Abstract Background Information on cannabinoids in breast milk and maternal cannabis use is limited. We quantified cannabinoids in plasma and breast milk of breastfeeding mothers and assessed cannabis use patterns. Methods This is a prospective study at a university hospital in a state with legal medical and recreational cannabis. Breast milk and plasma samples along with survey data were collected from volunteers using cannabis in the last 48 h at 2 weeks and 2 months postpartum. Results Twenty subjects were enrolled. Median age (IQR) was 27 (24–34) years. Median (IQR) instances of cannabis use in the last 7 days were visit 1: 17 (6–29) and...