"The finding of this study is that heavy, frequent, long-term marijuana use (more than 15 years of use on average) may subtly affect certain cognitive abilities, especially the ability to verbally learn and remember new information and information-processing speed," researcher Lambros Messinis, PhD, tells WebMD in an email.
"However, we do not yet definitely know if these deficits are reversible after longer periods of abstinence from marijuana," Messinis adds, calling the findings "still controversial."
Messinis works in Patras, Greece at the University Hospital Patras.
Marijuana Users Studied
Messinis' study included people who were 17-49 years old. Participants were split into three groups based on pot use:
- 20 people had smoked at least four marijuana "joints" per week for at least 10 years.
- 20 people had smoked pot at least four days per week for at least five years.
- 24 people had smoked pot 1-20 times, but not in the last two years.
None had used any other illicit drugs in the last year. If they had used other illicit drugs, they hadn't done so for more than three months of their entire lives.
Mental Skills Tested
After abstaining from pot for at least 24 hours -- and providing a urine sample confirming no other drug use -- participants took several tests of mental skills.
In a memory test, participants heard two lists of 15 words and tried to recall which words had been on the first list. Other tests checked attention and reaction times.
Participants with heavy, long-term marijuana use fared worst. For instance, here are the memory test's average results:
- Not current marijuana users recalled 12 out of 15 words.
- Heavy, frequent marijuana users for at least five years recalled nine out of 15 words.
- Heavy, frequent marijuana users for at least 10 years recalled seven out of 15 words.
On a test measuring executive functions that are important in making decisions, long-term users had 70% impaired performance, compared with 55% impaired performance for shorter-term users and 8% impaired performance for nonusers.
Messinis and colleagues took other factors into account, including age, IQ, and education.
"Duration of cannabis use ... appears to be related to neuropsychological performance in certain cognitive domains," the researchers write.