No, they should be legalised, regulated, and taxed and run as legitimate business.

Decriminalisation is not a good half way step, because it is more of "we tolerate this". It is harder to tax / regulate and manage legitimate businesses. For example, in Holland cannabis is decriminalised, but the coffee shops cannot (couldn't?) buy it legally and still had to get it from criminal networks.

The only way is complete legislation. No pick and choose. Cannabis legal, MDMA legal, heroin no no no. It would still leave a market for the blackmarket to control and those in are perhaps the most in need of legalised services. For example, anyone buying a gram of heroin a day probably needs help / advice / guidance more than someone buying half a gram of coke each Friday night.

I wouldn't even stop at legislation. In some cases, the drugs should be prescribed freely in the most severe cases of addiction to either help them taper off, or just to accept they will need to maintain a drug dependence until death to leave as relatively normal and productive life as possible. Which ends up better and cheaper for all