Background: The harm reduction effects of police drug diversion (PDD) are influenced by how people engage with diversion, which is moderated by their motivation to change. Underlying influences can be revealed by examining the contexts and mechanisms which trigger and support participants’ motivation for engagement and change.

Methods: The qualitative phase of a realist evaluation of three PDD programs in England used semi-structured interviews with police officers, diversion practitioners, and divertees. Collaborative, abductive analysis focused on contexts and mechanisms of motivation and the nature of engagement.

Results: Contexts featuring positive interactions were moderated by negative perceptions of intervention environments, course content and delivery, and feelings of dissatisfaction with drug policy and legislation. Short PDD programs without individually tailored support worked well in triggering consequential thinking to motivate some people involved in experimental and/or occasional drug use. More intensive, longer and individualized forms seemed to be more effective in supporting and building motivation among people with more complex needs. Neither form of PDD succeeded in motivating people who believed their substance use was beneficial, unproblematic, or who disagreed with current drug laws.

Conclusion: Findings suggest that PDD programs require careful tailoring to the diverse needs of people who are diverted.