All data were obtained for analysis from a large multisite database, involving 26,268 patients who came for evaluation of complex, treatment resistant issues to one of nine outpatient neuropsychiatric clinics across the United States
But "people with serious neuropsychiatric people who used cannabis have low blood flow to the brain" is both less clickworthy and less politically useful than "OMG pot rots yr brain!"
And I love this: "As a physician who routinely sees marijuana users..." Yeah, that's called "a physician". Cannabis use is common, every physician has seen patients who has used it.
Both Amen and this methodology are poorly regarded. He's in the addiction treatment industry -- look at this is an old marketing pitch of his quoted in a Quackwatch article:
How your brain and soul work together determines how happy you feel, how successful you become, and how well you connect with others. The brain-soul connection is vastly more powerful than your conscious will. Will power falters when the physical functioning of the brain and the health of your soul fail to support your desires, as seen by illogical behaviors like overeating, smoking, drug and alcohol abuse, and compulsive spending.