Numerous studies tout marijuana for its brain protective benefits, from helping brain cells grow to shielding the brain from chronic stress. However, a new study in the Journal of Alzheimer's Disease suggests long-term marijuana use may reduce blood flow to the brain, specifically the hippocampus, and increase the susceptibility for 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,” said Dr. Elisabeth Jorandby, co-author of the study, in a statement.

Compared to healthy participants, pot smokers displayed low blood flow in the hippocampus, which is involved in storing long-term memory, including all past experiences. It becomes especially vulnerable to damage at the early stages of Alzheimer’s since the disease leads to dysfunctional neurogenesis (growth of brain cells), which can cause memory impairment.

Read More: MRI Scans Show Smoking Weed Lowers Hippocampus Activity, Creating False Memories

Jorandby and her colleagues sought to determine if single photon emission computed tomography (SPECT), a sophisticated imaging study that evaluates blood flow and brain activity patterns, could distinguish marijuana user brains from healthy, control brains. The researchers obtained data from approximately 1,000 current or former marijuana users and about 100 healthy participants using SPECT at rest and during a mental concentration task. Not only did low blood flow in the hippocampus distinguish smokers from controls, but the right hippocampus was found to be the most affected. 

The researchers believe marijuana use may interfere with memory formation by inhibiting activity in this part of the brain. The study suggests marijuana use has damaging effects on the brain, particularly in areas important in memory and learning, which are known to be affected by Alzheimer’s. It’s one of the first brain regions to shrink in people with the disease.

Long-term pot smoking and even halting exercise for a few days show similar effects on brain blood flow. A 2016 study found reduced blood flow to the brain can occur in people who stop exercising for 10 days. The resting cerebral blood flow significantly dropped in eight brain regions, including the areas of the left and right hippocampus and several regions known to be part of the brain’s “default mode network” — a neural network known to deteriorate quickly with a diagnosis of Alzheimer’s disease.

Read More: Hippocampus Shape, Not Bulk Volume, Indicates How Well Our Memory Functions

There are lifestyle factors that can be controlled to reduce the risk of developing diseases such as Alzheimer’s. Whether it’s to smoke less or stop smoking, or exercise more, these studies suggest lifestyle choices influence brain health in more ways than previously believed.

"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." said Dr. George Perry, Editor in Chief of the Journal of Alzheimer's Disease.

The study proposes the idea that perhaps marijuana legalization is moving faster than research would support.

Source: Amen, DG, Darmal, B, Raji, CA et al. Discriminative Properties of Hippocampal Hypoperfusion in Marijuana Users Compared to Healthy Controls: Implications for Marijuana Administration in Alzheimer’s Dementia. Journal of Alzheimer’s Disease. 2016.