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.