The hidden route: associations between maternal patterns of cannabis use and concentration of cannabinoids in breastmilk

Abstract Background Cannabis consumption among reproductive aged women is increasing in Canada, especially after its legalization. Despite its growing prevalence, limited empirical data exists to guide evidence-based clinical recommendations. The objective of this study is to examine how maternal cannabis consumption characteristics such as timing of use (pregnancy, lactation, or both), frequency, and mode of use are associated with cannabinoids concentrations in breastmilk. Methods A cohort of 181 lactating individuals self-reporting cannabis consumption or not during pregnancy and/or lactation were recruited. Concentrations of Δ⁹-tetrahydrocannabinol (Δ⁹-THC), 11-hydroxy-THC (11-OH-THC), 11-nor-9-carboxy-THC (THC-COOH), cannabidiol (CBD), and cannabinol (CBN) were measured in their breastmilk samples collected between 4 and 16 weeks postpartum. Participants completed a baseline survey assessing their sociodemographic characteristics, pregnancy information, and patterns of cannabis consumption. Results Δ⁹-THC showed the strongest and most consistent associations: frequency of consumption during lactation was associated with higher breastmilk Δ⁹-THC (frequent use, 58-fold higher; infrequent use, 6-fold higher), as was consumption during both pregnancy and lactation (34-fold higher) and inhalation-based or combined modes during lactation (smoking, vaping, and combined use). Frequent consumption during pregnancy was also associated with higher Δ⁹-THC, whereas timing within pregnancy and mode of use during pregnancy were not. THC-COOH was associated with frequent consumption and use across both pregnancy and lactation, and CBN with frequent consumption during lactation. No pattern of consumption was significantly associated with 11-OH-THC or CBD concentrations after correction for multiple comparisons. All comparisons are expressed relative to no cannabis consumption in the corresponding period, except those involving period of use, which are relative to consumption during pregnancy only. Conclusions This study found that frequent cannabis consumption during lactation, particularly via inhalation-based or combined modes, and continued use across both pregnancy and lactation were most strongly associated with higher breastmilk Δ⁹-THC concentrations. These findings highlight the relevance of postnatal, not only prenatal, cannabis consumption to infant exposure and support a cautious, person-centered approach to cannabis consumption during pregnancy and lactation; larger studies are needed to determine the implications for infant development.