Pavlovian conditioning (also called classical conditioning) is a kind of learning. It is where we take a unlearned response and learn to make that response to a new stimulus. For years, the idea that drug overdose may be related to a particular form of classical conditioning has been suggested by learning theorists. A recent paper in Current Directions in Psychological Science, reviews the evidence;
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. I describe an explanation based on Pavlovian conditioning. 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. Overdose prevention strategies should incorporate information about the contribution of drug-associated cues to drug tolerance.
peakmemory 
I have been in the counseling profession since 2001. Opioid use has skyrocketed in the past three years. Pretty much everyone I assess who uses IV heroin also has Hep C. I think it is the combination of ease of access on the streets for heroin, and every Tom, Dick, and Harry doctor prescribing pain pills. Very sad epidemic now.
Thank you for the important work that you do.