Researchers out of Switzerland's University of Lausanne studied the marijuana habits of 3,400 participants beginning as young adults via data compiled over 25 years. About 84 percent of participants reported past marijuana use and 11 percent continued to smoke into middle age, per Medical Daily. To study usage on the same scale, they tallied a person's marijuana-years of exposure — a figure referring to every 365 days a person smoked weed, whether spread across a single year or seven.
While scientists found no link between exposure and brain function or processing speed, they did find verbal memory slackened slightly after the equivalent of five years of daily marijuana use.
Essentially, someone who smoked marijuana every day for five years could recall 8.5 out of 15 words they were told to memorize 25 minutes earlier, compared to nine words for non-users, per the Washington Post. But each year of exposure added more damage. Researchers estimate a person who smoked pot daily for 25 years would remember 2.5 fewer words than their non-smoking counterparts. (None of the study participants actually claimed 25 years of pot use — just 8 percent managed five years — but marijuana use is on the rise.)
Researchers can't confirm if marijuana causes memory problems — people with lower cognitive abilities may simply be more inclined to use marijuana — but it's possible THC affects how the hippocampus processes information, reports Live Science.
(In related news, here's how potent pot affects the brain.)