Results of a neuroimaging study show harmful effects of marijuana on the brain.

Using single-photon emission computed tomography (SPECT) in marijuana users and healthy nonusers, researchers found abnormally low blood flow in multiple brain regions in marijuana users, particularly in areas known to be affected by Alzheimer's pathology, and this "reliably" distinguished marijuana users from nonusers.

"While legalization of marijuana may open pathways to new treatment applications, this study shows reduced global cerebral perfusion in the brain of almost 1000 marijuana users, particularly in the hippocampus, a brain area vital for memory and learning and also vulnerable to reduced blood flow in Alzheimer's," study investigator Cyrus Raji, MD, PhD, from University of California, Los Angeles, told Medscape Medical News.

Prior studies have suggested that the hippocampus physically shrinks with marijuana use, but none have looked at perfusion in a group this large, he noted.

"The finding that smoking pot is toxic to the brain was not a surprise," first author Daniel Amen, MD, from Amen Clinics Inc, Costa Mesa, California, told Medscape Medical News. "The thing that was surprising was the global reduction in blood flow and the decreases particularly in the right hippocampus."

The study was published online November 24 in the Journal of Alzheimer's Disease.

"Troubling" Effects

The findings are based on brain SPECT studies at rest and during a concentration task in 982 individuals with a diagnosis of cannabis use disorder and 92 healthy controls.

Marijuana users showed lower cerebral perfusion on average (P < .05). Discriminant analysis distinguished marijuana users from controls with correct classification of 96% and leave-one-out cross-validation of 92%.

Hippocampal perfusion in marijuana users was 13% lower on concentration scans compared with that in controls, with a large effect size (Cohen's d = 0.99 for left and 1.03 for right). "Notably," say the researchers, right hippocampal hypoperfusion on concentration SPECT was the most predictive in separating marijuana users from nonusers.

"This work suggests that marijuana use has damaging influences in the brain — particularly regions important in memory and learning and known to be affected by Alzheimer's," coinvestigator Lantie Elisabeth Jorandby, MD, from Amen Clinics, said in a statement.

"Open use of marijuana, through legalization, will reveal the wide range of marijuana's benefits and threats to human health. This study indicates troubling effects on the hippocampus that may be the harbingers of brain damage," added Dr George Perry, editor in chief of the Journal of Alzheimer's Disease.

"Twenty-six states have legalized marijuana for recreational use, and quite frankly I am completely for legalization," Dr Amen added. "However, there are 10 states that have approved marijuana for dementia and that's just not smart."

"There is no literature that marijuana is helpful in Alzheimer's disease," he said. "There are animal studies that show that marijuana can help break up β-amyloid plaque formation, which is thought to be one of the causes of Alzheimer's disease. From those animal studies, some people have inferred that this is good for Alzheimer's disease. The problem is that all of the amyloid drug trials are failing, so it's much more complex than that."

J Alzheimers Dis. Published online November 24, 2016. Abstract

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