How Marijuana May Change Teen Brains

Marijuana abuse in teen years may change hippocampus shape and affect memory

/ Author:  / Reviewed by: Joseph V. Madia, MD Beth Bolt, RPh

(RxWiki News) Teens often experiment with substances like marijuana, but abusing marijuana may pose a threat to memory abilities.

A new study from Northwestern University found that teens who used marijuana heavily had an oddly shaped hippocampus. The hippocampus is a structure in the brain that is integral to short- and long-term memory.

Teens who were heavy marijuana users also performed poorly on memory tests. The authors of this study stopped short of saying that marijuana caused the changes in the hippocampus. However, they did note that the findings were suggestive.

Senior study author Dr. John G. Csernansky, the Lizzie Gilman professor and chair of psychiatry and behavioral sciences at Northwestern University Feinberg School of Medicine in Chicago, said in a press release, "The memory processes that appear to be affected by cannabis are ones that we use every day to solve common problems and to sustain our relationships with friends and family.”

A total of 97 subjects participated in this study. Most had used marijuana daily since the age of 16 or 17 and had smoked it for about three years.

These patients had stopped smoking marijuana at least two years before the study started and were in their early 20s.

These subjects included four groups. The first group was healthy controls who had not used marijuana. The second group was subjects who had abused marijuana. The third group included subjects with schizophrenia who had abused marijuana. The fourth group included subjects with schizophrenia who were otherwise healthy.

To study the patients' brain structures, Dr. Csernansky and team used computer programs and MRI studies. Each study patient also took a memory test.

First, the subjects listened to a one-minute story. Twenty to 30 minutes later, they were asked to recall as much information as possible from the story.

People in their early 20s who had smoked marijuana heavily as teens performed 18 percent worse on memory tests than adults of the same age who had never abused marijuana.

Dr. Csernansky and colleagues found that the longer a patient had used marijuana, the more abnormal the shape of his or her hippocampus. The physical damage to the hippocampus could include structures such as neurons or axons, the brain cells and nerve fibers.

None of the subjects in the test had used any other drug except marijuana.

Subjects who had schizophrenia and had abused marijuana performed 26 percent more poorly than subjects with schizophrenia who had not abused the drug. Schizophrenia is a brain disorder that can cause hallucinations and delusions. It is thought to be a genetic disorder, as it runs in families.

This study was published March 12 in the journal Hippocampus.

The National Institute of Mental Health funded this research. Dr. Csernansky and team disclosed no conflicts of interest.

Review Date: 
March 11, 2015
Last Updated:
April 6, 2015