Heroin Toxicity and Opiate Overdose
"A striking finding from the toxicological data was the relatively small number of subjects in whom morphine only was detected. Most died with more drugs than heroin alone 'on board', with alcohol detected in 45% of subjects and benzodiazepines in just over a quarter. Both of these drugs act as central nervous system depressants and can enhance and prolong the depressant effects of heroin."
Zador, Deborah, Sunjic, Sandra, and Darke, Shane, "Heroin-related deaths in New South Wales, 1992: toxicological findings and circumstances," The Medical Journal of Australia, 1996; 164 (4): 204-207.