> The converse is also true, for example legalizing cannabis in Canada has significantly increased demand for it

The relevant thing that link actually says is that more survey respondents admitted to cannabis use after legalization, the obvious problem being that before legalization they would be admitting to a crime, which will suppress response rates.

The same link also points out that the legalization happened right before COVID and then you have a major confounder because even if cannabis use is actually up, you don't know if it's because of legalization or people turning to cannabis over stress from COVID. Moreover, the reported usage increased during COVID but started to decline in 2023. This implies that either the apparent spike was COVID, or that it was something like media reports about recent legalization acting as temporary free advertising and causing a temporary increase in usage. Neither of those is evidence of a sustained increase in demand.

Meanwhile legal options do cause people to prefer legal sources over the black market, and then you get fewer people becoming addicts because the thing they thought they were buying was spiked with something significantly more addictive by a black market seller. Or the black market products have higher variation in the dose and then customers can't predict how much they're getting and occasionally take more than expected, leading to a higher rate of overdose and stronger dependency-inducing withdrawal.