Green’s Dictionary of Slang

(drugs) the early use of heroin, before actual addiction, during which period the user can stop without any real physical or mental pain.

1934		F. Tuohy Inside Dope 195: ‘Helldust’, ‘coke’, ‘the needles’ ‘honeymoon stage’ [...] all are of American origin.
1952		Lannoy & Masterson ‘Teen-age Hophead Jargon’ AS XXVII:1 27: HONEYMOON, n. The time when normal doses of heroin are taken.
1953		Anslinger & Tompkins Traffic In Narcotics 310: honeymoon stage. Drug-exhilaration during its most agreeable period.
1959		J.E. Schmidt Narcotics Lingo and Lore.
1971		E.E. Landy Underground Dict. (1972).
2001		ONDCP Street Terms 12: Honeymoon — Early stages of drug use before addiction or dependency develops.