Infiltrate the source and poison the crap. Execute all dealers from kingpin to street level. Repeat as necessary.
We see a little E Coli in Nestlé Toll House cookie dough and 300,000 cases are recalled and destroyed. I do not know, however, if drug contamination could be successfully pulled off because of the legal liability. If it was done surreptitious, where it could not be traced to the U.S. Government, it might work. I would not support something deadly. If the person temporarily turned green so they got the mark of the Beast that identified them as a drug user, that would be best. Once the user understood there was a chance they could turn green, it would be the same effect as those Toll House cookies. I am sure we have biochemists that could come up with a similar technology.