First, do no harm: An intensive experience sampling study of adverse effects to mindfulness training

Objective: The study of safety and adverse effects of mindfulness-based interventions (MBIs) is limited. We propose a novel reliable change index (RCI) approach to experience sampling (ES) data to begin to understand the common domains, frequency, severity, risk for, and context of adverse responding to mindfulness meditation practice and brief MBI. Methods: Over the course of a 21-day MBI among 82 meditation-naive participants, we estimated (i) momentary adverse effects during mindfulness meditation practice and (ii) sustained adverse effects in daily living following the intervention. Results: First, RCI analyses of experience sampling of mindfulness meditation document that 87% of participants demonstrated at least one momentary adverse effect during meditation, most commonly anxiety; and subjectlevel temporal variability or instability in experience samples of daily living did not account for momentary adverse effects attributed to mindfulness meditation sessions. Second, 25% of participants experienced a sustained adverse effect in daily living at post-intervention. Yet, neither momentary adverse effects to meditation nor vulnerability factors at pre-intervention predicted adverse effects at post-intervention. Conclusions: Findings illustrate that mindfulness meditation may be transiently anxiogenic for many participants, yet, these experiences are unlikely to constitute objective harm per se. Furthermore, observed deterioration in daily living post-intervention cannot be attributed to momentary adverse effects in response to mindfulness meditation. We speculate that observed deterioration in daily living post-intervention may thus be better explained by increased awareness to internal states following mindfulness training. Findings highlight the potential utility of applying a RCI approach to intensive ES measurement to quantify adverse effects of mindfulness training specifically and mental health interventions broadly.