According to a large-scale U.S. study on the effects of long-term use of cannabis, published in the American Journal of Epidemiology, the age-related decline of cognitive functioning "does not appear to be associated with cannabis use" (Lyketsos 1999).

Constantine Lyketsos and colleagues of John Hopkins Hospital in Baltimore conducted a follow-up study of 1,318 persons, divided in heavy users, light users, and nonusers of cannabis. All participants had completed a special test, the Mini Mental State Examination (MMSE), in 1981, 1982, and 1993-1996. The individual score differences between 1982 and 1993-1996 were calculated for each study participant. Within these 12 years the mean score decline was 1.2 points.

The Mini Mental State Examination (MMSE) is a brief and widely used, standardized method for assessing cognitive mental status. It assesses orientation, attention, immediate and short-term recall, language, and the ability to follow simple verbal and written commands. The maximum achievable score is 30.

Researchers found a decline in all age groups. There was "no significant differences in cognitive decline between heavy users, light users, and nonusers of cannabis." And there were also no sex differences in these subgroups.

This is the first large-scale prospective study to examine the effects of long-term cannabis use on cognitive function. Recent research has given support to the hypothesis that there may be subtle impairment of special "higher cognitive functions, which include the organization and integration of complex information involving various mechanisms of attention and memory processes" (Solowij 1999).

But these observations are hampered by the fact that they are based on retrospective studies with single measurements. In a commentary by Martha Clare Morris and colleagues of the Rush Institute for Healthy Aging in Chicago, the difficulties encountered with the use of single measurements of cognition and the importance of measuring changes are stressed (Morris 1999).

On the other hand one may argue, that the MMSE might not be an adequate tool to detect minor cognitive alterations, that are suspected to be caused by cannabis, especially a subtle impairment of functions of the frontal lobe of the brain.

Sources:
Lyketsos CG, et al: Am J Epidemiol (1999) 149:794- 800
Morris MC, et al: Am J Epidemiol (1999) 149:789-793
Solowij N, in: Kalant H, et al (eds): The Health Effects of Cannabis. Toronto 1999, 195-265