At present cannabis remains the most commonly used recreational drug among breastfeeding women. In the context of of changes to marijuana legislation in many US states and a previous study
that documented psychomotor effects breastfed infants a new study quantified cannabinoid concentrations in the milk of 54 nursing mothers after cannabis use.

  •  ∆9-THC was detectable in 34 (63%) up to ∼6 days after last reported use;
  • 5 samples had detectable levels of 11-hydroxy-Δ-9-tetrahydrocannabinol or cannabidiol
  • The sample with the highest concentration of cannabidiol (8.56 ng/mL) did not have measurable ∆9-THC.
  • Time since last use was a predictor of log ∆9-THC concentrations, as were the number of daily uses and time from collection to analysis

The American Academy of Pediatrics has now published a detailed clinical report report addressing the issue in its August 2018 of Pediatrics.