A new study warns that marijuana use may increase susceptibility to Alzheimer's disease, after finding the drug severely reduces blood flow in an area of the brain affected by the illness.

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Researchers suggest marijuana use may increase susceptibility to Alzheimer's by reducing blood flow in the hippocampus.

Published in the Journal of Alzheimer's Disease, the study reveals that individuals with a marijuana use disorder showed reduced blood flow in nearly all areas of the brain, compared with healthy controls.

What is more, the research team - including co-author Dr. Elisabeth Jorandby of Amen Clinics Inc. in California - found that the hippocampus saw the largest reduction in blood flow with marijuana use.

The hippocampus is the brain region associated with learning and memory, and it is the first region to be affected in patients with Alzheimer's disease.

In the United States, marijuana is becoming increasingly legalized for recreational and/or medicinal use.

With this in mind, researchers are in agreement that it is more important than ever to understand the possible harms of marijuana use, and Dr. Jorandby and colleagues caution that reduced brain blood flow may be one such effect.

Almost every brain region affected by marijuana use

When blood flow in the brain is reduced, this causes a reduction in the amount of oxygen that reaches brain cells, which can cause brain tissue damage and death.

According to the authors, few previous studies have assessed the effects of marijuana use on blood flow in the brain.

To address this research gap, the team used single photon emission computed tomography (SPECT) to measure the blood flow and brain activity of 982 individuals who had been diagnosed with a marijuana use disorder, alongside 92 healthy controls.

SPECT was used to measure participants' brain blood flow and activity during a mental concentration task and when at rest.

Compared with the healthy controls, the researchers found that subjects with marijuana use disorders showed significantly reduced blood flow in almost all brain regions, but the hippocampus fared worst.

In particular, the team identified abnormally low blood flow in the right hippocampus of subjects with marijuana use disorders as they completed the concentration task.

Talking to Medical News Today, study co-author Dr. Cyrus Raji, of the University of California-San Francisco, said the team was surprised by just how much marijuana use affected brain blood flow.

"Prior papers have suggested that marijuana can damage the brain. What surprised [us] was how low blood flow was in the brains of our cohort - virtually every brain area had reduced blood flow on perfusion imaging in relation to marijuana use," he told us.

Findings suggest marijuana has 'damaging influences in the brain'

The researchers note that marijuana use is believed to impede activity in this brain region to disrupt memory formation, and previous studies have associated weakened blood flow in the hippocampus with Alzheimer's disease.

"As a physician who routinely sees marijuana users, what struck me was not only the global reduction in blood flow in the marijuana users' brains, but that the hippocampus was the most affected region due to its role in memory and Alzheimer's disease," says Dr. Jorandby.

"Our research has proven that marijuana users have lower cerebral blood flow than nonusers. Second, the most predictive region separating these two groups is low blood flow in the hippocampus on concentration brain SPECT imaging.

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."

Dr. Elisabeth Jorandby

Study co-author Dr. Daniel Amen, founder of Amen Clinics Inc., believes the team's findings should act as a word of caution for marijuana users.

"Our research demonstrates that marijuana can have significant negative effects on brain function. The media has given the general impression that marijuana is a safe recreational drug, this research directly challenges that notion," he says.

Based on their results, Dr. Raji told MNT that frequent cognitive testing for patients prescribed medical marijuana is something that is "certainly worthy of careful consideration."

He added that the team now plans to conduct further research with the aim of pinpointing "actual risk levels of dementia" for individuals who use marijuana.

Read how marijuana may lower dopamine levels to trigger mental illness.