|Long-term heavy users of marijuana perform significantly worse on tests of mental agility and physical dexterity than short-term users or nonusers, even when they have abstained from smoking for more than 24 hours, new research shows.|
Scientists, led by Lambros Messinis, a neuropsychologist at University Hospital in Petras, Greece, tested three groups.
They were 20 long-term users who had smoked four or more marijuana cigarettes a week for at least 10 years, 20 short-term users who had smoked a similar amount for 5 to 10 years and, finally, 24 people, representing a control group, who had used marijuana no more than 20 times in their lives and not in the prior two years.
The long- and short-term users were drawn from participants in a drug rehabilitation program.
Even after controlling for I.Q., other drug use, age, sex, depression and other variables, long-term users scored significantly lower than control group members and shorter-term users on tests of verbal fluency, memory and coordination.
The exercises included naming objects when shown pictures of them, thinking up words with the same initial letter, listening to lists of words and later recalling them and drawing lines in the proper order among numbers and letters randomly spread on paper.
The study appears in the March issue of Neurology.
Dr. Messinis acknowledged that the results might have differed with marijuana users from the general population. Still, he said, the study was carefully controlled, and frequent heavy use appeared to have significant negative effects on performance.