New study reveals cannabis THC stays in breast milk for six weeks

smoking weed and breastfeeding
Researchers say the findings support official doctors' recommendations to mothers to abstain from marijuana during pregnancy and while breastfeeding. Photo credit: Getty.

If you're a weed-smoking mother who uses the 'pump and dump' method, we've got bad news - it might not be as effective as you thought. 

A new study published this week in Jama Pediatrics has found that tetrahydrocannabinol (THC), the psychoactive component in cannabis, stays in breast milk for up to six weeks.

Researchers at Children's Hospital Colorado say the findings further support official doctors' recommendations to mothers to abstain from marijuana use during pregnancy and while breastfeeding. 

Weed was legalised in Colorado, where the study took place, in 2012. 

"With the increasing utilisation of marijuana in society as a whole, we are seeing more mothers who use marijuana during pregnancy," said Dr Erica Wymore, MD, MPH, the lead researcher behind the study. 

"However, given the lack of scientific data regarding how long THC persists in breast milk, it was challenging to provide mothers with a definitive answer regarding the safety of using marijuana while breastfeeding and simply 'pumping and dumping' until THC was no longer detectable in their milk. 

"With this study, we aimed to better understand this question by determining the amount and duration of THC excretion in breast milk among women with known prenatal marijuana use." 

The researchers studied seven women with prenatal marijuana use who delivered their babies at Children's Hospital Colorado between 2016 and 2019. All the women had an intention to breastfeed, and were willing to abstain from marijuana use for six weeks after delivery. 

The study found that while the concentrations of THC varied from woman to woman - due to level of use, BMI and metabolism - THC was excreted in the breast milk of all up to six weeks later.

This is the first study examining THC in breastmilk and plasma among women with known marijuana use in pregnancy since a 1982 study in the New England Journal of Medicine.