All That Pot Smoking May Leave You Searching for a Few Words

If you’ve wondered what years of smoking pot might have done to your brain, a March 2016 study could be reassuring or anxiety-provoking, depending on your point of view.

The study, published in JAMA Internal Medicine, looked at people 18 to 30 years old who lived in Birmingham, Ala.; Chicago; Minneapolis; and Oakland, Calif., in 1985 and 1986. The researchers followed up with 3,385 of them after 25 years. At the beginning of the study, 84.3 percent of that group said they had used marijuana, but only 11.6 percent said they were still using it 25 years later (work, family and a more mature brain tend to make pot less desirable as people age).

At their 25-year study visit, when they were about 43 to 55, the participants took standardized tests to assess verbal memory, which is simply the memory of language; processing speed, which refers to how quickly people can perform easy or automatic cognitive tasks, such as solving simple arithmetic problems or copying words; and executive function, or higher-level cognitive skills used to coordinate other cognitive abilities, kind of like how CEOs keep track of all the departments in their companies.

The longer people had used marijuana, the worse they did on all three tests. However, after accounting for other factors that could affect performance on cognitive tests, such as education level, only the association between long-term marijuana use and verbal memory persisted. But the difference between the long-term users and people who were not was small, on average equal to remembering one word less out of a list of 15 for every five years of marijuana use.

“Personally, I thought it would not do anything,” said lead author Reto Auer, academic chief resident at the University of Lausanne, Switzerland, who was formerly at the University of California, San Francisco. “I was surprised that we found an association, actually, and that it was so consistent.”

In an editorial accompanying Auer’s paper, two substance abuse researchers from England and Australia noted that the study participants had not undergone cognitive testing when they entered the study, which they said raises a chicken-and-egg question: Does long-term marijuana use lead to the decline in verbal memory, or are people with poorer verbal memory to begin with more likely to have a pot habit?

Even if the researchers had tested their subjects at the beginning of the study, they still wouldn’t be able to answer that question because most of the long-term marijuana users had started before they entered the study, Auer said. Plus, he said, he doesn’t know why having poorer verbal memory would make people more likely to smoke pot.

Auer said he can’t predict whether the verbal memory gap will widen between people who had been long-term marijuana users and those who hadn’t. Study participants will be invited back for a 30-year visit, at which time they’ll again undergo cognitive testing. 

In addition, at the 25-year visit, more than 500 people received MRI scans of their brains, and Auer said he hopes many of them will return for the 30-year visit and get a repeat brain MRI. The researchers are looking at the scans for brain differences between the long-term users and other study participants, although, Auer said, the research team might not have enough subjects to reach any conclusions.

Given that an increasing number of states and the District of Columbia are legalizing medical marijuana and/or decriminalizing recreational pot use, more research is needed on its brain effects, Auer said. One problem, he said, is that while pharmaceutical companies fund research into their products, there’s no comparable industry money available to fund research studies of marijuana.