Green’s Dictionary of Slang

(US drugs) narcotics; narcotic addiction.

1938		D. Maurer ‘Lang. of the Und. Narcotic Addict’ Pt 2 in Lang. Und. (1981) 101/2: dr. white. A euphemism referring to drugs or drug-addiction.
1949		Monteleone Criminal Sl. (rev. edn) 76: dr. white Term used by narcotic addicts to camouflage a reference to narcotics.
1953		Anslinger & Tompkins Traffic In Narcotics 307: Doctor White [...] Drugs, especially cocaine, heroin, or morphine.