Heavy, chronic marijuana users suffer memory loss and attention problems that can affect their work, learning and life, researchers reported Tuesday.
But in an indication of the controversy surrounding a drug said to be used at least weekly by 7 million people in the United States, a medical expert not involved in the study questioned the findings and whether the alleged adverse impact of the drug persists.
The study was published in this week's Journal of the American Medical Association. It was based on patients seeking help for marijuana dependence at clinics in Seattle, Farmington, Conn., and Miami, between 1997 and 2000.
The study examined 51 people who had been using marijuana regularly for an average of 24 years, 51 short-term users and 33 nonusers who were included as controls for comparison purposes in the research.
"Long-term users ... performed significantly less well on tests of memory and attention than nonuser controls and shorter-term users with a mean of 10 years' use," the study said.
On one verbal learning test "long-term users recalled significantly fewer words than either shorter-term users or controls; there was no difference between shorter-term users and controls. Long-term users showed impaired learning, retention and retrieval compared with controls," it added.
The authors of the study, Nadia Solowij at the University of New South Wales, Sydney, Australia, and colleagues with the Marijuana Treatment Project Research Group said their study confirmed and extended previous findings of cognitive impairments among chronic heavy marijuana users.
"For habitual users, the kinds of impairments observed in this study have the potential to impact academic achievements, occupational proficiency, interpersonal relationships and daily functioning," the study concluded.
But in an editorial published in the same issue, Harrison Pope of Harvard Medical School said "a recent meta-analysis of neuropsychological studies of long-term marijuana users found no significant evidence for deficits in seven of eight ... ability areas and only a small effect size for the remaining area of learning."
In a separate statement he said the study does not explore whether the heavy users might have been taking other drugs that might have accounted for the deficits uncovered, or whether they might have been suffering from anxiety or depression that could cause the problems noted.
"Another recent study from our laboratory ... found virtually no significant differences between 108 heavy cannabis users and 72 controls -- screened to exclude those with current psychiatric disorders, medication use, or any history of significant use of other drugs or alcohol -- on a battery of ten neuropsychological tests after 28 days of supervised abstinence from the drug," he said.
Pope, who directs the Biological Psychiatry Laboratory at McLean Hospital in Belmont, Mass., added: "The safest thing to say at this point is that the jury is still out on the question of whether long-term marijuana use causes lasting impairment in brain function."