Forming ‘if-then’ plans has been shown to reduce self-harm among people admitted to hospital following an episode of self-harm.

To explore whether the same intervention, delivered online, could prevent future self-harm among a large community sample who had previously self-harmed.

UK adults were recruited to a randomised controlled trial and received either an intervention to reduce self-harm or one to reduce sedentariness (control group). Randomisation was stratified to ensure both groups were representative of the UK population. There were three primary outcomes: non-suicidal self-injury (NSSI), suicidal ideation and suicide attempts, assessed at baseline and 6 months post-intervention.

Participants (1040) were randomised to the intervention (n = 520) or control (n = 520) group. The vast majority of people formed implementation intentions in both the experimental (n = 459 (88.3%)) and control (n = 520 (100%)) condition. Overall, the intervention did not significantly reduce the frequency of NSSI, suicidal ideation or suicide attempts. Among people who had self-harmed in the past week at follow-up, mixed analysis of covariance revealed a significant interaction between time and condition for reflective motivation, F(1,102) = 7.08, P < 0.01, pn2 = 0.07, such that significantly lower levels of reflective motivation were reported at follow-up in the control condition, t(57) = 2.42, P = 0.02.

This web-based intervention has limited utility for reducing self-reported self-harm or suicidal ideation in adults with a history of self-harm. Further work is needed to improve the effectiveness of brief interventions for self-harm aimed at adults living in the community and to understand the conditions under which the intervention may or may not be effective.