Long-term use of marijuana does not lead to a decline in mental function, according to researchers at the Johns Hopkins University (JHU) in Baltimore. Individuals who confessed to having smoked marijuana, even heavily, have no more signs of impaired mental function than people who have never used cannabis (Constantine G. Lyketsos, et al., "Cannabis Use and Cognitive Decline in Persons Under 65 Years of Age," American Journal of Epidemiology, May 1999, vol. 149, p. 794-800; "Long-term Use Doesn't Hurt Mind, Study Says," Detroit Free Press, May 7, 1999; "The Straight Dope," Science News (US), May 22, 1999).

Scientists at JHU tracked the mental functioning of 1,318 Baltimore residents aged 18 to 64 over the course of 11 ½ years. After analyzing data gathered by the Mini-Mental State Examination, or MMSE, researchers concluded that there is no cognitive decline associated with marijuana use. The researchers said that cognitive decline is related to aging and starts in individuals younger than 30 years old. The decline is said to increase exponentially with each decade that passes, but attaining a higher level of education may reduce the severity of cognitive decline.

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