“It is unclear whether cannabis use causes cognitive decline; several studies show an association between cannabis use and cognitive decline, but quasi-experimental twin studies have found little support for a causal effect.

Here, we evaluate the association of cannabis use with general cognitive ability and executive functions (EFs) while controlling for genetic and shared environmental confounds in a longitudinal twin study.

CONCLUSIONS:

We found little support for a potential causal effect of cannabis use on cognition, consistent with previous twin studies. Results suggest that cannabis use may not cause decline in cognitive ability among a normative sample of cannabis users.”

https://www.ncbi.nlm.nih.gov/pubmed/31753729

“Overall, there was little evidence for causal effect of cannabis on cognition.”

https://www.sciencedirect.com/science/article/abs/pii/S0376871619304892?via%3Dihub