Women are being discouraged from using marijuana while breastfeeding, although more research is needed to truly understand the effects on infants. 

By Sarah Klein
August 27, 2018

Considering that marijuana is the most frequently used recreational drug among breastfeeding moms, experts know surprisingly little about what impact pot might have on nursing babies.

But a small new study published in the journal Pediatrics is helping to build on what we do know. The study, which analyzed breast milk samples from 50 mothers who used marijuana, found that the psychoactive part of weed, tetrahydrocannabinol (or THC), was detectable in breast milk up to six days after women last used the drug.

THC was found in 34 of those breast milk samples, or 63%. So how does it get there? "THC in marijuana is a very lipophillic (lipo: fat, phillic: loving) compound and gets deposited in the brain and fat tissues of the body," explains Sourabh Verma, MD, neonatologist at Hassenfeld Children's Hospital at NYU Langone. "It also has a low molecular weight. Both these properties combine to cause transfer of THC into human milk."

The concentration of THC found in breast milk is notably lower than what an adult would get through actively using marijuana, the study authors note, but it’s unknown how much of that THC (or other chemicals in marijuana) actually is absorbed by breastfed infants. "There is no information about how the amount transferred is related to the concentration of THC in the marijuana, the frequency of use, or the concentration in maternal plasma," explains Deborah Campbell, MD, chief of neonatology at Children's Hospital at Montefiore in the Bronx, New York.

But is marijuana exposure through breast milk safe at any level? Experts don't really know. “The extent of oral absorption in breastfeeding infants, metabolism and accumulation patterns, and pharmacologic effects of even low levels of cannabinoids … in infants are unknown and require further study,” wrote the study authors, a team of researchers from the University of California, San Diego.

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Most concerning is the question of brain development. Previous research examining marijuana use in pregnant women has suggested that prenatal exposure to pot may cause cognitive function problems in babies. "Growth of a newborn's brain is the highest during the first few months of life," Dr. Verma says. Therefore, the study authors wrote, “it is reasonable to speculate that [THC] exposure during breastfeeding, depending on the dose and timing, could influence normal brain development of a child.” 

Older research suggests that exposure to marijuana via breastfeeding during the first month of life was also linked to slower motor development, Dr. Campbell adds.

In the absence of hard facts, experts urge caution. “Present data are insufficient to assess the effects of exposure of infants to maternal marijuana use during breastfeeding. As a result, maternal marijuana use while breastfeeding is discouraged," states a new report covering marijuana use guidelines from the American Academy of Pediatrics (AAP), which were also published in Pediatrics.

Without any evidence that smoking or ingesting marijuana while lactating is safe or harmful, new moms and moms-to-be "need to be informed about the lack of definitive research and counseled about the current concerns regarding potential adverse effects of THC use on the woman and on fetal, infant, and child development,” the AAP report continues. 

This is in line with last year's guidance from the American College of Obstetricians and Gynecologists.“There are insufficient data to evaluate the effects of marijuana use on infants during lactation and breastfeeding, and in the absence of such data, marijuana use is discouraged,” an ACOG committee opinion update concluded in October 2017.

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Ultimately, both the UCSD researchers and the AAP are calling for more research on this topic, which will only grow more important as more states legalize medical and recreational marijuana use. “Because marijuana is the most commonly used recreational drug among breastfeeding women," the UCSD study authors wrote, "information regarding risks to breastfeeding infants is urgently needed.”

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