Long-Term Marijuana Smoking Doesn’t Impact Cognition, Study Says

Long-term use of marijuana does not lead to a decline in mental function, according to the results of a large-scale John Hopkins University study.

“There is no convincing evidence that [even] heavy long-term marijuana use impairs memory or other cognitive functions,” said NORML board member Dr. John P. Morgan of City University of New York (CUNY) Medical School. “During the past 30 years, researchers have found, at most, minor cognitive differences between chronic marijuana users and nonusers, and the results differ substantially from one study to another.”

The most recent John Hopkins study examined marijuana’s effects on cognition on 1,318 participants over a 15 year period. Researchers gave subjects specialized tests, called Mini-Mental State Examinations (MMSE), in 1981 and 1982. Subjects took follow-up MMSE tests 12 to 15 years later and scientists measured rates of cognitive decline among marijuana smokers and nonsmokers.

Researchers reported “no significant differences in cognitive decline between heavy users, light users, and nonusers of cannabis.” They also found “no male-female differences in cognitive decline in relation to cannabis.

“These results … seem to provide strong evidence of the absence of a long-term residual effect of cannabis use on cognition,” they concluded.

The study is the first to investigate the long term effects of marijuana on cognition in a large epidemiological sample.

Researchers did conclude that cognition declines over long time periods in all age groups, but found this decline “closely associated with aging and educational level, [and] … not … associated with cannabis use.”

The study appears in the May 1, 1999, issue of the American Journal of Epidemiology.

For more information, please contact John P. Morgan of CUNY Medical School @ (212) 650-8255 or Allen St. Pierre @ (202) 483-8751. To read an abstract of this study online, please visit: www.jhsph.edu/Publications/JEPI/may199/may1con.htm.