NEW YORK — Most adolescents who plan or attempt suicide have already received at least some mental health treatment, raising questions about the effectiveness of current approaches to helping troubled teenagers, according to the largest in-depth analysis to date of suicidal behaviors in US teenagers.

The study, posted online Tuesday by the journal JAMA Psychiatry, found that 55 percent of suicidal teenagers had received some therapy before they thought about suicide, planned it, or tried to kill themselves, contradicting the widely held belief that suicide is due in part to a lack of access to treatment.