The American Academy of Pediatrics opens its 2013 statement on The Transfer of Drugs and Therapeutics into Human Milk with an explanation for why physicians might tell lactating women not to breastfeed while on medication.
First, there may just not be any evidence – positive or negative – about the effects of a particular drug on human milk. It's not hard to understand this, as it would be difficult to run ethical studies if there is any doubt about the potential harm of a drug to an infant. Additionally, the drug studies that physicians do base their breastfeeding recommendations on often come from animal studies, which only imperfectly apply to humans.
The AAP asserts that physicians should consider a host of factors when prescribing medications to lactating women, which include "the need for the drug by the mother, the potential effects of the drug on milk production, the amount of the drug excreted into human milk, the extent of oral absorption by the breastfeeding infant, and potential adverse effects on the breastfeeding infant."
Given all of the factors to consider when prescribing a medication, it's no wonder there can be disagreement on whether any particular medication is safe, or why a woman's physician advises her to stop breastfeeding while her midwife or lactation consultant says it's okay.
When looking for drug information, new mothers might question whether a drug is "safe or unsafe" for breastfeeding. That question, however, is an oversimplification. Furthermore, given the vast array of medications available and each individual physician's experience, the "safe or unsafe" question might be answered differently by different people. When making the choice to take a medication and pause or discontinue breastfeeding, nursing mothers might do well to think less in terms of absolutes and more in terms of concentration.
In the case of nursing mothers, dosing is especially complicated. The prescribing physician has to consider not just the right dose for the mother, but also a safe dose for the child who may receive some of that drug through the mother's breast milk.
To make calculations about how much of any particular drug will be present in breast milk, the physician has to consider how that drug travels to different parts of the body. In pharmacology, this is called distribution. Some medications are water-soluble, meaning that they dissolve in water, while other medications are fat-soluble.
The solubility of a drug determines how that drug will be distributed throughout the body. Alcohol, for example, is water-soluble. Furthermore, it dissolves quickly in water, which is why it moves so rapidly from the intestines to the bloodstream, and from the bloodstream throughout the body. Alcohol's solubility is part of the reason it is found only in trace amounts in breast milk and generally regarded as safe to drink while breastfeeding.
Many classes of pain medications are also water-soluble. Women who have had C-sections are often given morphine, also a water-soluble drug, because it appears only in trace amounts in breast milk.
Medications are said to be "lipophilic" when they have a tendency to concentrate in fat. Because breast milk contains a high proportion of fat, it will also contain a high proportion of a drug that dissolves in fat. Therefore, fat-soluble medications are often prescribed with caution.
SSRIs, which are prescribed for anxiety and depression, are lipophilic and have relatively long half-lives, meaning that they both dissolve in fat and take longer to metabolize. Although the infant exposure to SSRIs through breast milk is still relatively low, there have not been long-term studies of this exposure, so women who need these drugs may be discouraged from breastfeeding.
The solubility of a medication is just one of a host of factors physicians need to consider when making a recommendation about breastfeeding while on that medication. New drugs are introduced all the time, and drugs that were previously thought to be safe for mothers to take while breastfeeding are sometimes found to be unsafe (codeine is one such example).
LactMed, a database maintained by the U.S. National Library of Medicine, is regularly updated with new drug information and can be used to help evaluate whether a particular medication is safe for a nursing infant.
But the concentration of a drug in the mother's milk is only one way to think about concentration. It's also important for prescribing physicians to consider "concentration" as a mental state. Narcotics like morphine can interfere with awareness, so even if they won't pass into the bloodstream, they may temporarily incapacitate the mother.
While that mother's milk may be completely safe for the infant, that mother's diminished concentration may make it less safe for her to be holding – and therefore nursing – the baby. A more conservative approach might be to feed a baby expressed milk or formula until the medication wears off.
In other cases, physicians might be weighing the mother's ability to safely care for her baby against her ability to nurse that baby. This is often the consideration when prescribing antidepressants which may require cessation of breastfeeding.
Although women should feel comfortable asking questions about alternative medications that may make it easier to continue breastfeeding, it's worth noting that – at a time when everything now appears to revolve around the baby – the mother's doctor will focus on the mother. Because of this, a physician may make recommendations about the mother's health that may be momentarily at odds with her plans to breastfeed.
There are rare times when the best decision may be to pause breastfeeding or stop breastfeeding altogether. At those times, it's important to consider one last form of concentration. Infant formula is also a "concentration," a combination of proteins, fats, and carbohydrates.
The decision to use formula while on a potent medication can feel like a failure to many mothers, who are flooded with messages about the importance of breastfeeding. But if short-term formula use can ensure a long-term benefit for the mother, that mother can be a healthy parent to her baby for life.