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Canadian researcher compared the intelligence quotient (IQ) of 15 current heavy users of cannabis, 9 current light users, 9 former regular users and 37 non-users in a group of 70 young people. Participants had been followed since birth and now were 17-20 years old.
Current marijuana use was significantly correlated in a dose-related fashion with a decline in IQ when compared to the IQ measured at age 9-12. In current heavy users the IQ showed a decrease of 4.1 points, compared to gains in IQ points for light current users (5.8), former users (3.5) and non-users (2.6).
The authors concluded that current cannabis use "had a negative effect on global IQ score only in subjects who smoked 5 or more joints per week" and that "marijuana does not have a long-term negative impact on global intelligence."
Former users had smoked marijuana regularly in the past but not for at least 3 months. Current heavy use was defined as smoking at least 5 joints per week. Light users smoked less than 5 joints per week.
Source: International Association for Cannabis as Medicine
Current marijuana use was significantly correlated in a dose-related fashion with a decline in IQ when compared to the IQ measured at age 9-12. In current heavy users the IQ showed a decrease of 4.1 points, compared to gains in IQ points for light current users (5.8), former users (3.5) and non-users (2.6).
The authors concluded that current cannabis use "had a negative effect on global IQ score only in subjects who smoked 5 or more joints per week" and that "marijuana does not have a long-term negative impact on global intelligence."
Former users had smoked marijuana regularly in the past but not for at least 3 months. Current heavy use was defined as smoking at least 5 joints per week. Light users smoked less than 5 joints per week.
Source: International Association for Cannabis as Medicine
