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The number of teenagers sent to emergency rooms more than quadrupled after marijuana was legalized in Colorado — mostly for mental health symptoms, researchers reported Thursday.

They found 639 teenagers who went to one hospital system in Colorado in 2015 had either cannabis in their urine or told a doctor they’d been using cannabis. That’s up from 146 in 2005, before the use of marijuana was legalized in Colorado.

The number of teens visiting Colorado ERs with cannabis in their system rose after the state legalized marijuana use. Lucy Nicholson / Reuters

"The state-level effect of marijuana legalization on adolescent use has only begun to be evaluated," said the University of Colorado's Dr. George Sam Wang, who led the study.

Wang said people believe marijuana is safe — but it is not.

"The perception of risk has gone down quite a bit," Wang told NBC News.

"We know that marijuana use at a young age can affect adolescent brains."

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Colorado legalized medical marijuana use in 2010 and made the recreational use of marijuana legal in 2014. Other studies have shown that ER visits involving marijuana use went up after those dates, especially among children and tourists visiting Colorado.

Cannabis is legal for medical use in 28 states and Washington, D.C., and eight states plus D.C. have legalized the recreational use of marijuana and marijuana-derived products.

Wang and colleagues looked at the university hospital’s emergency department and urgent care records between 2005 and 2015 for the study, presented to a meeting of the Pediatric Academic Societies in San Francisco.

Related: Marijuana Users Risk Schizophrenia

They found 3,443 ER visits by 13- to 21-year-olds to six ERs and urgent care centers over that time. Two-thirds involved psychiatric symptoms, Wang said. More than half also had evidence of other drugs in their systems as well, including alcohol, amphetamines and cocaine.

A federal survey published late last year found 6 percent of high school seniors are using marijuana daily. Wang said it’s important to find out just how harmful it is to them.

Wang has also studied the effects of legalization on younger kids, who are more likely to get hold of marijuana by accident.

Related: Fewer Teens Drink or Use Drugs

In 2016 Wang found that the average rate of marijuana-related visits to the children’s hospital doubled after legalization. Poison center calls about marijuana went from nine in 2009 to 47 in 2015.

The children got the drug from parents, grandparents, neighbors, friends, babysitters or other relatives. Most of the time, the kids ate food containing marijuana. Their symptoms included drowsiness, dizziness, vomiting, agitation, dangerous heart rates and seizures.

Studies suggest that using marijuana and alcohol together impairs driving more than either substance alone and that alcohol use may increase the absorption of THC, the psychoactive chemical found in marijuana.