While it raises concerns, a few things to note: It is one primary study that has just been released and is undergoing further peer-review. Results need to be replicated by other researchers with better controls and varied methods. Why was the control group (92) so much smaller than the cannabis group (982)? This is unusual for this type of study. Possible selection bias, this was heavy current use of cannabis by people who managed to be diagnosed with "cannabis use disorder". Reduced blood flow is a pharmacological effect of cannabinoids. They can constrict blood vessels. The researcher may simply be observing something that for the most part wears off with abstinence (it can take days/weeks for the heaviest of users to clear the system). Other research has suggested cannabinoids may help treat and prevent Alzheimer's.