As a bonus health of patients(addicts) is not deteriorating so rapidly due to crap they take along with actual drugs, so bonuses go back to whole society in form of lower costs of general healthcare. Imagine cigarettes that would be killing 10x more slowly/less often. Smokers would often die of old age and not in 1-5 years of super expensive agony. Safer methods of applications can be used.
The benefits just mount for whole society. All it takes is general acceptance that there will always be some part of society that will want to get high / calm the existential pain, be it physical of psychical.