BACKGROUND: Heroin overdose is a major cause of premature death. Naloxone is an opioid antagonist that is effective for the reversal of heroin overdose in emergency situations and can be used by nonmedical responders. OBJECTIVE: Our aim was to assess the cost-effectiveness of distributing naloxone to adults at risk of heroin overdose for use by nonmedical responders compared with no naloxone distribution in a European healthcare setting (United Kingdom). METHODS: A Markov model with an integrated decision tree was developed based on an existing model, using UK data where available. We evaluated an intramuscular naloxone distribution reaching 30% of heroin users. Costs and effects were evaluated over a lifetime and discounted at 3.5%. The results were assessed using deterministic and probabilistic sensitivity analyses. RESULTS: The model estimated that distribution of intramuscular naloxone, would decrease overdose deaths by around 6.6%. In a population of 200,000 heroin users this equates to the prevention of 2,500 premature deaths at an incremental cost per quality-adjusted life year (QALY) gained of £899. The sensitivity analyses confirmed the robustness of the results. CONCLUSIONS: Our evaluation suggests that the distribution of take-home naloxone decreased overdose deaths by around 6.6% and was cost-effective with an incremental cost per QALY gained well below a £20,000 willingness-to-pay threshold set by UK decision-makers. The model code has been made available to aid future research. Further study is warranted on the impact of different formulations of naloxone on cost-effectiveness and the impact take-home naloxone has on the wider society.