You mean the stealing for money for high priced black market drugs?
You mean the shootouts/high speed chases where people run from the cops because they have a bag on them?
Or do you mean lighting up a joint?
The thing is, it's very easy to look at the statistics of violent crimes before and after. Also the vast majority of law enforcement agencies classify their crimes so you get detailed statistics of what kinds of crimes occur on a monthly/yearly basis.
Let's say that drugs are illegal and get prosecuted, and are counted as crime in the statistics.
If you legalize them, your amount of crime in your statistics drops, but the actual acts are the same in real world.