Breastfeeding is the biologically superior way to feed an infant. However, harmful substances in the mother’s blood can transfer to and accumulate in her breast milk. A recent study found that tetrahydrocannabinol (THC), the primary psychoactive ingredient in cannabis, accumulates in breast milk after a single use, persisting for several hours.

The study included 20 healthy young women who were breastfeeding their infants and used cannabis at least once weekly. Participants collected samples of their breast milk after abstaining from cannabis use for at least 12 hours and at several intervals over an eight- to 12-hour period after use.

Researchers found that participants' breast milk contained THC even after 12 hours of abstention. THC concentrations peaked about two hours after cannabis use; however, in some cases, concentrations peaked as early as half an hour after use. Concentrations were higher with multiple uses and remained elevated throughout the 12-hour study period. On average, infants received approximately 16.2 micrograms per kilogram of body weight of THC over 12 hours.

These findings suggest that THC accumulates in breast milk after a single use of cannabis. The effects of THC on the developing infant brain are unclear. However, research on rodents has identified lasting, mild effects on memory and learning following a single, extremely low dose of THC equivalent to 0.2 micrograms per kilogram in humans – far less than the average dose the infants in this study received. Other harmful compounds, including nicotine, caffeine, and heavy metals, can pass into breast milk, too. Learn more about breast milk and breastfeeding in our overview article.

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