University of Mississippi researchers tracked the potency of nearly 62,800 marijuana samples seized in the U.S. by government officials since 1975.
They found that in 2007, seized marijuana averaged 9.6% THC (marijuana's active ingredient) by dry weight. That's up from 8.75% in 2006 and from 0.88% in 1977.
"The increases in marijuana potency are of concern since they increase the likelihood of acute toxicity, including mental impairment," Nora Volkow, MD, director of the National Institute on Drug Abuse (NIDA), says in a news release.
"Particularly worrisome is the possibility that the more potent THC might be more effective at triggering the changes in the brain that can lead to addiction ; however, more research is needed to establish this link between higher potency and higher addiction risk," Volkow says.
By Miranda Hitti
Reviewed by Louise Chang
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