March 18, 2022

By Tom Insel, MD, and Matthew Hirschtritt, MD, MPH

Good morning, and welcome back to our weekly Research Roundup. Unfortunately, the studies we review this week are mostly cautionary tales. Here’s what we’ve got: Most people who die by suicide have never been in mental health care. Spending time in a juvenile detention facility may increase the odds of criminal justice involvement as an adult. And perhaps most concerning, new research from a careful study of cannabis shows that chronic use reduces IQ and hippocampal volume, big time. Not a week with a lot of good research news, but we’ll keep hunting for studies that point to solutions. If you’re aware of some, please send them our way.

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This Is Your Brain on Pot

Across the country, a growing number of states are allowing medical or recreational use of marijuana and more people than ever are using it. In 2020, nearly 50 million Americans – 18% of the total population – reported using cannabis in the past year. Among 18-to-25-year-olds, the number was even higher: about one-third.

While cannabis can be helpful for many health conditions – including wasting associated with AIDS, nausea associated with chemotherapy, and severe pain – chronic use has been associated with both short-term and prolonged cognitive impairment in areas like executive functioning, attention, and processing speed long after the high has worn off. Now, new data show just how profoundly long-term cannabis use can impair cognition.

A team led by Madeline Meier of Arizona State University, Tempe, used data from the Dunedin Longitudinal Study, which tracks the health of more than 1,000 New Zealanders born in 1972 or 1973, nearly all of them white. Based on their self-reported drug use starting at 18, researchers classified participants into six groups: long-term users who used at least weekly; lifelong nonusers; long-term tobacco users and long-term alcohol users who were not  heavy cannabis users; midlife recreational cannabis users, who started late and use less than weekly; and cannabis quitters, who weren’t using at 45 but had used weekly or had a history of dependence in the past.

The researchers reviewed participants’ past scores on tests of IQ, working memory, verbal comprehension and learning, and processing. Most participants also had an MRI to measure the volume of their hippocampus, a brain structure involved with memory, navigation and other functions.

Their findings? Long-term cannabis users weren’t doing so well at age 45:

  • They’d lost about 5.5 IQ points, falling from average in childhood to below-average in adulthood, a bigger loss than the midlife recreational users and cannabis quitters, as well as the long-term tobacco and alcohol users.
  • They performed worse than the other groups – including long-term alcohol users – on almost all cognitive tests, even after controlling for sex and childhood IQ. Alarmingly, cannabis quitters performed as poorly as long-term cannabis users on all tests.
  • Among long-term users, the more frequent their use, the lower their IQ scores and cognitive-functioning test performance. Dosage matters.
  • Long-term cannabis users had smaller hippocampi than most groups, but notably not smaller than cannabis quitters.

So what’s the takeaway? Using cannabis is not good for your brain, and this study suggests it’s even worse than long-term alcohol use. Discouragingly, cannabis quitters showed some of the same cognitive deficits as the long-term users, suggesting that frequent cannabis use may have long-lasting and irreversible negative impacts. Even midlife recreational users suffered a substantial blow to their IQ over time. These data add to mounting evidence that cannabis use is not benign. As legal use becomes more common, public health campaigns should highlight the real hazards of cannabis.

Long-Term Cannabis Use and Cognitive Reserves and Hippocampal Volume in Midlife. American Journal of Psychiatry, March 8, 2022.

– Matthew Hirschtritt


The Challenge of Reducing Suicide

Reducing suicide is a national mental health priority. Although suicide deaths have dropped a bit during the past two years, the rate is still roughly 30% higher than two decades ago. Efforts to prevent suicides usually combine a range of approaches. Primary prevention efforts seek to lower the risk for everyone by, for example, making bridges and buildings safe.  Secondary prevention efforts focus on reducing suicide for people identified as being at risk, such as those in care for suicidal thoughts or those who have made a previous attempt.

A new report on suicide demonstrates the limitations of secondary prevention. Allison E. Bond and colleagues from Rutgers University studied the history of care for 234,652 people who died by suicide from 2003 to 2018. Half of these deaths involved firearms. Only about a third of these people had a lifetime history of treatment and, of those who died by firearm, only 20% were in current treatment. Most had no previous suicide attempt or lifetime history of suicidal ideation. For those who died by firearm, only 10.7% had made a previous attempt.

While the goal of eliminating suicide by groups like Zero Suicide and others are commendable, these data demonstrate how difficult reducing suicide will be if our focus is only on improving health care. Even if every firearm suicide were prevented for people in current treatment, the overall rate would drop only 20%. The implication is that we need a public health approach that focuses on primary prevention, including safe storage of firearms, if we want to reduce mortality from suicide. Secondary prevention is important, but it will not be enough. This was a lesson we can take from the history of reducing automobile deaths, where safer cars and safer roads proved more effective than focusing on drivers after an accident.

Mental Health Treatment Seeking and History of Suicidal Thoughts Among Suicide Decedents by Mechanism, 2003-2018  JAMA Network Open, March 14, 2022

-Tom Insel


Getting Stuck in the Justice System

The primary goal of the juvenile justice system in the U.S. is supposed to be rehabilitation. Does it work?

William Copeland of the University of Vermont and colleagues set out to answer that question. They tracked 1420 youth who’d been part of the longitudinal Great Smoky Mountains Study from 1993 to 2000 and then re-interviewed them years later when they were 19 to 30 years old to learn whether they’d been arrested or otherwise been involved with the criminal justice system as adults.

Of the 1420 young people in the study, 226 had been involved with justice system as youth, and they were 2.5 to 3.3 times more likely than those who were not involved with the justice system to later engage in some form of criminal behavior as adults or to have records of arrest or time booked in jail. Even more strikingly, youth who were detained or placed in juvenile detention facilities were 5.1 to 14.5 times more likely to be charged with felonies or misdemeanor charges or to spend time in jail as adults. As the authors conclude, “Juvenile justice involvement may catalyze rather than deter from adult offending.”

The US has the highest rate of incarceration in the world. This study adds to the growing national sentiment to decrease reliance on incarceration to address criminal behavior – especially among youth. The National Juvenile Justice Network offers multiple recommendations to divert youth from justice involvement, including focusing on prevention, investing in accessible mental health resources and school counseling, involving and empowering families, and removing youth with mental illness from the justice system.

These results suggest one step toward reducing the number of detained adults: Whenever possible, keep youth out of the justice system.

Adult criminal outcomes of juvenile justice involvement. Psychological Medicine, March 10, 2022.

Matthew Hirschtritt


If you or anyone you know is considering suicide, call the National Suicide Prevention Lifeline at 1-800-273-8255. And if you’re a veteran, press 1.



Tom Insel, MD, is a psychiatrist, neuroscientist, and former director of the National Institute of Mental Health (NIMH). He is a donor to MindSite News and chair of its Editorial Advisory Board. Dr. Insel’s financial conflict of interest statement, which includes equity and advisory roles in several early-stage mental health technology companies, can be found here.

Matthew Hirschtritt, MD, MPH, is a clinical psychiatrist with the Permanente Medical Group, Inc., a mental health services researcher with the Division of Research, Kaiser Permanente Northern California, and assistant program director of the Kaiser Permanente Oakland Adult Psychiatry Training Program. His current research focuses on identification and treatment of patients with first-episode psychosis, as well as implementation of a telehealth-based mental health evaluation and referral program.

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