1. Details of peer‐support intervention in each included study.

Study ID	Peer‐support intervention
	Treatment duration	Who delivered/led the intervention	Element of peer support
Castelein 2008	8 months	People with schizophrenia or related psychotic disorder	Guided peer support group; participants decided the topic of each session; each session had the same structure discussing daily life experiences in pairs; it is to provide peer‐to‐peer interaction.
Cook 2012a	8 weeks	Peer instructors	Peer‐led, mental illness education intervention called Building Recovery of Individual Dreams and Goals through Education and Support (BRIDGES). Classes were delivered interactive, and included group discussion, illustrative anecdotes and structured exercises designed to apply information to everyday situations. Course topics included recovery principles and stages, strategies for building interpersonal and community support systems, brain biology and psychiatric medications, diagnoses and related symptom complexes, traditional and non‐traditional treatments and relapse prevention and coping skills.
Cook 2012b	8 weeks	Peer instructors	Peer‐led illness self‐management intervention called Wellness Recovery Action Planning (WRAP). Course work included lectures, group discussions, personal examples from the lives of the educators and participants, individual and group exercises, and voluntary homework assignments. Session 1: introduction of key concepts of WRAP; session 2 and 3: development of personalised wellness strategies; session 4: introduction of a daily maintenance plan to use every day to stay emotionally and physically healthy; session 5: educating of early warning signs; session 6 and 7: creation of a crisis plan specifying signs of impending crisis, names of individuals willing to help, and types of assistance preferred; session 8: post crisis support.
Druss 2010	6 sessions	Peer specialists	6 group sessions led by peer specialists, the following topics were discussed: overview of self‐management; exercise and physical activity; pain and fatigue management; healthy eating on a limited budget; medication management; finding and working with a regular doctor.
Eisen 2012	12 weeks	Peer facilitators	Peer facilitators used written recovery material such as the Spanior Recovery Workbook available from the Boston University. Peer leaders also shared their personal experiences as veterans with mental illness.
Van Gestel‐Timmermans 2012
Goldberg 2013	13 weeks	People with mental illness	Living well group; the first 3 sessions of the living well intervention focus on the basic strategies of self‐management; the remaining weekly sessions focus on training in specific disease management techniques and skills.
Kelly 2014	6 months	People with mental illness	Manualised intervention. Navigators encouraged development of self‐management of healthcare through a series of psychoeducation and behavioural strategies.
Mahlke 2017	6 months	People with mental illness	1‐to‐1 peer support in addition to standard care. Peer supporters contacted patients within the first week after randomisation and then established 1‐to‐1 meetings. The minimum number of meetings required to build a supporting relationship and be effective for the patient, based on the experiences in delivering support by the peers themselves.
Qian 2015	5 weeks	People with mental illness	Peer support and psychoeducation.
Reynolds 2004	5 months	People with mental illness	The transitional discharge model; this peer support provided friendship, understanding and encouragement for the discharged patient.
Rowe 2007	4 months	People with mental illness	Citizenship intervention plus valued‐roles projects. Consist of classes with topics related to social participation and community integration (citizenship classes), followed by projects designed to foster participants' acquisition of valued social roles (valued‐roles projects).
Sells 2008	12 months	Peer providers	Peer‐based group; use past experiences with recovery as a tool for understanding, role modelling and hope building for others.
Van Gestel‐Timmermans 2012	12 weeks	People with mental illness	Each session had the same structure and was organised around a specific, recovery‐related theme, such as the meaning of recovery to participants, personal experiences of recovery, personal desires for the future, making choices, goal setting, participation in society, roles in daily life, personal values, how to get social support, abilities and personal resources, and empowerment and assertiveness.