Objective:

To assess Δ9-THC concentrations in breast milk and saliva, and THC-COOH concentrations in urine among postpartum women with cannabis use at the time of delivery, and to explore correlations between these biological matrices during early lactation.

Approach:

• Study Design: A longitudinal observational study conducted at Vall d’Hebron University Hospital in Barcelona, Spain, from April 2022 to December 2023.
• Participants: Thirteen postpartum women aged over 18 years with a positive urine test for cannabis at delivery and intent to breastfeed.
• Sample Collection: Saliva, urine, and breast milk samples were collected at 24 hours, 48 hours, and one week after birth.
• Analysis Method: Δ9-THC and THC-COOH concentrations were analyzed using liquid chromatography–tandem mass spectrometry (LC-MS/MS).

Key Findings:

• Urinary THC-COOH concentrations decreased progressively but remained quantifiable across all study stages among participants who abstained from cannabis.
• Δ9-THC concentrations in breast milk decreased over time and were below the limit of quantification one week postpartum.
• Salivary Δ9-THC concentrations were generally low and frequently below the limit of quantification.
• Breast milk Δ9-THC concentrations at the first sampling stage were significantly correlated with salivary Δ9-THC and urinary THC-COOH concentrations.

Interpretation:

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.

Limitations:

• Small sample size of thirteen participants.
• Short duration of the study may not capture long-term trends.
• Limited generalizability due to specific population studied.

Conclusion:

Larger prospective studies are needed to confirm these findings.