Accessible summary What is known on the subject People with mental disorders have increased risk of dying from diabetes and cardiovascular diseases compared to the general population. Diabetes and cardiovascular diseases are preventable by improved lifestyle regarding smoking, alcohol consumption, physical activity and dietary behaviours. Forensic mental health service users are treated for longer periods of time compared to non‐forensic mental health service users, giving the opportunity to affect the lifestyle for a substantial period of time. What the paper adds to existing knowledge This review gathers existing research on forensic mental health service users’ lifestyle regarding smoking, alcohol consumption, physical activity, and dietary behaviours and factors influencing it. The lifestyle was found to be unfavourable with many patients being smokers, having problematic alcohol consumption, being physically inactive and eating a diet of poor nutritional value and rich in calories. Therefore, it seems likely that an unfavourable lifestyle is one reason for the excess mortality from diabetes and cardiovascular diseases. Smoking cessation and improving dietary habits was perceived difficult, but nicotine replacement and practical advice was suggested to support a change. What are the implications for practice The treatment period gives an opportunity to improve the lifestyle of forensic mental health service users to prevent diabetes and cardiovascular diseases in this high‐risk group. We recommend a holistic approach, when planning the prevention activities, since activities that are perceived fun are more likely to succeed. Abstract IntroductionPeople with mental disorders have increased risk of dying from diabetes and cardiovascular diseases, both of which can be prevented by lifestyle. AimTo review existing research, in order to investigate the characteristics of, and factors that influence forensic mental health service users’ (FMU) health behaviours. MethodWe searched PubMed, CINAHL, PsycInfo and Scopus for primary research on FMU’s health behaviours regarding smoking, alcohol consumption, physical activity and dietary behaviours, and factors that influence them. ResultsWe found 13 eligible studies. The findings consistently indicated the presence of unfavourable health behaviours in FMU: Smoking, problematic alcohol consumption, physical inactivity and a high‐calorie diet of poor nutritional value. Changing smoking and dietary habits was perceived as difficult, but nicotine replacement and practical advice were suggested to support change. DiscussionThe existing research on FMU’s health behaviours is sparse. In particular, there is a lack of research on factors that influence health behaviours. From our findings, it seems likely that FMU’s unfavourable health behaviours contribute to their increased risk of dying from diabetes and cardiovascular diseases. Implications for practiceFMU’s health behaviours should be improved to prevent diabetes and cardiovascular diseases in this high‐risk group.