This review is the first international review to comprehensively bring together economic evaluations of preventive interventions specifically for opioid misuse, cannabis and illicit drug use. The review distinguishes itself from other reviews that did not have a specific focus on cannabis use, opioid misuse and illicit drug use prevention.^15,16 We found that there are preventive interventions that are supported by cost-effectiveness evidence that are of fair to good quality. In particular, school-based (LST, Project ALERT and SET) and family-centred programmes (ParentCorps and ISFP), or combinations of both (SFP:10–14 + All Stars, SFP:10–14 + LST), have demonstrated promising cost-effectiveness results across cannabis use, opioid misuse and illicit drug use prevention. Other preventive interventions with cost-effectiveness evidence are SBIRT for illicit drug use prevention and the Network System to Prevent Doctor-Shopping for Narcotics for problem opioid use prevention. Although multiple preventive interventions for cannabis use, opioid misuse and illicit drug use have been found to have positive effects, particularly in the short term,¹¹ this review found that there are still limited numbers of full economic evaluations in this area. Most of the studies have been published in the past decade in the USA, with no economic evaluations found for low- and middle-income countries. Another review by Le et al¹⁶ also identified a similar lack of economic evaluations in low- and middle-income countries in their review of economic evaluations of mental health prevention and promotion.

Although the review intended to look into a broad range of preventive programmes, no studies were identified that focused on supply reduction, as all studies focused on demand reduction. Most programmes included in this review were universal and targeted young people (<18 years) through school settings. This is logical given that delaying initial consumption is typically the goal of cannabis use, opioid misuse and illicit drug use prevention. The majority of studies used economic modelling to simulate the cost-effectiveness of interventions. Economic models allow researchers to estimate population-level costs and effects over extended time periods. The models for substance use prevention projected costs and outcomes from 1-year to lifetime time horizons. Longer time horizons provided favourable cost-effectiveness results, from avoidance of longer-term costs to health and social systems. This trend is similar to alcohol and tobacco prevention interventions with lifetime time horizons included in the Nystrand et al¹⁵ review. A limitation of using a longer time horizon is that models use assumptions that may limit confidence in conclusions.

There were a variety of perspectives reported. The most common perspective was societal, which would be appropriate given the costs associated with substance use that fall outside the healthcare system, such as judiciary and productivity costs.³² This is consistent with the recommendation promoted by Neumann et al,³³ as societal perspective reflects public interest. Some studies chose to evaluate narrow health sector or employer perspectives, most likely because of the difficulties of collecting costs for criminal justice and lost productivity associated with substance use. Using narrower perspectives are not necessarily inferior to a societal perspective, as choices regarding intervention or policy adoption depend on what level of evidence a decision maker requires. For instance, if an employer plans to implement a prevention programme, benefits of increased productivity may be enough for decision-making. This is the same when health payers use a healthcare perspective in deciding which programmes should be implemented.

All CBA studies reported cost-savings for the interventions evaluated, meaning the dollar value of the benefit gained was greater than every dollar being spent on the intervention. For CEA and CUA, Hajizadeh et al²⁴ showed cost-savings and increased QALYs, which points toward ParentCorps intervention dominance. Interpretation of other ICERs would depend on a WTP threshold. Of the studies, four reported WTP thresholds, and three reported ICERs falling below the threshold designating the interventions as cost-effective.