PARIS — (CNN) As attitudes about marijuana shift around the world, researchers are warning parents that it’s risky to keep it around children, especially those who are too young to know what it is.

The number of children who were admitted to emergency rooms for marijuana intoxication increased by 133% in France over an 11-year period, according to a new study.

Marijuana intoxication can occur when a child accidentally ingests a marijuana product or inhales marijuana smoke. Symptoms can vary based on the child’s age and size but often include sleepiness, difficulty breathing, seizures or even coma. Effects usually last six to 24 hours.

Cannabis is illegal in France, but it has the highest rate of marijuana use in Europe, said Dr. Isabelle Claudet, lead author of the study, published Monday in the journal Pediatrics.

“And that means we are facing an increase in emergency admissions of marijuana intoxication and an increase in severe symptoms seen in children,” said Claudet, a pediatric emergency physician in Toulouse.

She and other researchers analyzed the number of French children under 6 admitted to pediatric emergency departments because of unintentional cannabis intoxication and the number of cannabis-related calls involving children to French poison control centers.

From 2004 to 2014, 235 children were admitted to ERs with cannabis intoxication, and there was a 133% — or 13 times — increase in the admissions rate for it. The number of calls to poison control centers related to cannabis exposure in children increased by 312% in the same period.

What concerns Claudet the most is that the concentration of the psychoactive chemical tetrahydrocannabinol, or THC, has been increasing in cannabis products in France.

“THC concentration in cannabis products has increased from 9% in 2004 to 20% in 2014,” she said. “I believe that’s why we’re facing more adverse effects in children.”

Over the 11-year span, the severity of symptoms in children admitted to emergency departments because of marijuana intoxication also increased.

Twenty times more severe cases were reported in 2014 compared with 2004, and and four times more severe cases were reported in 2014 compared with 2013. Of the 32 children reported to have gone into comas, 53% were admitted in 2014, and there were more cannabis-related admissions than any other type of pediatric emergency room admission.

The main cannabis product circulating on the French market is hashish or cannabis resin, a solid and very concentrated form of cannabis extract, usually sold in the form of sticks or balls. Users break off pieces, roll them in tobacco papers and smoke them.

Claudet believes the best way to decrease the number of pediatric marijuana intoxication cases in France is to decrease the concentration of THC in cannabis through regulation.

“And we have to also warn consumers and parents that it could be very dangerous for children to eat such products,” she said. “Because usually, parents think it’s not very harmful because they’re smoking it, and it relaxes them. But if a child ingests one stick or ball, they can become comatose.”

What about children in the US?

Recreational marijuana is legal in eight US states and the District of Columbia, and medical marijuana is legal in 29 states and D.C. Cannabis product trends in the United States are quite different from those in France, though high concentration of THC is still a concern, especially in edible products.

“The results of this Pediatrics study also makes sense with what we’ve been seeing in Colorado. With the marijuana industry increasing, there’s a lot of new products, a lot of concentrated products, that people vape or ingest, like the edible products, that are high in THC,” said Dr. G. Sam Wang, a pediatric toxicologist at Children’s Hospital Colorado in Aurora.

“And so we’ve been seeing a lot of kids who are more symptomatic and more intoxicated,” said Wang, who was not involved in the French research.

Wang has conducted several studies of US trends in pediatric marijuana intoxication, as well as trends in Colorado.

He found that in states where recreational or medical marijuana is legal, the number of pediatric marijuana intoxication cases reported to poison control centers increased by 30% each year from 2005 to 2011. Children in states where marijuana was legal had more severe symptoms and were more likely to be admitted to a critical care unit compared with those in states where marijuana was not legal.

In his Colorado study, Wang found that in the two years leading up to when recreational marijuana became legally available for purchase in 2014 and the two years after, rates of marijuana exposure cases in children increased. On average each year, there was a 34% increase in calls to poison control centers about marijuana exposures in Colorado and a 19% increase across the US. Forty-eight percent of the cases in Colorado were attributed to the ingestion of an edible marijuana product.

Wang said the higher number of cases in states where marijuana is legal can in part be attributed to the increased availability of marijuana products and the attractiveness of edible product labels, which often look like normal candy or cakes to a child.

“Usually, kids get into things that become more available, and usually, that happens when it’s a household product, like those laundry detergent pods, which were attractive,” Wang said. “It’s kind of the same situation with marijuana, where we think in states with legal marijuana, probably more households have it in their home, especially the food or edible marijuana products with bright labels, that makes it easier for kids to get into them.”

The US National Organization for the Reform of Marijuana Laws, or NORML, says that it is concerned about the inadvertent ingestion of cannabis products by children in both the US and France but that more regulation could help provide a solution: “It is our contention that the frequency of these incidences can be mitigated by the imposition of stricter regulatory controls, such as more stringent and overt product labeling and by limiting such products to single serving sizes.”

Wang believes the key to ensuring the safety of children living around marijuana products is a combination of outreach and education. He tells parents to treat the marijuana like any other prescription or over-the-counter drug and keep it out of reach of children.

“Parents need to understand that kids can actually get sick from this stuff,” he said.

Several states with legal recreational marijuana, including Colorado, Oregon and Washington state, have also made child-resistant packaging a requirement for certain products.

Wang thinks this is also a step in the right direction, especially as other states or countries may be looking to legalize marijuana.

“They need to think about the unintended consequences of legalization, such as marijuana intoxication in children. So whether that’s education, packaging regulations, dose regulations,” he said.

“Colorado had some of that in the beginning of legalization, but we also made mistakes. But I’m hoping that other places can learn from those mistakes and implement programs or rules at the outset.”

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