New mothers who smoke cannabis and breastfeed could be getting their babies high and affecting their brain development.
A study has revealed the chemical in marijuana which gives smokers a high can stay in women's breast milk for up to six days.
THC, which is the psychoactive part of cannabis, can stick to fat molecules in a woman's milk meaning it passes into the baby during breastfeeding.
And the experts who did the research say it is possible the drug could have an impact on an infant's brain development.
They recommend women quit using marijuana while breastfeeding to make sure their babies are not harmed.
The study by scientists at the University of California in San Diego tested breast milk samples of 50 women who smoked marijuana.
Of 54 samples taken, 34 of them – 63 per cent – still contained THC up to six days after the milk was produced.
The researchers explained THC can pass to breast milk because marijuana's active compounds like to bind to fat molecules, of which there are a lot in breast milk.
This stickiness suggests the compounds can end up in breast milk, raising concerns about the potential effects on nursing babies.
It's not known if the amounts of THC detected in the study pose any risk but researchers are studying children whose mothers' were involved to try to find out.
SAFE LEVELS FOR INFANTS UNKNOWN
Dr Christina Chambers, the lead researcher in the study, said: "We found the amount of THC that the infant could potentially ingest from breast milk was relatively low.
"But we still don't know enough about the drug to say whether or not there is a concern for the infant at any dose, or if there is a safe dosing level.
"The ingredients in marijuana products that are available today are thought to be much more potent than products available 20 or 30 years ago."
Two small studies from the 1980s had conflicting results on whether pot use affects breastfed infants.
One found no evidence of growth delays, as the other found slight developmental delays in breastfed infants, but their mothers had used pot during pregnancy too.
And while cannabis use is likely to be lower in the UK and Australia where it is outlawed, the drug is legal in nine US states and available on prescription in 31 states.
According to US government data, about one in 20 women report using marijuana during pregnancy.
Estimates for use among breastfeeding mothers vary, but a study in Colorado, where recreational marijuana is legal, put the number at almost 20 per cent among women in a government supplemental food program.
Experts said it is important to not risk women choosing marijuana instead of breast feeding.
Dr Chambers added: "We don't have strong, published data to support advising against use of marijuana while breastfeeding.
"If women feel they have to choose, we run the risk of them deciding to stop breastfeeding – something we know is hugely beneficial for both mum and baby."
The University of California's findings were published in the journal Pediatrics.