Legalization may provide a net health benefit for Mexican gang members. Whether legalization will provide a net health benefit for American citizens is still a point of contention.
For example, the murder rate in prison seems to be actually lower than outside. There's also less risk of dying in a car accident, etc.
Before you get outraged, note that I'm only talking about health effects, not happiness or achievement.
With criminalisation we see drives to stronger forms of the drug (EG: bootleggers were mostly not selling beer, but spirits). Decriminalisation and legalisation would allow people to grow and actively market weaker forms of the drug.
Criminalisation pushes people towards unhealthier forms of drug taking because it's legally tricky to provide harm reduction information or harm reduction devices. Most people smoke cannabis; and most people mix it with tobacco. That's a pretty unhealthy way to take it. Legally being allowed to sell vaporisers would help.
I get confused about what is or isn't "drug paraphernalia" and whether it's legal or not.
Might end up with a few less killed violently. Might end up with more money to spend on social programs instead of militarised police. Might end up with much more tax from economic activity brought out of the black market. Might end up with less contaminated produce.
All of these could end up being better for the health of American citizens.
And even if not you're bringing your problems in-house rather than shipping them off to poorer places and causing suffering there.
Both of these can be solved by legalizing and regulating recreational drugs.