Commentary on Mehta et al. (2012): Natural history of injecting drug use


It is a truth universally acknowledged that heroin addiction is a ‘chronic relapsing problem’[1]. The duration of this problem, however, is a more open question. Few prospective studies have investigated the process of recovery, relapse and final cessation [2]. Investment in long term follow-up is required; this is difficult to do but rarely disappoints scientifically—from classic early studies by Vaillant and Robins and studies in California, Baltimore, Amsterdam and Edinburgh [3–9]. With one exception these cohorts emphasise both the prolonged duration and substantial morbidity and mortality associated with injecting. Uniquely the follow-up of people returning from armed service in Vietnam [5] reported high rates of cessation on return to the US.

What is lacking still, however, is a generalized model of injecting duration or natural history similar to other chronic problems, such as progression from HIV infection to AIDS defining illness [10]. In part, this is because we lack sufficient long-term follow-up (or cohort) studies to synthesise estimates of the characteristics and duration of injecting; but also in part because there are multiple biases in our current evidence base [2]. These include selection bias—as most studies under-represent people with short periods of injecting and tend to recruit people with established injecting careers. This means that follow-up studies of ‘established’ heroin users may over-estimate the duration of injecting—e.g. studies in Switzerland have estimated an annual cessation rate of ∼4% (i.e. mean duration of 1/0.04 ∼ 25 years) [11]. Population surveys of drug use suffer a contrasting problem that also adds bias: underestimating and under-representing prevalent cases of people who inject drugs and people with long drug injecting careers and sampling a substantial proportion of ex-injectors who injected on a few occasions or very infrequently [12,13].

This leads us to Shruti and colleagues impressive and valuable study of injecting cessation in people who inject drugs in India [14]. Valuable because there are even less data on injecting natural history from transitional and developing countries than for developed countries [15]. Furthermore, the potential insights on duration—which if supported by other studies—have far reaching implications for prevention strategies. Inevitably, there is some bias that needs to be teased out. For instance, the cohort observes that approximately 50% ceased injecting after one year with 24% subsequently relapsing (suggesting that 38% may cease within a year). If this was true from injecting onset and for all injectors in Chennai then rather crudely the average duration of injecting would be under 3 years (i.e. 1/0.38). In addition, the observed sample (with an average 8 years injecting) would represent the top 5% of injectors (i.e. 1-exp−0.38*8) in terms of their injecting duration. Sites with shorter injecting durations can reduce harms, such as injecting related transmission, quicker than sites with longer injecting durations once preventive interventions had been introduced [16]. In an earlier study of the Chennai cohort they observed a background prevalence of 25% and 55% for HIV and HCV and then subsequently an HIV and HCV incidence of 0.5 per 100 person-years and 1.7 per 100 person-years respectively which suggests that current risk of HIV and HCV have reduced. These reductions were driven largely by short injecting duration (and cessation from injecting) [17].

The role of drug treatment in the Chennai cohort was marginal—with only 5% exposed to treatment and the authors' discuss that they found no association between treatment and cessation [14]. A study in Edinburgh also found no association between opiate treatment and long term cessation (in fact there was a negative association) [9]. Opiate substitution treatment, however, does improve survival—especially after prolonged treatment [18]. Perhaps the worst situation—as suggested by the cohort in Chennai—is the provision of opiate detoxification as the sole drug treatment—for if detoxification does not hasten cessation it is very likely to increase mortality [19].

Shruti and colleagues are to be congratulated. This study adds to a small but interesting body of work that suggests that natural history of people who inject drugs vary in important ways geographically. There is a need to for such studies across the world, to better understand the consistencies in use careers, the methodological variables that affect the careers documented, and begin to elucidate the factors that determine variations in the length and nature of these careers.

Declaration of interests