The authors theorized that the fact that teens who used both drugs had the highest depression risk a year out might indicate that the combination ends up being more than the sum of its parts in terms of boosting risk.
However, as the authors did not tally how often a drug was used, they could not compare differences in risk among frequent users and infrequent users.
The team noted that earlier animal and human studies suggested that ecstasy and speed use can have a long-term negative impact; however, the new study could not show whether depression was a long- or short-term aftereffect. They also noted that other drugs alongside ecstasy and speed might have had an effect on depression.
"But it's also important to know that we did control for a previous history of depressive symptoms among these students," Fallu noted. "And we still found a clinically significant association between this drug use and depressive symptoms."
A host of experts weighed in on the findings.
"Certainly this study raises some really interesting points," said Dr. Michael Mithoefer, a psychiatrist in private practice in Charleston, S.C. "And of course parents should be concerned about kids this age using these drugs. But at the same time we don't really know what the cause and effect might be here. Because one problem with this kind of retrospective study is that you never really know what's different about those people who decide to use substances to being with, as opposed to those who didn't."
Steven Shoptaw, a clinical psychologist and professor in the department of family medicine at University of California, Los Angeles, concurred.
"One way to think about this finding is that this kind of drug use could be considered a marker for vulnerability for depressive symptoms," Shoptaw said. "But we can't say, from this, that it's the drugs causing depression.
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