Breastfeeding has positive health consequences for both the breastfed infant and the nursing mother.(1,2) Although information on drug use during lactation is available through sites such as LactMed,(3) available information is often incomplete. Unlike pregnancy, in which large numbers of pregnant women need to be studied to assure safety, measurement of drug concentrations in breastmilk in a relatively few subjects can provide valuable information to assess drug safety. This article reviews methods of measuring and predicting drug passage into breastmilk.

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Medication use during breastfeeding shortens the duration of breastfeeding often because of overly cautious information given by healthcare providers. No comprehensive review of the literature on infant adverse reactions from drugs in breastmilk has been published. All published studies and case reports on adverse events in infants caused by medications (excluding drugs of abuse) in breastmilk were identified and analyzed. Of 100 case reports evaluated, none were considered to be "definite" using a standard ranking scale; 47% were "probable" and 53% were "possible." Drugs with central nervous system activity accounted for half of all reports. All 3 reported fatalities involved central nervous system depressants, but each had extenuating circumstances. At least 63% of reported cases were in neonates and 78% were in infants 2 months or younger; only 4% of adverse reactions occurred in infants older than 6 months of age. Published studies expand on and generally reinforce the analysis of case reports. By taking a few simple precautions in drug selection and considering the infant's age, breastfeeding rarely needs to be discouraged or discontinued when a mother needs drug therapy.

These data do not suggest that women who breastfeed while taking fluoxetine are likely to note unusual behavior in their infants that they consider related to use of the medication. However, although there was no excess of infants in the fluoxetine group with postnatal weight measurements >2 standard deviations below the mean, these data indicate that breastfeeding while taking fluoxetine is associated with reduced growth that may be of clinical importance in situations in which infant weight gain is already of concern.

Use of plant and drug products to enhance lactation is widespread, and numerous papers have been published in the medical literature claiming efficacy for various products. This paper will review and evaluate the published literature on the most widely used pharmaceuticals that are used as galactagogues. Breastfeeding physiology is reviewed with the aim of creating a framework for understanding galactagogue pharmacology. Published articles were selected and evaluated using the principles of evidence-based medicine, and were also evaluated using the principles of good lactation management. Only three studies on oxytocin and seven studies on dopamine antagonists were found to be useful. Oxytocin is probably not useful as a galactagogue, except possibly in rare circumstances of tetraplegic mothers. Dopamine antagonists appear not to enhance milk supply if mothers are given good lactation support and employ these practices. The safety of the dopamine antagonists has not been adequately evaluated, so their use should be avoided unless other measures have failed.

The types and rate of adverse drug reactions experienced by breastfed infants whose mothers are taking medications has not been well defined. This article reviews the literature on adverse drug reactions in infants since a previous review in 2002. Case reports and studies of adverse drug reactions in breastfed infants whose mothers were taking a prescribed or over-the-counter medication were selected. Fifty-three case reports and 16 studies were located. Serious acute adverse drug reactions from drugs in breastmilk appear to be uncommon. Infants under 2 months of age, and especially those under 1 month, appear to be most susceptible. Similar to previous reviews, free iodine, opioids, and the use of multiple central nervous system drugs simultaneously were identified as drugs of concern. A few narrowly focused studies are now available on long-term effects of maternal drug therapy on breastfed infants and they are mostly reassuring.

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More than half of women take medications during breastfeeding, predisposing their infants to medication exposure via breast milk. As a result, adverse drug reactions may emerge in the infant, although they are rarely reported. Disposition of maternal drugs in breast milk is described with several key parameters, which include relative infant dose (RID): infant drug intake via milk (weight‐ and time‐adjusted) expressed as a percentage of the similarly adjusted mother's dose. Most drugs show RID values of <10%, indicating that drug concentrations in infant serum do not reach a level known to be therapeutic in adults unless drug clearance is markedly lower than the adult level on a weight basis. RID is a function of milk‐to‐(maternal) plasma drug concentration ratio (MP ratio) and maternal drug clearance. Therefore, MP ratio between drugs must be interpreted not by itself but with maternal drug clearance of each drug. This is why some drugs such as phenobarbital show an MP ratio of <1 but an RID as high as 50–70%, while morphine shows an MP ratio of 2 but an RID in the range of 5%. Using RID, we interpreted case reports of infant adverse outcomes, and we observed cases with relatively low infant serum concentrations of drug, consistent with low RID, as well as those with near‐ or above‐adult therapeutic serum concentrations, with or without increased drug intake (i.e. high RID). It is important to consider both pharmacokinetic and pharmacodynamic factors in interpreting adverse outcomes in infants breastfed by a mother taking medications.