The findings were particularly significant considering the movement by several states to make cannabis (marijuana) available as a medicinal drug, and questions regarding its potential toxicity over long-term usage.
Published in the July issue of the Journal of the International Neuropsychological Society, the study involved a quantitative synthesis of 15 previously published research studies on the non-acute (residual) effects of cannabis on the neurocognitive performance of adult human subjects.
The studies included 704 long-term cannabis users and 484 non-users. The neurocognitive performance measurements included simple reaction time, attention, verbal/language, abstraction/executive functioning, perceptual/motor skills, motor skills, learning and forgetting.
"Surprisingly, we saw very little evidence of deleterious effects. The only exception was a very small effect in learning new information," said Igor Grant, M.D., the study's senior author, a UCSD professor of psychiatry, and director of the Center for Medicinal Cannabis Research (CMCR), a collaborative, state-supported program between UCSD and UC San Francisco, that oversees 11 studies of the safety and efficacy of medicinal cannabis to treat certain diseases.
In describing the negative effects in the study, the research team said the problems observed in learning and forgetting suggest that chronic long-term cannabis use results in selective memory defects. They added that "while the results are compatible with this conclusion, the effected size for both domains was of a very small magnitude."
Grant added that the minimal side effects seen "raised the question of practical significance. If we barely find this tiny effect in long-term heavy users of cannabis, then we are unlikely to see deleterious side effects in individuals who receive cannabis for a short time in a medical setting."
In addition, Grant said that heavy marijuana users often abuse other drugs, such as alcohol and amphetamines, which also might have long-term neurological effects. This raises the question of the extent to which the other drugs contributed to the minimal problems found in learning and forgetting in the marijuana users.
The paper's authors also noted that many of the research studies examined had significant limitations, either with small numbers of subjects or insufficient information about potential confounding factors, such as exposure to other drugs or presence of neuropsychiatric factors such as depression or personality disorders.
They noted that only studies that begin with the examination of children and young adolescents before they enter the period of risk to cannabis exposure, can sufficiently reduce the influence of these additional factors.
In addition to Grant, the paper's authors included doctoral students Raul Gonzalez, M.S., and Catherine L. Carey, M.S. and Loki Natarajan, Ph.D., UCSD HIV Neurobehavioral Research Center (HNRC) and UCSD Department of Family and Preventive Medicine, and Tanya Wolfson, M.A., UCSD HNRC.
The study was supported by the CMRC.