Frequent marijuana use associated with long-term memory loss

Previous research suggests smoking marijuana daily may affect short-term memory and that even casual use of marijuana can lead to brain changes. Now, another study shows that teenagers who smoke regularly but then kick the habit in a couple of years may see their long-term memory take a hit too.

The study, published Thursday in the journal Hippocampus, analyzed 97 study participants— some who had a history of smoking marijuana as teenagers and others who didn’t— and found that two years after abstaining, those who abused the substance scored worse on tests that assessed their long-term memory. Those who smoked did so for three years, and more than 80 percent of the smoking group reported using marijuana every day, said lead study author Matthew J. Smith, assistant professor of psychiatry and behavioral science at Northwestern University Feinberg School of Medicine.

In their initial study on the potential link between marijuana use and short-term memory loss, published in the December 2013 edition of the journal Schizophrenia Bulletin, researchers looked primarily at subcortical parts of the brain: the striatum, the globus pallidus and the thalamus. The current study used MRI scans to analyze changes in the hippocampus, the part of the brain that lies close to our emotional and hearing centers, and is involved in the storage and recall of long-term memories.

“[The] hippocampus itself serves as a major player of the formation of our new memory or long-term memory and spatial navigation, abilities that we need in everyday life,” Lei Wang, assistant professor of psychiatry and behavioral sciences at Northwestern University, and one of three senior study authors for the paper, told

The marijuana plant is thought to have at least 70 cannabinoids, or compounds, that include the active ingredient associated with marijuana’s psychotropic effects, tetrahydrocannabinol (THC). The hippocampus is dense with cannabinoid receptors, so when THC enters the body, it targets this area of the brain.

“Due to the high level of receptors in the hippocampus that attract THC, I think it makes sense that we’re seeing that that part of the brain looks different within someone who abused THC for several years as an adolescent,” Smith told

None of the study participants reported taking any drugs besides marijuana, and researchers adjusted for nicotine and verbal IQ differences, which may have affected the results, in their analysis.

The study group consisted of individuals who had been previously diagnosed with schizophrenia and those who were psychologically healthy. Of the latter group, 44 had no history of smoking marijuana while 10 did have a history of using the drug. Among the participants with schizophrenia, 28 had no history of marijuana use and 15 did.

People diagnosed with schizophrenia were included because other research indicates that these patients are known to have short- and long-term memory problems, and are more likely to use marijuana at some point in their lives. Scientists haven’t concluded why that is exactly: Some studies suggest schizophrenia’s underlying genetic factors may render these patients more likely to try the drug, while other research says marijuana’s ability to induce psychosis— a symptom of schizophrenia that is marked by delusions and hallucinations— may actually promote the onset of the condition.

In this research, study authors observed that the schizophrenia patients who reported using marijuana scored the worst on memory tests, which involved a standardized method of hearing a story and being asked to recall certain details 20 to 30 minutes later. Schizophrenia patients who reported smoking marijuana daily scored 26 percent worse on the long-term memory tests compared to the schizophrenia patients who didn’t smoke weed.

Among the psychologically healthy patients, those who smoked marijuana scored 18 percent worse on the memory tests than those who didn’t smoke.

The participants— now in their early- to mid-20s— reported smoking marijuana when they were between ages 16 and 17, and Smith said age likely played a role in the formation of abnormal brain changes linked to their marijuana use. Doctors say the brain doesn’t stop growing until a person reaches at least his or her mid-20s or 30s.

Smith cited what he called a landmark study published in April 2012 in the journal Proceedings of the National Academy of Sciences that followed about 1,040 individuals from birth until age 38, and found that— even when taking education into account— smoking marijuana was associated with a linear decline in IQ that wasn’t fully reversed after abstinence.

A recent federal study indicates that teen alcohol and drug abuse, including marijuana use, has progressively declined over the years. However, Smith pointed out that the THC concentration in unregulated marijuana— a factor his team wasn’t able to take into account— has nearly tripled since the 1990s, meaning those who do buy marijuana on the street may not know how much they’re harming their memory.

“We’re in the Prohibition-era of marijuana, and I say this as an analogy because during Prohibition there was no alcohol available in restaurants or that could be sold, and people were brewing it at home and because they were brewing it at home, they didn’t know what the alcohol content was,” Smith said. “And we do know more potent THC is linked to poorer memory and a higher risk of psychotic-like experiences.”