Breastfeeding parents who use cannabis have detectable levels of tetrahydrocannabinol (THC) in their milk even after abstaining from the drug for more than 12 hours, new research has found. And while it’s unclear if the cannabinoid accumulates in levels that are sufficient to affect infants’ development, the study authors warn that it may be impossible to predict how long to wait after getting high before it’s safe to breastfeed.

The main psychoactive compound found in the cannabis plant, THC is associated with a range of negative outcomes when infants are exposed prenatally. However, little is known about how the molecule affects the composition of human breast milk.

To investigate, the researchers collected milk from 20 lactating mothers, all of whom used cannabis at least once a week and had babies under the age of 6 months. Samples were taken at five different timepoints after cannabis use, with a baseline sample provided following a period of at least 12 hours without getting stoned.

Surprisingly, the study authors found, “After abstaining from cannabis for at least 12 hours, milk [THC] concentrations were still measurable for all participants.” 

“These results indicate that infants consume measurable amounts of [THC] from breastfeeding, even after 12 hours of abstaining from cannabis,” they conclude.

Offering a possible explanation for this persistence, study author Courtney Meehan explained in a statement, "Human milk has compounds called lipids, and cannabinoids are lipophilic, meaning they dissolve in those lipids. This may mean that cannabinoids like THC tend to accumulate in milk – and potentially in infants who drink it.”

Further analysis of the data then revealed that there is no clear time-point at which THC levels peak following cannabis use. For mothers who only ingested the drug once during the study period, concentrations of the cannabinoid increased for between 30 minutes and 2.5 hours before receding.

Those who used weed multiple times during this period, meanwhile, saw a continual increase in the amount of THC in their breast milk.

“These results indicate that there is no clear window of time for mothers to breastfeed after cannabis use without some degree of THC exposure for the infant,” write the authors.

Unlike with alcohol, then, there appears to be no obvious point at which parents can be confident that the exogenous cannabinoids in their milk have returned to baseline. Typically, women are advised to wait at least two hours after consuming booze before breastfeeding, yet these findings indicate that similar guidelines cannot be applied to cannabis.

"There was such a range. If you're trying to avoid breastfeeding when the concentration of THC peaks, you're not going to know when THC is at its peak in the milk," said lead author Elizabeth Holdsworth.

Despite all of this, the researchers stress that the concentration of THC in breast milk remained low at all timepoints. Assuming babies drink 150 milliliters (5 ounces) of milk per kilogram of body weight per day, the authors calculate that their cannabinoid intake will always be below the minimum level associated with adverse effects.

However, they go on to explain that “no assumption of safety” should be inferred from this finding, since the impact of repeated low doses of THC in infants has not been properly studied.

Ultimately, then, Meehan insists, "Breastfeeding parents need to be aware that if they use cannabis, their infants are likely consuming cannabinoids via the milk they produce, and we do not know whether this has any effect on the developing infant."

The study is published in the journal Breastfeeding Medicine.