The risk of initiating injecting drug use is eight times higher for patients who receive long-term prescription opioid medication for non-cancer pain compared with that of opioid naïve patients, a large retrospective cohort study from Canada has found. However, the five-year cumulative probability of injecting drug use initiation in this group was only 4%.

The authors used data from a large administrative data source, the Integrated Data and Evaluative Analytics (IDEAs) cohort, covering approximately 1.7 million people in British Columbia to investigate any association between prescription opioid treatment for non-cancer pain and initiation of injecting drug use among individuals without a history of substance misuse at baseline.

Prescription of opioid medication for non-cancer pain was identified from PharmaNet, a province-wide database, for drugs dispensed between 2000 and 2015.

Prescribing episodes were classified by length and intensity of opioid use into acute, episodic, and chronic.

Injecting drug use initiation was identified by means of a high specificity algorithm.

A total of 59,804 participants (14,951 from each opioid use category) were included in the matched cohort. Around half, 30,140, of the participants were male and the median age was 44. Median follow-up was 5.8 years. Overall, 1,149 participants commenced injecting drug use during the study period.

Half of the chronic opioid episodes were longer than two years.

The cumulative probability of injecting drug use initiation at five years was highest for participants with chronic prescription opioid use and lowest in those who were opioid naïve:

• Chronic prescription opioid use (4.0%)

• Episodic use (1.3%)

• Acute use (0.7%)

• Opioid naïve (0.4%)

Initiation of injecting use was eight times higher, hazard ratio (HR) 8.4 (95% CI: 6.4-10.9), for those with chronic prescription opioid use compared with those who were opioid naïve.

Initiation of injecting drug use was more frequent at higher opioid doses and younger ages.

In a sensitivity analysis limited to individuals with a history of chronic pain, cumulative risk for those with chronic use (3.4% within five years) was lower than the primary results. However, the hazard ratio was higher; HR 9.7 (95% CI: 6.5-14.5).

The risk of developing substance misuse problems while on long-term prescribed opioid medication is well documented, and a systematic review of prescription opioid treatment for chronic pain estimated average rates of opioid misuse and addiction to be 21-29% and 8-12%, respectively.1 However, little is known about the risk of initiation of injecting drug use.

The study authors conclude: ‘The rate of injection drug use initiation among individuals who received chronic prescription opioid treatment for non-cancer pain was infrequent overall (3-4% within five years), but

8-10 times higher than opioid naïve individuals. Rate of initiation was more frequent with greater doses and at younger ages.’

However, they caution that their findings ‘should not be used as a reason to inappropriately taper or discontinue long term prescription opioid treatment.’

Though initiation of injecting use was relatively uncommon in those prescribed opioid medication for the management of chronic non-cancer pain, GPs should be aware of this potential harm when considering prescribing opioids.

Dr Jez Thompson