Canadian researchers found that of nearly 9 000 Ontario adolescents, 20% said they'd previously had a concussion, and almost 6% had suffered at least one in the past year.
Sports accounted for more than half of those recent concussions, according to findings reported in a research letter in the June 26 issue of the Journal of the American Medical Association.
And teens who drank alcohol or smoked pot at least occasionally had three to five times the concussion risk of their peers who were drug- and alcohol-free.
The results give a broader idea of how common kids' concussions are, since past studies have focused on narrower groups -- such as athletes or kids who land in the hospital, said lead researcher Gabriela Ilie, of the injury prevention research office at St. Michael's Hospital in Toronto.
Experts have called concussion a "silent epidemic", Ilie said, but there is relatively little data on how common the traumatic head injuries are among teenagers. "We know it's a big public health problem, but we don't exactly how big is 'big'," she said.
Her team's findings are based on a questionnaire of students in grades 7 to 12 -- average age 15 -- from the general Ontario population. So the figures capture kids who are not on sports teams, or didn't go to the hospital for treatment.
"This is the first study I'm aware of that looked at the general population," said Kenneth Podell, co-director of the Methodist Concussion Center at the Methodist Hospital System in Houston.
Still, it's hard to know how accurate the figures are, he said, because kids reported on their own history. And any time studies rely on people's memories, it is a limitation, Podell noted.
He also saw an issue with how the study defined "concussion" -- a blow to the head that knocked the child out for at least five minutes or resulted in an overnight hospital stay.
But contrary to popular belief, most concussions do not knock people out. "We know that about 90% of sports-related concussions do not cause a loss of consciousness," Podell said.
So the 20% lifetime prevalence in this study could be an underestimate.
Podell and Ilie said it's not surprising that sports accounted for so many concussions. The study did not pinpoint which sports were the culprits, but it's known that football, ice hockey, soccer and basketball are among the riskiest activities. Even some non-contact sports, such as gymnastics and cheerleading, carry a concussion risk.
It's not clear why kids who drank or smoke pot showed a greater concussion risk, said Ilie. "All we know is, there was an association," she noted.
Podell said the finding is, again, not surprising. "We know that drinking and drug use is associated with a higher risk of injuries in general," he pointed out.
According to the US Centers for Disease Control and Prevention, more than 173 000 US children and teens are seen in emergency departments each year because of traumatic brain injuries, including concussion, sustained in sports or recreational activities, such as bike riding.
But the total number -- including kids not seen in the ER -- is probably much bigger: the CDC estimates that across age groups, up to 3.8 million Americans suffer a sports-related concussion each year.
Concussion symptoms include headache, dizziness, nausea, ringing in the ears, fatigue and confusion -- though these problems may not become noticeable until hours after the jolt to the head.
Ilie suggested that parents consult their doctor any time their child suffers a blow to the head. Typically, concussion symptoms improve within a few days, but if they do not -- or actually start to worsen -- parents should get medical help, Podell said.
In general, experts say kids with concussions should be symptom-free and get a doctor's OK before returning to sports. The biggest concern is that if they sustain another knock to the head while still recovering from the first concussion, they could suffer so-called second-impact syndrome, which can cause potentially fatal bleeding inside the skull and brain swelling.
Experts are still unsure whether kids' concussions -- especially multiple concussions -- can raise their risk of degenerative brain conditions, such as Alzheimer's disease, later in life.
Podell noted that it's an important but difficult question to study. "You have to look at people over decades," he said, "and many other things happen in a person's life during those years."
Learn more about concussions from the US Centers for Disease Control and Prevention.