History of medication-assisted treatment and its association with initiating others into injection drug use in San Diego, CA

Medication-assisted treatment (MAT) remains the gold standard of biomedical care for opioid use disorder, and is effective in reducing the frequency of injecting among people who inject drugs (PWID) [1‐4]. This is important given the growing scientific consensus that PWID play a key role in the expansion of injection-related epidemics by exposing and directly initiating injection-naïve persons into injection drug use (IDU) [5, 6]. With an estimated 12 million PWID worldwide, and an increasing prevalence of opioid-related morbidity and mortality in North America and elsewhere, the prevention of opioid IDU initiation (e.g. heroin) has major public health implications [1, 2, 7, 8]. This is particularly the case as the period immediately following IDU initiation has been shown to be associated with a higher risk of HIV and HCV acquisition [9].

MAT includes opioid agonist treatment (i.e., methadone; also known as opioid substitution therapy), in combination with counseling and behavioral therapies to treat opioid use disorder [1‐4]. Given that MAT is associated with reductions in the frequency of opioid injecting among PWID as well as street-based injecting in particular, we hypothesize that MAT enrollment may have a secondary preventive impact on the risk that PWID expose and initiate others into injecting [3‐10]. This study therefore investigated the potential association between a history of MAT enrollment and reporting injection initiation assistance among PWID.

Participants (n = 354) were predominantly male (n = 249, 70%), with a mean age of 47 years (Interquartile Range [IQR]: 38–55), and a median of 24 years of IDU (IQR: 13–35). Thirty-eight percent of participants (n = 135) reported ever providing injection initiation assistance, and 39% (n = 137) reported ever having been enrolled in MAT. The proportion of MAT enrollment for males was 67.9% (n = 93) and 32.1% (n = 44) for female and transgender participants. The majority of participants (n = 304, 86%) reported having ever injected heroin.

As shown in Table 1, there was a significantly higher proportion of participants ≤30 years old who reported ever providing injection initiation assistance compared with older participants aged 31–50 and ≥51 years (69.0% vs. 37.7% vs. 32.9%, Fisher’s exact p < 0.01).

As shown in Table 2, each year increase in age was associated with a decreased odds of ever providing injection initiation assistance (Wald χ² = 13.27, degrees of freedom [df] = 1, p < 0.01), while reporting a higher number of years since first injection was associated with an increased odds initiating others (Wald χ² = 5.53, df = 1, p = 0.02). PWID reporting a history of MAT enrollment had significantly decreased odds of initiating others into injecting (Wald χ² = 4.04, df = 1, p = 0.04; see Table 2). Additionally, the inclusion of an interaction term in a separate multivariate model did not significantly impact the association between gender and a history of MAT enrollment (χ² = 1.13, df = 1, p = 0.29; data not shown).

Along with its effectiveness in supporting the management of opioid use disorder [1‐4], these preliminary results suggest that MAT enrollment may also be associated with a reduced risk that PWID initiate others into IDU. Specifically, results suggest that among PWID participants, a history of MAT enrollment was associated with a 38% reduction in the odds of having reported initiating others into IDU. This suggests a need to further explore potential pathways by which MAT enrollment may influence the risk that PWID provide injection initiation assistance.

In line with other studies, we also found that each year increase in age was associated with a decreased risk of providing injection initiation assistance [16, 17]. However, in contrast to these studies we observed an association between a higher number of years since first injection and an increased risk of providing injection initiation assistance.

Multiple studies have reported on gender differences in injection initiation, including data suggesting that men are most often initiated by men comparted to women [12‐15, 18]. Additionally, data suggest that some gender-responsive programs may influence the capacity of PWID to engage with supplementary health services offered during MAT enrollment [11, 19‐25]. However, the effect of a history of MAT enrollment on providing injection initiation assistance did not differ significantly between male and female participants in our sample. Further quantitative and qualitative studies are needed to more clearly delineate potential differences by gender with respect to injection initiation risk and uptake of MAT.

To our knowledge, this is the first study to investigate the potential impact of MAT enrollment on providing injection initiation assistance. As such, and due to the exploratory nature of this analysis, results should be interpreted cautiously. First, survey items assessing lifetime initiation of others into IDU were limited to the final follow-up of a 24-month observational cohort study, and we were therefore unable to identify the temporal ordering of the dependent and independent variables, and, as such, cannot confirm the direction of the causal association. It may be the case that both enrollment in MAT and avoiding the initiation of others into IDU are both proxy markers of increased capacity by participants to manage their opioid use and we note that this will be the subject of future longitudinal study from our group. Second, providing injection initiation assistance is a highly stigmatized behavior and likely resulted in under-reporting of this behavior [13, 26]. However, there is no reason to believe that differential under-reporting occurred among PWID based on MAT enrollment history; thus, the effect of this bias is likely to be toward a null finding. Despite these limitations, this study provides preliminary evidence particular to opioid users of an association between MAT enrollment and the provision of injection initiation assistance that should be investigated in longitudinal study.

Given the harms associated with recent increases in opioid use across North America [1, 4], this study highlights the need to further investigate the potential impact of MAT as a preventive intervention to reduce not only the incidence of negative injection-related health outcomes experienced by opioid users but also incident cases of IDU initiation.