How Medications Affect Breastmilk January 20, 2013 21:26

At the pharmacy, I get a lot of questions about medications while breastfeeding.  While it is always safest to avoid medications while breastfeeding, sometimes mothers are left without a choice if their condition puts their own health at risk.  Although many medications are safe to use when you're breastfeeding, most drugs will get into your milk to some degree and may even affect your milk supply. To be safe, check with your child's doctor before taking any kind of medication, even over-the-counter drugs.  The mechanism of how drugs enter breast milk is described below in addition to some general guidelines that I follow when counseling my patients.

Transfer of drugs into breast milk is influenced by protein binding, lipid solubility and ionization

This sounds pretty scientific but basically this means that nearly all drugs transfer into breast milk to some extent.

Notable exceptions are heparin and insulin which are too large to cross biological membranes. The infant almost invariably receives no benefit from this form of exposure and is considered to be an 'innocent bystander'.

Drug transfer from maternal plasma to milk is, with rare exceptions, by passive diffusion across biological membranes. Transfer is greatest in the presence of low maternal plasma protein binding and high lipid solubility.

In addition, milk is slightly more acidic than plasma (pH of milk is approximately 7.2 and plasma is 7.4) allowing weakly basic drugs to transfer more readily into breast milk and become trapped secondary to ionization.

What you should know is that milk composition varies within and between feeds and this may also affect transfer of drugs into breast milk. For example, milk at the end of a feed (hindmilk) contains considerably more fat than foremilk and may concentrate fat-soluble drugs.

As a general rule, maternal use of topical preparations such as creams, nasal sprays or inhalers would be expected to carry less risk to a breastfed infant than systemically administered drugs.

This is due to lower maternal concentrations and therefore lower transfer into breast milk.

However, the risk to the infant must be considered in relation to the toxicity of the drug used, the dosage regimen and the area of application. For example, use of corticosteroids nasal sprays or inhalers in standard doses would be considered compatible with breastfeeding.

Infants have lower drug clearance (elimination) than adults

Drug clearance in the infant is a particularly important consideration and premature infants have a severely limited ability to clear drugs.

Within a few days of delivery, term infants have kidney filtration rates approximately one-third of adult values after adjusting for difference in body surface area, and premature infants have even more impaired clearance.

Generally, adult kidney filtration rates (adjusted for the difference in surface area) are attained by five to six months of age.

Minimize risk to the breastfed infant by reducing drug exposure

The overall risk of a drug to a breastfed infant depends on the concentration in the infant's blood and the effects of the drug in the infant. If, after assessment of the risks and benefits, the decision is made to breastfeed while the mother is using a drug, the infant should be monitored for adverse effects such as failure to thrive, irritability and sedation.

However, it is difficult to identify adverse reactions occurring in neonates. 

Feeding immediately prior to a dose may help to minimize infant exposure as concentrations in milk are likely to be lowest towards the end of a dosing interval.  Or,it may be reasonable to reduce infant exposure by alternating breast and bottle-feeding. For drugs that are not considered safe in breastfeeding, breast milk may be expressed and discarded for the treatment duration. Breastfeeding may be resumed after the drug has been eliminated from the maternal blood stream. A period of approximately four half-lives (the time it takes for half of the drug to clear the body) will reduce maternal concentrations to around 10% of steady-state (full) concentrations.

Atkinson HC, Begg EJ, Darlow BA. Drugs in human milk. Clinical pharmacokinetic considerations. Clinical Pharmacokinetics 1988;14:217-40.

Bennett PN and the WHO Working Group, editors. Drugs and human lactation. 2nd edition. Amsterdam: Elsevier, 1997.

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