"The findings were kind of a surprise. One might have expected to see more impairment of higher mental function," said Dr. Igor Grant, a UCSD professor of psychiatry and the study\'s lead author. Other illegal drugs, or even alcohol, can cause brain damage.
His team analyzed data from 15 previously published, controlled studies into the impact of long-term, recreational cannabis use on the neurocognitive ability of adults.
The studies tested the mental functions of routine pot smokers, but not while they were actually high, Grant said.
The results, published in the July issue of the Journal of the International Neuropsychological Society, show that marijuana has only a marginally harmful long-term effect on learning and memory.
No effect at all was seen on other functions, including reaction time, attention, language, reasoning ability, and perceptual and motor skills.
Grant said the findings are particularly significant amid questions about marijuana\'s long-term toxicity now that several states are considering whether to make it available as a medicinal drug.
In California, growing marijuana for medical purposes is legal under a voter-approved law.
The UCSD analysis of studies involving 704 long-term cannabis users and 484 nonusers was sponsored by a state-supported program that oversees research into the use of cannabis to treat certain diseases.
Anecdotal evidence has shown that marijuana can help ease pain in patients with diseases like multiple sclerosis or prevent severe nausea in cancer patients, but the effects have yet to be proven in controlled studies, Grant said.
The UCSD research team said the problems observed in learning and forgetting suggest that long-term marijuana use results in selective memory defects, but said the impact was of a very small magnitude.
"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," Grant said.
In addition, he noted that heavy marijuana users often abuse other drugs, such as alcohol and amphetamines, which also might have long-term neurological effects.
Some of the research studies used in the analysis were limited by the numbers of subjects or insufficient information about factors like exposure to other drugs or whether participants suffered from conditions like depression or personality disorders.
"If it turned out that new studies find that cannabis is helpful in treating some medical conditions, this enables us to see a marginal level of safety," Grant said.
