(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 unintentional 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% 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?