Heroin overdose deaths in the Unites States more than tripled from 2010 to 2014, reaching almost 11,000 per year. Despite the use of the term 'overdose,' many of these victims died after self-administering an amount of opiate that would not be expected to be fatal for these drug-experienced, and drug-tolerant, individuals. Various explanations of this overdose mystery have been proposed."

Source :The Heroin Overdose Mystery Current Directions in Psychological Science, Dec 2016

Author of the paper, Professor Shepard Siegel at the Department of Psychology, Neuroscience and Behaviour, McMaster University, US, suggests that these types of reaction might be an extreme version of a Pavlovian response :

Organisms associate cues present at the time of drug administration with the systemic effect of the drug. These drug-predictive cues come to elicit responses that attenuate the effect of a drug. Such anticipatory conditional responses mediate chronic tolerance. If the drug is administered in the presence of novel cues, tolerance fails to occur and the victim suffers an overdose."