By Dr. Brian McDonough, Medical Editor

PHILADELPHIA (CBS) – The numbers are startling when it comes to concussions in teenagers – that is, if a Canadian study of over 2900 teens is to be believed.

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The study is one where teenagers self-reported concussions and, according to their reports, 20% of them said they had traumatic brain injury.

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In the survey a traumatic brain injury was defined as a head injury in which the student was unconscious for at least 5 minutes or hospitalized overnight. Twenty percent said they had traumatic brain injury in the past, 5.6% of them in the past year.

Sports injuries accounted for more than half of those recent cases, and were more common in boys.

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Not all incidents were related to sports, teenagers who reported drinking alcohol and smoking marijuana in the past year had higher odds of a traumatic brain injury in the past year than those who did not.