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Evaluating Medications for the Lactating Woman

Thomas W. Hale, Ph.D.

from Breastfeeding Abstracts, November 2004, Vol. 24, No. 1, pp. 3 & 4.

Virtually all breastfeeding mothers will at some time require the use of a medication. In the early postpartum period, the use of medications may become a contentious issue, with physicians sometimes hesitant to allow mothers to breastfeed while using needed medications, and mothers resistant to discontinuing breastfeeding. Fortunately the average transfer of most medications into human milk is exceedingly low, and therefore mothers are sometimes justified in resisting suggestions to discontinue unless the reasoning of the physician is sound and based on good science.1 But how do health professionals evaluate the situation and make sound judgments concerning safety of the medication in each particular mother/infant? The data below should provide some useful guidelines.

Milk Production and Drugs

The clinical dose of medication transferred to the infant is largely a function of how much milk the infant ingests and the amount of medication present in the milk. Most mothers with young infants produce approximately 150 cc of milk per kilogram of infant weight per day. This amounts to about 700cc daily. The actual amount varies enormously by mother, and even from one week to the next. As the infant ages and with the institution of solid feedings, the actual rate of milk production normally declines to less than a few hundred cc per day at one year or later. Thus milk transfer is a highly variable function.

However, because the clinical dose of medication transferred to the infant is largely a function of how much milk the infant ingests, after 6 months and in those infants who start on solids, the older the infant, the relatively less medication they receive. In a 14-month-old breastfeeding baby, the volume of milk provided is often so low that the dose of maternal medication transferred to the infant is minimal to nil. So stage of lactation is an important factor in determining the risk of using various drugs in breastfeeding women. After about 6 months, the later the stage of lactation, the lower the dose of medication transferred to the infant. Also, the older infant the better it can metabolize and eliminate various drugs.

In the first 72 hours of milk production (colostral phase), there are numerous gaps between the lactocytes in the breast, and drugs and proteins may transfer into human milk relatively easily. However, the absolute amount of colostrum is generally so low (30-60 cc/day) that the relative dose of medication the infant receives is actually quite low during this period. It is only after milk production is high (600 cc/day) that one must begin to worry about the dose of medication transferred.

Relative Risk Factors to Infants

The risk to infants is solely determined by the amount of the drug transferred via the milk. While this is often difficult to determine with accuracy, we know that most infants ingest around 150 cc of milk per kilogram per day. With this estimate and the concentration of medication in human milk we can estimate rather closely the clinical dose transferred each day. While perhaps fewer than half of all medications have been studied in breastfeeding mothers, published studies are available for many popular drugs in breastfeeding mothers.2, 3 Unfortunately, none of this data is published in prescribing information or the PDR so a mother may have to search for this information. On average, less than 1-4% of the maternal dose transfers to the infant. Very few drugs exceed this level, and those few include the CNS-active drugs, the antidepressants, and a few other very lipid-soluble drugs. In general, we do not become overly concerned about a medication until its transfer to the infant approaches 10% or more of the maternal dose.4 Termed relative infant dose (RID), this figure is measured by dividing the infant’s dose via milk (mg/kg/day) by the mother’s dose (mg/kg/day). There are fortunately very few drugs that exceed this safety level of 10%. Metronidazole (Flagyl,which is really quite safe), atenolol, potassium iodide, lithium and a few others may exceed this 10% level. In general, if the RID of a medication is less than 10%, it is probably pretty safe to use.

Know your medication

It is important that the advisor understand just how toxic a medication is to the mother and infant, as this can directly affect the risk of breastfeeding. It should be obvious that mothers requiring hazardous medications such as anticancer drugs, methotrexate, certain antimanic drugs such as lithium, and others, should at least temporarily discontinue breastfeeding. Obviously, the more toxic a medication, the greater the degree of risk to the infant. Therefore, avoid taking a highly toxic medication while breastfeeding if at all possible, and avoid breastfeeding if a medication must be taken that is considered extremely toxic.

Drugs that have the potential to suppress the production of milk are also relatively contraindicated. Medications such as bromocriptine and cabergoline are potent inhibitors of milk production. Perhaps less active, but still important lactation suppressants include estrogens in birth control products5 and possibly pseudoephedrine in cold remedies.6 These should be avoided if possible.


Fortunately, the transfer of most medications into human milk is quite low. We have thousands of studies presently available which suggest that most medications can be safely used in breastfeeding mothers. Some caution is obviously needed with certain medications such as lithium, anticancer agents, and radioactive products, but the vast majority of medications transfer into milk in subclinical levels and pose little problem to a healthy breastfed infant.

A mother and her physician must always weigh the risk to the infant of the medication in milk against the enormous benefits offered by breastfeeding. Mothers should resist discontinuing breastfeeding until they have consulted with a physician who is versed in the safety of medications in breastfeeding mothers. Almost without exception, drug information inserts warn against the use of the medication in breastfeeding mothers. Pharmaceutical firms do not want the liability, so they universally warn patients to not breastfeed. But these recommendations are often not supported by research. By working together, the mother and her physician can almost always find a safe and suitable drug to treat her condition while maintaining this most important breastfeeding relationship.


  1. Hale, T. W. and K. F. Ilett. Drug therapy and breastfeeding. From theory to clinical practice. London: Parthenon Press, 2002
  2. Hale, T. W. Medications and mothers' milk. 11 th ed. Amarillo. TX: Pharmasoft, 2004.
  3. Briggs, G. G., R. K. Freeman and S. J. Yaffe. Drugs in pregnancy and lactation: A reference guide to fetal and neonatal risk. 4 th ed. Philadelphia, PA: Lippincott, Williams and Wilkins, 1994.
  4. Bennett, P. N. Use of the monographs on drugs. In: Drugs and Human Lactation. Amsterdam: Elsevier, 1996.
  5. Lonnerdal, B., E. Forsum,and L. Hambraeus. Effect of oral contraceptives on composition and volume of breast milk. Am J Clin Nutr. 1980; 33:816-24.
  6. Aljazaf, K., T. W. Hale, K. F. Ilett et al. Pseudoephedrine: Effects on milk production in women and estimation of infant exposure via breastmilk. Br J Clin Pharmacol. 2003; 56:18-24.
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