The authors also point out that there’s a much stronger link between violent behavior and alcohol than there is for many illegal drugs: “There is a good deal of evidence showing an association between alcohol intoxication and pharmacologically induced violent crime,” they write.

They added: “There is little direct association between marijuana or opiate use and violent crime. … it is also possible that for some would-be offenders, the pharmacological effect of certain drugs (marijuana and heroin are often given as examples) may actually reduce violent tendencies.”
https://www.washingtonpost.com/news/wonk/wp/2016/09/07/why-medical-marijuana-patients-cant-buy-guns/