I've heard (but I don't have a link right now) that legalization of marijuana has already drastically reduced the amount of marijuana being brought into the country, so I expect further legalization to increase this effect. Marijuana is one of, if not
the most lucrative drugs for traffickers.
I don't think rehab will have nearly as big of an effect as legalization/regulation. We should be looking for safer alternatives to more dangerous, often abused drugs. For examine, "magic mushrooms" are generally safer than other psychedelics, and LSD can be relatively safely used under supervision (and has potential for medical uses as well), so we should be considering those for legalization as well once marijuana is legalized.
It's far easier to control a problem if you control distribution than if you don't. Attacking it from a rehab angle doesn't solve the biggest problem with drugs: trafficking. Legalization solves that aspect, and taxes on drugs could fund rehab, so it's a win win situation.
Therefore, I want drug legalization as the primary focus for curbing drug-related violence specifically, and violent crime generally, since I believe it will have the largest positive effect for zero cost (in fact, it should be a net win).