been ingested by the neonate, its clinical impact on the neonate depends on several factors. Firstly, the baby’s exposure is reliant on the drug’s absorption from the baby’s gut. Drugs which have diffused into breastmilk after parenteral administration to the mother may be poorly absorbed orally by virtue of their physical characteristics. Examples include gentamicin, cefotaxime and adrenaline.

Secondly, the baby’s ability to eliminate the drug will influence its overall effect. Neonatal kidney and liver functions are not as efficient as an adult’s, which may cause drug accumulation. Furthermore, drugs which may have been safe for the mother to take during pregnancy may not be safe in breastfeeding. This is because, during pregnancy, elimination of the drug occurs through the mother’s clearance systems, whereas after the birth the baby’s systems alone must clear any drug passed through the milk.

Arguably the most important consideration in using drugs while breastfeeding is the degree of possible toxicity to the baby after exposure through breastmilk. Some drugs have a higher inherent potential for harm than others. Examples of drugs which should be avoided in breastfeeding for this reason are cytotoxics, cyclosporin, and ergotamine. Drugs which are routinely prescribed in higher doses to babies than they would be exposed to through breastmilk are unlikely to cause harm, although monitoring is recommended in these cases.

Maternal Considerations

Potential harm to the neonate is not the only reason certain drugs should be avoided in breastfeeding. A number of drugs impede the breastfeeding process itself by reducing milk production. These include pseudoephedrine, phenylephrine, diuretics, oestrogen, bromocriptine, and cabergoline.

General considerations for treatment of a breastfeeding mother include firstly avoiding drug therapy where alternatives are available, or if the drug is not clinically indicated. Consideration may also be given to interrupting breastfeeding if the course of treatment is less than 48 hours and the infant would be at risk from exposure. Resuming breastfeeding may be difficult if it is stopped for any longer than this. It may be useful for the mother to express and discard the milk during the interruption so her milk supply is not reduced. Interrupting breastfeeding may not be an option depending on the preferences of the mother, and the willingness of the infant to take a bottle.

Drug Selection and Dosing Guidelines

The following recommendations apply to drug selection for a breastfeeding mother:

  • Use topical treatment where available (i.e. patches, inhalers, and creams)
  • Use the safest drug within a therapeutic group, and one which is marketed for, or used in, infants under two years old (if applicable)
  • Choose drugs with the shortest half-lives, highest molecular weight, or protein binding affinity
  • Choose drugs with poor oral absorption
  • Avoid newly marketed drugs
  • Avoid the use of two or more drugs with similar adverse reactions, and
  • Use the lowest dose which gives the desired outcome.

The risk to the infant may be further minimised by tailoring the dosage times with regard to feeding times. The timing of the drug’s peak plasma level should be considered and breastfeeding should be avoided at this time. This may not be practical for babies who feed every couple of hours. Similarly, single daily dose preparations may be taken just before the infant’s longest sleep interval to minimise the infant’s exposure.


There are many variables to consider when treating breastfeeding mothers, and decisions must be made on a case-by-case basis. It is important to consider not only how much drug will pass into breastmilk, but also the magnitude of the potential impact the drug will have on the baby and/or the lactation process.

Treatment strategies must be individualised, with a view to minimising the risk to the infant and maintaining breastfeeding wherever possible. A strategy which enables the mother to receive appropriate treatment, while simultaneously allowing her to continue breastfeeding with minimal neonatal risk, will ensure optimal health outcomes for both the mother and baby.


  1. Australian Breastfeeding Association. Breastfeeding FAQS. Glen Iris: Australian Breastfeeding Association; 2014. Available from Accessed 4 Mar 2014.
  2. Spencer JP, Gonzalez LS, Barnhart DJ. Medications in the breast-feeding mother. Am Fam Physician 2001; 64(1): 119–26.
  3. Tutorial 12: Drugs in Breastfeeding. In: SHPA NSW Medicines Information Special Interest Group. Australian Medicines Information Training Workbook. 1st ed. Collingwood: Society of Hospital Pharmacists of Australia (SHPA); 2011.

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