A small study looked at how much THC could be passed on to infants.

Breastfeeding women who smoke marijuana transfer low levels of cannabis’ main psychoactive ingredient, THC, to their children via breast milk, according to a 2018 study published in the journal Obstetrics and Gynecology.

Researchers took samples of breast milk from eight anonymous test subjects who regularly used cannabis, and tested the milk for the presence of delta-9-tetrahydrocannabinol (THC) and its metabolites.

The milk was tested 20 minutes after the study subjects smoked marijuana and then at one, two, and four hours post-use. THC levels were highest at one hour after study subjects smoked cannabis.

Infants who breastfed exclusively ingested an estimated 2.5 percent of the maternal dose of THC, the study found. That translated to an estimated daily infant dose of 8 micrograms of THC per kilogram per day, according to the study’s findings.

The infants in the study ranged in age from 3 to 5 months old.

“The levels are low in the milk, and even less would be absorbed by the infants,” study author Tom Hale, PhD, professor of pediatrics at the Texas Tech University School of Medicine, executive director of the school’s InfantRisk Center, told Healthline.

THC affects the central nervous system for about two hours, and typically takes 20 to 36 hours to be eliminated from the body.

While the researchers in this study were able to measure the THC in breast milk, they were unable to collect blood samples from the infants to see if they had measurable levels of THC in their bodies.

Each study subject smoked 0.1 grams of cannabis, containing a 23.18 percent concentration of THC.

“This dose was chosen after extensively reviewing older studies wherein an average cannabis cigarette contained approximately 0.6 g of cannabis, containing approximately 3.55 percent [THC] tetrahydrocannabinol,” the study noted.

All of the women in the study used cannabis obtained from the same legal medical marijuana dispensary in Colorado.

“The study was carefully controlled: We knew exactly what they smoked and when they smoked it,” said Hale. He noted that set the study apart from earlier research, where dosage was uncontrolled.

While THC was measured in the breast milk, it’s not clear what this could mean for breastfeeding infants.

“It remains unclear what exposure to cannabis products during this critical neurobehavioral development period will mean for the infant,” the study concluded.

However, Hale’s message to cannabis-smoking mothers is the same as it would be to women who smoke tobacco while breastfeeding: Don’t do it.

“While it’s legal to use cannabis in Colorado, it’s not legal if you’re pregnant or breastfeeding,” said Hale. For example, if a blood test reveals any level of THC in a child, the treating physician is obligated under state law to inform child protective services.

“It’s still a reportable offense,” said study co-author Dr. Teresa Baker, a practicing OB-GYN at Texas Tech University School of Medicine.

The study noted that there is increased concern among healthcare professionals as more states have legalized medical marijuana. Therapeutic use of the drug is now legal in 34 states as well as the District of Columbia, Guam, Puerto Rico, and the U.S. Virgin Islands.

In the past, the drug’s legal status made studies like Hale’s difficult or impossible to conduct.

Now that marijuana has become widely legalized, marijuana users like those in Hale’s study have become more accessible, and researchers are finally able to see exactly how the drug affects people.

“We are still lacking so much fundamental knowledge on the basic clinical effects of cannabis use, which is so at odds with the drive to legalize the drug,” Dr. Harshal Kirane director of Addiction Services at Staten Island University Hospital, told Healthline.

“There’s growing evidence of potential harmful or dangerous aspects of cannabis use that people need to be informed about so they can make decisions about its use,” he said.

Studies in the past decade have greatly expanded knowledge about the endocannabinoid system and its role in healthy development.

“Long-term microdose exposure of THC could disrupt that development,” said Kirane. “I think that’s a real possibility.”

Additionally, prior research has identified a two- to threefold increase in risk of later psychiatric disorders associated with cannabis use prior to puberty, said Kirane.

“The [child] dosage seen in this study compared to an adult is small, but what impact it has on the developing brain is very unclear,” he said.

Editor’s note: This story was originally reported by Robert Curley on April 9, 2018. Its current publication date reflects an update, which includes a medical review by Karen Gill, MD.