Introduction Pesticide self-poisoning causes severe health and socioeconomic hardship in low- and middle-income countries, including Sri Lanka. A stepped-wedge cluster randomised controlled trial (cRCT) has been designed to test whether ‘gatekeeper’ training for pesticide vendors reduces pesticide self-poisoning in rural Sri Lanka (Vendor cRCT). Ensuring intervention fidelity in RCTs is essential for consistently replicating interventions, accurately assessing their impact and improving outcomes. Thus, the overarching goal of this proposed study is to explore to what extent the pesticide vendors use the ‘gatekeeper’ strategies.

Methods and analysis A multimethod qualitative research design is being used to explore the pesticide-selling behaviours of vendors after the ‘gatekeeper’ training. A subsample of the Vendor cRCT Study population is being recruited using a stratified purposive sampling method in all six intervention districts in Sri Lanka to ensure that the sample is representative of the pesticide vendors in the study area. Participant diaries, observations and focus group discussions are being adopted to collect data. Data triangulation will be performed and data will be analysed thematically.

Ethics and dissemination The study was approved by the Ethics and Research Committee, Faculty of Medicine and Allied Sciences, at the Rajarata University of Sri Lanka (ERC/2023/09). All participants will provide informed consent. Findings will be disseminated in scientific peer-reviewed journals and conference presentations.