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Marijuana is the most commonly used recreational drug in pregnant and breast-feeding women; however, the neurobehavioral risks to infants exposed to cannabis in breast milk remain unknown. In a study involving 50 breast-feeding mothers who reported marijuana use, researchers in California measured cannabinoid concentrations in 54 samples of breast milk to determine the relation between the mother's most recent use of marijuana and the potential dosage to her infant. Thirty-two participants (64%) used inhalation as the only means of administration; 44 reported at least daily use.
Cannabis metabolites were found in all samples, with higher concentrations in those from women who exclusively used inhalation. The psychoactive ingredient in marijuana, Δ-9-tetrahydrocannabinol (Δ-9-THC), was detected in 34 samples (63%). There was a strong correlation between Δ-9-THC concentrations in breast milk and hours since last marijuana use. The half-life of Δ-9-THC was estimated to be 27 hours, implying that clearing the drug from human milk could take about 6 days (5 half-lives).
Bertrand KA et al. Marijuana use by breastfeeding mothers and cannabinoid concentrations in breast milk. Pediatrics 2018 Sep; 142:e20181076. (https://doi.org/10.1542/peds.2018-1076)
Comment
Cannabis metabolites are highly lipophilic, so it makes sense that they would be found in the fat of human milk — and probably also in the fat-rich infantile brain. Accordingly, the potential effects of these metabolites on the developing infant brain are of the utmost concern. The next step will be to measure concentrations of cannabinoid metabolites in infant plasma and correlate these levels with developmental outcomes. It's still most prudent to urge pregnant and breast-feeding women to abstain from inhaling or ingesting any marijuana or cannabis products.