Green’s Dictionary of Slang

(US drugs) tablets/capsules of demerol.

1956		Jess Stearn Sisters of the Night 59: ‘If you can’t buy H or M, why, demmies will do the trick.’.
1977		L. Young et al. Recreational Drugs.
1981		D.E. Miller Bk of Jargon 337: demis: Demerol.