BACKGROUND AND AIMS: Randomised controlled trials in Europe and Canada have shown that supervised heroin assisted treatment (HAT) is an effective treatment option for people with long-term heroin addictions for whom the standard opioid substitution treatments (OST) have not been effective. This review aims to evaluate the effectiveness of supervised HAT and analyse the significance of context and implementation in the design of successful HAT programmes. METHODS: PubMed, CENTRAL, Embase, and Web of Science were searched to identify randomised controlled trials (RCT) and systematic reviews evaluating supervised HAT compared to any other OST. Studies were eligible for inclusion if they were published in English, evaluated a supervised form of HAT, and included illegal drug use and/or health as a primary outcome measure. There were no restrictions on publication date. The following outcomes of the included studies were analysed using narrative synthesis and meta-analysis where possible: retention, street drug use, health, and social functioning. RESULTS: Nine randomised controlled trials spanning eight studies (n = 2331) and three systematic reviews met the inclusion criteria. Seven of the eight studies compared HAT to methadone maintenance treatment (MMT). One study compared HAT to injectable hydromorphone in a double-blind non-inferiority trial. Meta-analysis was performed on pooled results of retention across all included studies and found that HAT has a statistically significant effect on retention [Z = 7.65 (P > 0.0001)]. Five of the eight included studies found that supervised HAT reduces participants' use of illegal drugs more significantly than MMT. Evidence of improved health in participants receiving supervised HAT compared to other OSTs was inconsistent; positive effects were observed in three of the included studies (n = 1626). CONCLUSION: When compared to methadone maintenance treatment (MMT), heroin assisted treatment (HAT) more consistently retains people with heroin addictions in treatment and reduces their consumption of illicit drugs. TRIAL REGISTRATION: PROSPERO registration: CRD42022341306.