The ratio of non‐fatal to fatal heroin overdose

Sir—Heroin overdose, and interventions to reduce overdose-related mortality and morbidity, have received a great deal of attention in the preceding decade 1. To date, studies have focused either on fatal 2 or non-fatal 3 overdose. However, it is important to understand the relative rate or ratio between these two events from a public health perspective. For example, a recent evaluation of the public health benefits arising from a trial of a medically supervised injecting centre in Sydney required an estimation of the numbers of lives saved by overdose management within the centre. In order to obtain such an estimate, the proportion of overdose events that result in death in the general community had to be calculated. The exact number of non-fatal overdoses among the total heroin-using population is unknown, so it is not possible to give a definitive ratio between these two events. However, we can make some evidence-based estimates. A recent review of international longitudinal studies estimated the annual mortality rate due to overdose to be 0.8%4. Separately focusing on Australia, this study also estimated that in 1997 the country had 74 000 heroin users 4; and there were 600 opioid overdose deaths in that year 5, giving an annual fatal overdose rate identical to that produced by the review of longitudinal studies—0.8%. Furthermore, a large number of studies on non-fatal overdose have been conducted in Australia; they also show a remarkable degree of consistency in self-reported rates of non-fatal overdose 3,6–9. An examination of 10 Sydney studies conducted between 1995 and 2001, based upon a total of 2315 interviews with heroin users, revealed the following self-reported annual non-fatal heroin overdose rates: 19%6, 23%6, 24%6, 25%7, 25%8, 28%6, 28%9, 29%3, 30%6, 30%6. Thus, in all studies rates were between 19 and 30%, with a median of 25% per annum. Using the median figure of 25% from these studies, and the annual overdose death rate of 0.8%, the ratio of non-fatal to fatal overdose can be calculated as 25% divided by 0.8%. This gives a ratio of 31.3 : 1, with a range of 23.8 : 1–37.5 : 1 based upon the lowest and highest reported non-fatal overdose rates. Put another way, this means that approximately 3.1% of overdose events (i.e. 1/32.1) result in death (range 2.6–4.0%). This estimate of 3.1% is broadly consistent with the self-report of heroin users that death had resulted in 5% of cases at the last overdose they witnessed 10. From another perspective, this ratio seems to be validated. It has been recently estimated that in New South Wales the ratio of ambulance overdose-related call-outs to overdose deaths in the community is 12 : 1 11. However, not all overdoses result in ambulance attendance. In one of the few studies that has examined this issue, an ambulance attended in 51% of the most recent non-fatal overdoses reported by heroin users 3. If we assume that there are 12 ambulance attendances to every overdose death, and that ambulances attend in 51% of non-fatal overdose events, we may extrapolate from this figure to obtain the ratio of all overdoses (those with and without ambulance attendance) to deaths. This would give a ratio of non-fatal to fatal overdose of 23.5 : 1, i.e. 12 / 0.51 : 1. This death rate of 4.1 per 100 overdoses is very close to the 2.6–4.0% death rates mentioned earlier, providing convergent evidence that approximately one in 20 to one in 30 overdose events result in death. These analyses are not definitive. However, they do provide an estimate of the relationship between these two events. As noted earlier, these ratios provide data that may assist in the evaluation of the public health benefits of an intervention such as a medically supervised injecting centre. They would be of broader use in estimating general overdose-related morbidity and the benefits of community overdose interventions. Alternatively, a knowledge of non-fatal overdose rates among a population may be of use in predicting the expected number of overdose fatalities. The issue is one that warrants further research in order to improve the evaluation of heroin-related interventions.