1 Department of Educational and Counselling Psychology, McGill University, Montréal, Québec, Canada.
2 Department of Psychology, Carleton University, Ottawa, Ontario, Canada.
3 Department of Applied Psychology & Human Development, University of Toronto, Toronto, Ontario, Canada.
4 Campus Wellness and Support Services, Concordia University, Montréal, Québec, Canada.
5 Student Wellness Hub, McGill University, Montréal, Québec, Canada.

Mindfulness instruction comprising both formal (FM) and informal (IM) mindfulness practice is increasingly offered to university students. FM involves sustaining attention on thoughts, emotions and bodily sensations through structured practices, while IM involves incorporating mindfulness into daily activities. However, recent evidence suggests that FM may pose challenges for students with recent non-suicidal self-injury (NSSI), whereas the flexibility and brevity inherent in IM may be better tolerated. This randomized controlled trial compared a FM induction, IM induction and control task among university students with (n = 103) and without (n = 123) past-year NSSI in terms of acceptability and pre-post state mindfulness, stress and well-being. Notably, results did not differ as a function of NSSI history. Two-way ANOVAs revealed that only IM was consistently preferred over the control task. Furthermore, three-way mixed ANOVAs revealed that-when assessed using brief Visual Analogue Scales-state well-being increased in all conditions, state mindfulness increased after both IM and FM, and state stress only decreased after IM. Notably, these differences by condition appeared to be of short duration as they were not found with lengthier measures. Results highlight the potential promise of IM and the importance of measurement selection when assessing the transient effects of mindfulness inductions in research.