The consumption of cannabis, even long-term, poses few adverse effects on cognitive performance, according to clinical trial data to be published in the scientific journal Addiction.

Investigators at the University of Melbourne and the Australian National University, Center for Mental Health Research assessed the impact of cannabis use on various measures of memory and intelligence in over 2,000 self-identified marijuana consumers and non-users over an eight-year period. Among cannabis consumers, subjects were grouped into the following categories: ‘heavy’ (once a week or more) users, ‘light’ users, ‘former heavy’ users, ‘former light’ users, and ‘always former’ — a category that consisted of respondents who had ceased using marijuana prior to their entry into the study.

Researchers reported: “Only with respect to the immediate recall measure was there evidence of an improved performance associated with sustained abstinence from cannabis, with outcomes similar to those who had never used cannabis at the end point. On the remaining cognitive measures, after controlling for education and other characteristics, there were no significant differences associated with cannabis consumption.”

They concluded, “Therefore, the adverse impacts of cannabis use on cognitive functions either appear to be related to pre-existing factors or are reversible in this community cohort even after potentially extended periods of use.”

Separate studies have previously reported that long-term marijuana use is not associated with residual deficits in neurocognitive function. Specifically, a 2001 study published in the journal Archives of General Psychiatry found that chronic cannabis consumers who abstained from the drug for one week “showed virtually no significant differences from control subjects (those who had smoked marijuana less than 50 times in their lives) on a battery of 10 neuropsychological tests. … Former heavy users, who had consumed little or no cannabis in the three months before testing, [also] showed no significant differences from control subjects on any of these tests on any of the testing days.”

Additionally, studies have also implied that cannabis may be neuroprotective against alcohol-induced cognitive deficits. A 2009 study by investigators at the University of California and San Diego reported that binge drinkers who also used cannabis experienced significantly less white matter damage to the brain as compared to subjects who consumed alcohol alone.

For more information regarding the impact of cannabis on brain function, see NORML’s factsheet ‘Cannabis and the Brain: A User’s Guide,’ here.

Such is the case with a pair of just-published studies slated to appear in the journal Schizophrenia Research. The first study, conducted by a team of researchers at various New York state hospitals, the Yale University School of Medicine, and the National Institutes of Mental Health assessed whether there exists a causal association between cannabis use and the age of onset of psychosis in patients hospitalized for the first time for an episode of schizophrenia.

Despite previous media claims to the contrary, researchers concluded:
“Although the onset of cannabis use disorder preceded the onset of illness in most patients, our findings suggest that age at onset of psychosis was not associated with cannabis use disorders. Previous studies implicating cannabis use disorders in schizophrenia may need to more comprehensively assess the relationship between cannabis use disorders and schizophrenia, and take into account the additional variables that we found associated with cannabis use disorders.”

A separate study slated for publication in the same journal assessed the cognitive skills of schizophrenic patients with a history of cannabis use compared to non-users. Authors reported that patients with a history of marijuana use “demonstrated significantly better performance on measures of processing speed, verbal fluency, and verbal learning and memory” compared to abstainers. Marijuana use was also associated with better overall GAF (Global Assessment of Functioning) scores compared to those of non-users.

Authors concluded: “The results of the present analysis suggest that (cannabis use) in patients with SZ (schizophrenia) is associated with better performance on measures of processing speed and verbal skills. These data are consistent with prior reports indicating that SZ patients with a history of CUD (cannabis use disorders) have less severe cognitive deficits than SZ patients without comorbid CUD. … The present findings also suggest that CUD in patients with SZ may not differentially affect the severity of illness as measured by clinical symptomatology.”

Both study’s findings are in line with previous (though virtually unreported) research indicating that marijuana is unlikely to instigate incidences of schizophrenia in the general population, that cannabis use among patients with the disease is associated with higher cognitive function, and that at least some schizophrenics find subjective relief from symptoms of the illness by using pot. Nonetheless, odds are the nobody from the mainstream media will be champing at the bit to report on them.

Bottom line: marijuana’s complex relationship with schizophrenia is far from understood, and likely won’t be for some time. But that doesn’t give the MSM a free pass to only promote one side of the story.

The consumption of cannabis, even long-term, has a “minimal” impact on brain function, according to a systematic literature review just published online by the journal Psychological Medicine.

An international team of investigators from the United Kingdom, Spain, Brazil, Australia, and Switzerland conducted a systematic review of the effects of cannabis on brain structure and function.

Authors wrote, “We reviewed literature reporting neuroimaging studies of chronic or acute cannabis use published up until January 2009. … Sixty-six studies were identified, of which 41 met the inclusion criteria. Thirty-three were functional (SPECT/PET/fMRI) and eight structural (volumetric/DTI) imaging studies. … Only three of the structural imaging studies found differences between users and controls.”

Investigators concluded, “Minimal evidence of major effects of cannabis on brain structure has been reported,” noting that marijuana users and controls perform similarly on cognitive tasks.

According to a 2001 study published in the journal Archives of General Psychiatry, long-term cannabis smokers who abstained from pot for one week “showed virtually no significant differences from control subjects (those who had smoked marijuana less than 50 times in their lives) on a battery of 10 neuropsychological tests.” Investigators added, “Former heavy users, who had consumed little or no cannabis in the three months before testing, [also] showed no significant differences from control subjects on any of these tests on any of the testing days.”