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March 22, 2021 -- Tetrahydrocannabinol, or THC, the main psychoactive component of marijuana, remains detectable in breast milk for weeks,. new data shows.

The estimated half-life of THC in breast milk is 17 days, according to the study results, and could remain for more than 6 weeks.

"To limit THC effects on fetal brain development and promote safe breastfeeding, it is critical to emphasize marijuana abstention both early in pregnancy and post-partum," Erica M. Wymore, MD, , an assistant professor of pediatrics and neonatology at the University of Colorado in Aurora, and colleagues wrote. The group published their results online March 8 in JAMA Pediatrics.

Wymore told Medscape the detectable levels of THC suggest any use is of concern. "We wish we had more data on the potential effects on the neurocognitive development of children, but for now we must discourage any use in pre-pregnancy, pregnancy, and breastfeeding, as our national guidelines recommend," she said.

The findings support current guidelines from national organizations, including those from the American Academy of Pediatrics, the American College of Obstetricians and Gynecologists, and the Academy of Breastfeeding Medicine. All say new mothers and mothers-to-be should not use marijuana.

Many women use marijuana during pregnancy. That tendency and the lasting presence of THC in breast milk led the authors to question whether women who use marijuana should simply discard their breast milk until THC is cleared.

"These findings make the recommendations for mothers to discard breastmilk until THC is undetectable unrealistic for mothers committed to breastfeeding,” they wrote.

However, not all experts are equally concerned. Neonatal pharmacologist Thomas R. Hale, PhD, a professor of pediatrics at Texas Tech University School of Medicine in Lubbock, said a previous study by his group showed that THC levels in breast milk peaked within 60 minutes of a moderate dose of inhaled marijuana and fell to quite low levels over the next 4 hours.

"So you can see that just because a mom is drug screen-positive, the clinical dose transferred to the infant is probably exceedingly low," he told Medscape.

Hale also stressed that judgments about drugs in this context should weigh the risk of the drug against the risk of not breastfeeding. "All of us caution women not to use cannabis when pregnant or breastfeeding," Hale said. "But when the decision has to be made as to whether a mom breastfeeds or not if she is drug screen–positive, a lot of other factors must be analyzed to make such a decision."

Study Cohort

For the study, Wymore and colleagues screened 394 women who gave birth between Nov. 1, 2016 and June 30, 2019. Of those, 25 women, with a median age of 26 years enrolled.

Of the 25 enrolled mothers, 12 had not used marijuana while pregnant or nursing.. Those who continued to use marijuana were younger than the overall sample, with a median age of 21, and were less likely to have attended college (23%) than abstainers (58%).

Hale noted that although THC was detectable in milk, the levels were exceedingly low. "This is where the risk assessment comes in. There's a lot of hysteria in the cannabis field right now, and we're going to need time and a lot more studies to really be able to predict any untoward complications."

Wymore, however, countered that THC levels were low only in those who abstained and that her concerns are not just about breastmilk levels but the health effects on children over the course of prepregnancy, pregnancy, and lactation. "[Hale's] message makes it difficult for clinicians to counsel mothers since it goes against national guidelines," she said. "We need to be consistent."

But Wymore and other experts acknowledge the dilemma to be faced in that breast milk clearly offers substantial benefits for infant and child health. "The risks of an infant's exposure to marijuana versus the benefit of breast milk must be considered," said Amy B. Hair, MD, assistant professor of pediatrics and neonatal medicine at Baylor College of Medicine in Houston, who was not involved in the Colorado study. "And it's unrealistic, as the study suggests, for mothers to discard breast milk for 6 weeks."

Nevertheless, calling the study findings "troublesome," Hair said the legalization of marijuana in some states gives the public the impression it's safe to use marijuana even during pregnancy and while nursing.

"Research studies, however, are concerning for potential detrimental effects on brain growth and development in infants whose mothers use marijuana during pregnancy and breastfeeding," she said.

Wymore stressed that more U.S. marijuana dispensaries counsel women on the potential harms during pregnancy. This is the case in Canada, she noted, where recreational and medicinal marijuana has been legal since 2018 and more than 90% of outlets (vs two thirds of their U.S. counterparts) advise women not to use marijuana during pregnancy or lactation, even for nausea.

"This is where many women are getting their information on cannabis," she said. "We learned the hard way with alcohol and we don't want to make the same mistake with marijuana."