Heavy use of marijuana by teens and young adults with mood disorders, such as depression and bipolar disorder, is linked to an increased risk of self-harm, suicide attempts and death, a study found.

Unintentional overdoses, suicide and homicide were the three most frequent causes of death, according to the study published in JAMA Pediatrics.

"The perception is that marijuana is safe to use, but we need to educate parents and kids that there are risks involved, particularly with heavy and high potency cannabis use," said study author Cynthia Fontanella, an assistant professor in the department of psychiatry and behavioral health at the Ohio State University's College of Medicine.

"And clinicians need to intervene to identify and treat cannabis use disorder as well as kids with mood disorders," Fontanella said.

Cannabis use disorder, also known as marijuana use disorder, is associated with dependence on the use of weed. People are considered dependent on weed when they feel food cravings or a lack of appetite, irritability, restlessness and mood and sleep difficulties after quitting, according to the National Institute on Drug Abuse.

"People who begin using marijuana before the age of 18 are four to seven times more likely to develop a marijuana use disorder than adults," NIDA advises. About 4 million people in the United States met the diagnostic criteria for a marijuana use disorder in 2015, the NIDA estimates.

Experts say that number will have grown due to a rise in potency in today's varieties of weed, along with the legalization of recreational marijuana for adults in 15 states and medical use in 36 states.

Studies in adults show a strong association between overuse of weed and suicide attempts and death. A study of adult same-sex twins found those who were dependent on marijuana were nearly three times more likely to attempt suicide than their twin who was not dependent on weed.

The new study used Ohio Medicare data to identify both cannabis use disorder and self-harm attempts and outcomes in youth between the ages of 10 and 24 years old. The study could only show an association between cannabis dependence and negative outcomes, not a direct cause and effect.

Prior studies show children with mood disorders are highly likely to use and abuse marijuana, Fontanella said, partly because they don't like the side effects of many prescribed medications.

"Mood stabilizers and psychotic medications can cause weight gain, say up to 30 or 40 pounds ... stiffness of their neck or eyes ... and it can cause sedation," Fontanella said. "So, they may not use their medication and may self-medicate with cannabis to treat the mood disorders."

It could also be that using weed might contribute to the development of mood disorders, however.

"Research shows cannabis use is associated with early onset of mood disorder, psychosis and anxiety disorders, so it can lead to the onset of severe mental illness," Fontanella said.

At this point, however, science is not sure which comes first, partly because few if any studies have been done in teens and young adults.

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