Why Addicts Overdose: Learned Tolerance

At first glance, the question of why addicts overdose seems absurd given the apparent straight-forwardness of the situation; an addict enjoys taking a drug, over time takes more of it, and eventually takes more than his body can handle which results in an overdose. However, things are not quite so simple. Since the victims of overdose are typically long-term users rather than novices, we can expect that they would have an extensive history with the substance and as a consequence will have developed a significant tolerance to the drug, and (looking specifically at heroin) this means that the user would need high levels of the drug to induce a fatal respiratory depression. When we compare heroin addicts who died from an overdose versus those who died through homicide, we find that the majority of victims in the "overdose" group had no higher levels of morphine in their blood than the comparison group¹. The conclusion of this study was that, for the majority of overdose victims, the death could not be attributed to a toxic quantity of morphine in the blood. Even 30 years ago, the problems with the standard "overdose" story were succinctly summarised by Brecher², who said:

1. the deaths cannot be due to overdose,
2. there has never been any evidence that they are due to overdose,
3. there has long been a plethora of evidence demonstrating they are not due to overdose.

TOLERANCE

To understand why people have claimed that it is a misnomer to attribute these deaths to the traditional understanding of "overdose", we have to look at the factors that influence the development of drug tolerance and why the usual processes of tolerance failed. Tolerance is usually defined as the decreasing effects of a drug through repeated administrations, but even as far back as the 60's researchers were arguing that a complete explanation of tolerance requires an element of learning. This was argued on the basis that there were findings that could only be explained from a learning perspective; for example, the observation that the analgesic effect of morphine can persist in rats even after a number of drug-free months³.

Due to the way we normally conceive of 'tolerance' and our reliance on the purely physiological model, the idea that learning affects our biological tolerance to drugs can be quite a difficult concept to get our heads around. However, after looking at how classical conditioning can affect our response to the placebo effect (and the functioning of our immune system), we can look at how classical conditioning could play a role in drug tolerance. By looking at how classical conditioning was proposed to work by Pavlov, with a previously neutral stimulus (e.g. a bell) being paired with a unconditioned stimulus (e.g. food) and the neutral stimulus taking on the value of the unconditioned stimulus to produce the same effects (the sound of the bell becoming capable of making a dog salivate in the same way food does), we can begin to understand how classical conditioning could affect drug tolerance.