Marijuana users have abnormally low blood flow in virtually every part of the brain, new scans reveal.

Sophisticated imaging of 1,000 cannabis users' brains found all of them had widespread restrictions or build-ups of blood flow.

Many had abnormal blood levels in areas affected by Alzheimer's disease, such as the hippocampus.

The findings, published in the latest Journal of Alzheimer's Disease, are an ominous warning as the United States rapidly embraces recreational and medical marijuana legalization.

It comes just weeks after the White House's surgeon general, Dr Vivek Gupta, warned legalization is moving faster than research.

Health dangers? Sophisticated imaging of 1,000 cannabis users' brains found 1,000 marijuana users had widespread restrictions or build-ups of blood flow (file image)

The researchers at Amen Clinics analyzed data from a broad database including 26,268 patients across the US between 1995 and 2015.

The patients - from California, Washington, Virginia, Georgia and New York - all had complex treatment-resistant issues, and all underwent single photon emission computed tomography (SPECT) - a sophisticated imaging study that evaluates blood flow and activity patterns while undergoing concentration tests.

One thousand of the patients were marijuana users.

Comparing those patients' brain scans with 100 healthy controls, the researchers saw a stark difference in blood flow levels.

Every marijuana user had significantly lower blood flow in the right hippocampus compared to the controls.

Marijuana use is thought to interfere with memory formation by inhibiting activity in this part of the brain.

Co-author Dr Elisabeth Jorandby said even she was shocked by the findings, despite dealing with marijuana patients on a routine basis.

'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,' she said.

'Our research has proven that marijuana users have lower cerebral blood flow than non-users.

'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 George Perry, editor in chief of the Journal of Alzheimer's Disease said: '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.'

Dr Daniel Amen, founder of Amen Clinics, said: '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.