Breastfeeding: Choosing a medicine

Last updated: Monday, December 11, 2023

The evidence to support the use of individual medicines in breastfeeding mothers is usually based on data of limited quantity and quality. Nonetheless, most medicines are considered compatible with breastfeeding, although there are some exceptions. As described on the previous page, advising a woman not to breastfeed is not a ‘no risk’ option because of the proven benefits over formula feeding.

Almost all medicines pass into breast milk, but the extent to which this occurs varies, as does the ability of a medicine to harm the neonate or affect lactation. It is therefore important to understand the factors that impact on these processes. There may be unknown mechanisms that influence transfer into breast milk, but known factors include:

Maternal plasma concentration

Drugs with a low maternal plasma concentration are less likely to pose a problem to breastfed infants. Some drugs are not absorbed from the maternal gut at all (e.g. nystatin, Fybogel) or are absorbed very poorly (e.g. oral vancomycin). Others have short half-lives, are cleared very quickly, and are therefore less likely to accumulate in breast milk (e.g. epoprostenol, lidocaine, ibuprofen).

Molecular weight of the drug molecule

Medicines may be more likely to transfer into breast milk if they have a low molecular weight. 

Protein binding

Drugs that are highly protein bound are less likely to pass from the bloodstream into breast milk. Highly protein bound drugs include warfarin and propranolol.

Acid-base balance

Breast milk is slightly acidic compared to blood. Drugs that are weak bases can become ionised in milk, this makes them more water soluble and so less able to diffuse back out into blood, which can result in accumulation within milk. Examples of weak bases include morphine and tramadol. Conversely, weak acids tend not to accumulate in milk such as ibuprofen.

Fat solubility

Lipophilic drugs preferentially dissolve in the fatty globules in milk, although fat is only a small proportion of the overall milk volume. More importantly, lipophilicity allows passage across the lipid alveolar epithelium of the breast, whereas passage of water-soluble drugs is inhibited by this membrane. Lipophilic drugs include benzodiazepines and amitriptyline.

Absorption from infant’s gut

Many non-oral drugs given to a mother enter her milk, but due to their physicochemical properties are not significantly absorbed from the infant’s gut or are broken down within it. Examples include gentamicin, dopamine and insulin.

Lactation inhibition

Drugs that can inhibit milk production may make breastfeeding difficult or impossible by interfering with hormonal mechanisms or fluid balance. Examples include bromocriptine, high-dose diuretics, anabolic steroids, some antipsychotics, and moderate/heavy regular alcohol intake. However, drugs at normal medicinal doses may be less likely to interfere with lactation once it is established, which occurs around 6-8 weeks post-partum.

Toxicity of the drug

Some drugs are potentially very toxic to babies, even in small amounts, and must be avoided when breastfeeding (e.g. some cytotoxic agents, cocaine). Others are natural to the body and unlikely to be harmful unless given in huge doses (e.g. iron, potassium, vitamin C). Some drugs are commonly administered uneventfully to full-term neonates in doses much bigger than they could be exposed to via breast milk (e.g. penicillins, aciclovir, fluconazole). In cases where a drug with potential neonatal side effects is still used whilst breastfeeding, infants should be monitored and/or alternatives with lower milk transfer chosen if possible.

It is not known whether drugs affecting CNS neurotransmission can cause behavioural or emotional problems in the exposed neonate in later life (e.g. antidepressants, antipsychotics, antiepileptics).

Neonatal clearance

If the newborn has already been exposed to a drug in pregnancy without ill effects, this may give healthcare professionals greater confidence in encouraging the mother to breastfeed. However, during pregnancy the mother clears all drugs from the infant's circulation; after delivery drug elimination relies solely on the neonate's clearance mechanisms. Therefore, even if a newborn has already been exposed to a drug during pregnancy without ill effect, this is not an indicator of the drug’s safety during breastfeeding.

Neonatal kidney and liver function is not optimal at birth, and drugs given via breast milk may accumulate. This is of special concern where drugs have CNS depressant effects (e.g. opioids). This risk of accumulation is greater in premature infants or in those with kidney or liver disease, particularly if the drug has a long half-life as well (e.g. fluoxetine, some antipsychotics). Even in a healthy, full-term infant it might be better to select a shorter-acting medicine for use during breastfeeding if you have a choice.