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Delta-9-tetrahydrocannabinol (THC) persisted for weeks in breast milk when mothers used marijuana during pregnancy, even after successfully abstaining, researchers found.

Among 12 women who used during pregnancy and then stopped after delivery, everyone showed detectable levels of THC in their breast milk lasting 6 weeks, with median concentration during the first week postpartum of 3.2 ng/mL (interquartile range 1.2-6.8, maximum 26.14), reported Erica Wymore, MD, MPH, of the University of Colorado School of Medicine in Aurora, and colleagues in a research letter.

With samples taken two to five times weekly, median levels of THC were 5.5 ng/mL at postpartum week 2, then declined over 6 weeks. At that point, the median was 1.9 ng/mL and was detectable in all participants.

Wymore and colleagues calculated the mean elimination half-life at 17 days (SD 3) for seven participants who abstained for at least 5 weeks.

In all, the researchers recruited 25 women who had used during pregnancy and said they planned to stop once their babies were born; but 13 did not follow through on that intention.

"We report challenges in abstention and prolonged excretion of THC in breast milk greater than 6 weeks among women with prenatal marijuana use," Wymore and colleagues wrote.

The findings make previous recommendations to discard breast milk until THC is no longer detected "unrealistic for mothers committed to breastfeeding," the researchers added. They didn't say so explicitly, but that would appear to leave bottle-feeding as the only safe option.

The difficulties in abstention noted in this study may be multifactorial, they said. Potential reasons for this challenge include the use of marijuana as self-medication for disorders, normalization of use due to legalization, and suboptimal resources to treat cannabis use disorder.

Daniel Corsi, PhD, a perinatal epidemiologist at the University of Ottawa who was not involved in this research, said that the findings bring attention to the lipophilic nature of THC. The chemical will concentrate in breast milk, he added, which is high in fatty acids.

"It seems that cannabinoids will continue to transfer to breastmilk even after maternal discontinuation," Corsi told 番茄社区app in an email. "This finding is essential to communicate clinically to women who may be using cannabis in pregnancy."

THC, the marijuana component responsible for its euphoric effect, is known to cross the placenta; previous studies showed that it enters breast milk as well. The chemical is stored in body fat, and is slowly released over time. While it's recommended that individuals abstain from marijuana use during pregnancy and lactation, it's unclear how long THC remains detectable in breast milk after cessation.

Wymore and colleagues conducted a prospective, observational, pharmacokinetic study to help fill that gap. They recruited women who used marijuana during pregnancy and presented for delivery from June 2016 to November 2019. Participants were included in the analysis if they expressed intent to breastfeed and abstained from marijuana use for the 6-week study period.

Participants self-reported details on their substance use patterns. Inhalational consumption was most common, and patients reported consuming marijuana more than twice a week. Participants who reported continued marijuana use during the study were younger than the overall sample (median age 21 vs 26 overall), and less likely to have gone to college than patients who succeeded in abstaining.

One limitation cited by the authors was that they couldn't quantify participants' original THC intake.

Further research is necessary to understand how other forms of marijuana consumption affect THC levels in breast milk, the group indicated, as well as their effects on child health outcomes. But ceasing use, they stated, is important for patients who are expecting.

"To limit THC effects on fetal brain development and promote safe breastfeeding, it is critical to emphasize marijuana abstention both early in pregnancy and postpartum," they wrote.

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