The levels of use of opioids, amphetamines and cocaine and associated levels of harm: summary of scientific evidence

This report presents the findings of a literature review to identify the most frequently occurring patterns of use and their relation to harm in users of opioids, powder and crack cocaine, and meth/amphetamine. The behavioural factors that were studied included: frequency of use, duration of use, routes of administration, drug type, dose, severity of dependence, and (the presence of) polydrug use. Research on stimulants covers a relatively broad spectrum of patterns and severity, and thus provides some indications of the levels of use that are more harmful than others. Similar evidence is relatively scarce for opioids, where the overwhelming majority of studies concentrate only on the most risky injecting and addictive use.
For cocaine and amphetamines, it appears that weekly and higher frequency of use and patterns involving heavy periods of continuous use (bingeing) are related to increased prospective risk or actual existence of harms. Similar conclusion cannot be made for opioids, although research provides some indication of controlled use of heroin on a weekly and monthly (or less frequent) basis. Similarly, some evidence exists that crystal forms of stimulants - crystal methamphetamine and crack cocaine - are often positively associated with more harmful patterns of use and more severe consequences, whereas very little attention is paid in the literature to the different forms of heroin/opioids. Routes of administration range from injecting, through smoking and inhaling, to snorting and oral consumption when ranked from the riskiest to less risky routes, although the less risky routes of snorting and swallowing are not considered to be risk-free behaviours. Frequency and duration of use are likely moderators of harms associated with routes of administration. Injectors are at higher risk of transmission of drug-related infectious diseases and death, the former being a function of the frequency and patterns of injecting.
Polydrug use, although not a primary concern of the present review, proved to be an extremely significant confounding factor of any harm associated with use of these substances. It indicates a particular level of compulsivity and is closely associated with higher levels of dependence and with the risk of overdose.