How Long-Term Marijuana Use Affects Memory

Long-term marijuana use causes memory, the speed of thinking and other cognitive abilities to get worse over time, but cognitive abilities are also affected in short-term pot smokers who use marijuana frequently.

Researchers studying heavy marijuana users (four or more joints per week) in Greece found that frequent marijuana users performed worse than non-users on a test of cognitive abilities. Those who had smoked for more than 10 years had more problems with their thinking abilities than those who had used for five to 10 years.

"We found that the longer people used marijuana, the more deterioration they had in these cognitive abilities, especially in the ability to learn and remember new information," said study author Lambros Messinis, Ph.D., of the Department of Neurology of the University Hospital of Patras in Patras, Greece. "In several areas, their abilities were significant enough to be considered impaired, with more impairment in the longer-term users than the shorter-term users."

The researchers made the following observations:

• Compared to non-smokers, marijuana users performed worse in recall, recognition and executive functions of the brain.​
• In making decisions, long-term users showed a 70 percent impaired performance, compared to 55 percent for short-term users and eight percent for non-users.
• In a test to remember a list of words that were read to them earlier, non-users remembered 12 of the 15 words, short-term users remember an average of nine words, and long-term users remembered an average of seven.
• Frequent smokers performed worse on tests of cognitive abilities, including divided attention (ability to pay attention to more than one stimulus at a time) and verbal fluency (number of words generated within a time limit).

The study involved people ages 17 to 49 in a drug abuse treatment program in Athens, Greece. Twenty were long-term users, 20 shorter-term users and 24 control subjects who had used marijuana at least once, but not more than 20 times and not in the past two years.

Those who had used any other drugs, such as cocaine or stimulants, during the past year or for more than three months throughout their lives, were not included in the study. Before the tests were performed, all participants had to abstain from marijuana for at least 24 hours, according to the researchers.