Purpose Dual Diagnosis Anonymous (DDA) is a peer-led programme developed in the USA, which aims to address mental and addictive disorders in an integrated manner. This study is part of a mixed-methods evaluation of the first DDA pilot in the UK, and the purpose of this study is to explore the impact and mechanism of change of the programme through the perspective of DDA attendees, facilitators and the funding commissioners. Design/methodology/approach Six DDA members were interviewed three times over a period of 12 months, the facilitators were interviewed twice and the commissioner was interviewed once. The qualitative longitudinal data were analysed using a trajectory thematic analysis. Findings DDA attendance was perceived to have had a positive impact on five main areas: acceptance of self, of others and from others; social functioning; self-development; recovery progression; and feeling of hope. The possibility of addressing both mental health and addiction at the same time was a key factor in the recovery process. The facilitators observed that DDA had contributed to integrate members into employment and education, while the commissioner stressed the importance of joint commissioning and sustainability. Originality/value The longitudinal approach provided a unique insight into the recovery process of DDA members. Being able to address the mental health as well as the substance use problems was considered to be a fundamental strength of DDA in comparison to the single purpose peer-support fellowships.