Researchers, whose findings appeared in the journal JAMA Psychiatry, said those numbers are similar to the prevalence of lifetime suicidal thinking and attempts reported by adults, suggesting that the teenage years are an especially vulnerable time.

"What adults say is, the highest risk time for first starting to think about suicide is in adolescence," said Matthew Nock, a psychologist who worked on the study at Harvard University.

The results are based on in-person interviews of close to 6,500 teens in the United States and questionnaires filled out by their parents. Along with asking youth about their suicidal thinking, plans and attempts, interviewers also determined which teens fit the bill for a range of mental disorders.

Just over 12 percent of the youth had thought about suicide. Four percent had made a suicide plan and four percent had attempted suicide.

Nock and his colleagues found that almost all teens who thought about or attempted suicide had a mental disorder, including depression, bipolar disorder, attention deficit hyperactivity disorder (ADHD) or problems with drug or alcohol abuse.

More than half of the youth were already in treatment when they reported suicidal behavior, which Nock said was both encouraging and disturbing.

"We know that a lot of the kids who are at risk and thinking about suicide are getting (treatment)," he told Reuters Health. However, "We don't know how to stop them - we don't have any evidence-based treatments for suicidal behavior."

The findings leave many questions unanswered.

Because most youth who think about suicide never go on to make an actual plan or attempt, doctors need to get better at figuring out which ones are most at risk of putting themselves in danger, according to Nock.

Once those youth are identified, researchers will also have to determine the best way to treat them, since it's clear a lot of current methods aren't preventing suicidal behavior, he said.

According to the U.S. Centers for Disease Control and Prevention, suicide is the third leading cause of death for people between the ages of 10 and 24, killing about 4,6000 young people annually.

Although girls are more likely to attempt suicide - a patter confirmed by Nock's study - boys have higher rates of death by suicide because they typically choose more deadly methods, such as guns. Source: bit.ly/11fTtDz

(Reporting from New York by Genevra Pittman at Reuters Health; editing by Elaine Lies)

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