Breast milk Δ9-tetrahydrocannabinol in cannabis users during the postpartum period: correlation between breast milk, maternal urine and saliva samples during early lactation.

Introduction: Cannabis use during pregnancy and the postpartum period has increased in recent years, raising clinical concerns regarding maternal and infant health, particularly during lactation. However, evidence regarding Δ9-THC concentrations in breast milk during the early postpartum period and their relationship with other biological matrices remains limited. Objective: This study aimed to assess Δ9-THC concentrations in breast milk and saliva, and 11-nor-9-carboxy-Δ9-tetrahydrocannabinol (THC-COOH) concentrations in urine, among postpartum women with cannabis use at the time of delivery. A secondary objective was to explore correlations between these biological matrices during early lactation. Methods: A longitudinal observational study was conducted at Vall d'Hebron University Hospital (Barcelona, Spain) between April 2022 and December 2023. Thirteen postpartum women aged over 18 years with a positive urine test for cannabis at delivery and intent to breastfeed were included. Saliva, urine, and breast milk samples were collected at 24 hours, 48 hours, and one week after birth. Δ9-THC concentrations in breast milk and saliva and THC-COOH concentrations in urine were analyzed using liquid chromatography-tandem mass spectrometry (LC-MS/MS). Results: Among participants who remained abstinent during the first postpartum week, urinary THC-COOH concentrations progressively decreased but remained quantifiable across all study stages. In contrast, Δ9-THC concentrations in breast milk decreased over time and were below the limit of quantification (LOQ) one week postpartum. Salivary Δ9-THC concentrations were generally low and frequently below the LOQ. Breast milk Δ9-THC concentrations at the first sampling stage were significantly correlated with salivary Δ9-THC and urinary THC-COOH concentrations, whereas no significant correlations were observed at later stages. Conclusions: This preliminary study suggests that Δ9-THC concentrations in breast milk may decline rapidly after postpartum cannabis cessation, becoming non-quantifiable within the first postpartum week among participants who discontinued use after delivery. In contrast, urinary THC-COOH remained quantifiable for a longer period. Salivary Δ9-THC showed limited concordance with breast milk Δ9-THC and should therefore be interpreted cautiously as a potential surrogate marker. Larger prospective studies are needed to confirm these findings and to support evidence-based breastfeeding counseling for women with recent cannabis use.