A recently published study titled “Investigating the causal effect of cannabis use on cognitive function with a quasi-experimental co-twin design as found that there is “little evidence for causal effect of cannabis on cognition.” It showed clear evidence that cannabis use “does not cause a decline in cognitive ability among a normative cannabis using sample.”

It’s important to mention that studies can’t show whether cannabis actually caused changes in IQ. Instead, because there are so many other factors present in our environment that could be the cause of lowering IQ, even among individuals who use cannabis regularly, some of whom might have a lower IQ due to other factors. This is important to take into consideration if you ever come across a study suggesting that cannabis use lowers IQ.

“A co-twin control study is uniquely designed to examine causal relationships,” explains the study’s lead author Dr. Jessica Megan Ross, at University of Colorado Boulder. “Identical twins share 100% of their genes and shared environmental factors (e.g., grew up in the same household). Thus, comparing within twin pairs allows us to control for genetics and numerous environmental factors.” (source)

Ross’ twin study also tracked cannabis use and cognitive from adolescence through early adulthood but used 428 pairs of twins rather than individuals. By tracking twins who differed in their cannabis use, Ross and her team were able to see whether cannabis use caused changes within twin pairs. If cannabis really causes neurotoxic effects on the teenage brain,  then cannabis using twins should have lower scores on cognitive function than their non-cannabis using counterpart. But that isn’t what they found.

“Our study refutes the results from these studies which suggest that cannabis use is associated with poorer cognitive function.”

Dr. Jessica Megan Ross, Institute for Behavioral Genetics, University of Colorado

Instead, they found that in cases where one twin used cannabis, both twins were more likely to suffer drops in IQ.

“Our study refutes the results from these studies which suggest that cannabis use is associated with poorer cognitive function,” explains Ross. “Our study did find significant associations between cannabis use and cognition phenotypically. However, when we controlled for genetics, shared environmental factors between twins, and other substance use, these significant associations disappeared.” (source)