Despite growing interest in the therapeutic potential of 3,4-methylenedioxymethamphetamine (MDMA), no targeted measure to systematically assess side effects of MDMA-assisted psychotherapy (MDMA-AP) exists.

Our aim was to develop an MDMA-Assisted Psychotherapy Side Effects Tool (M-SET) to capture side effects over the course of MDMA-AP.

Informed by a systematic review and a review of other relevant questionnaires, we drafted a list of potential side effects. Face and content validation were obtained via a modified two-round online Delphi process involving experts in MDMA-AP and the neuropsychopharmacology of MDMA.

Twelve experts consented to participate over two rounds of Delphi panel deliberations (response rate: Round 1 = 83–92%, Round 2 = 75%). The Delphi panellists were asked to keep, discard, modify or suggest additional items. The final version of the M-SET consists of 165 items across four questionnaires that collect information at screening, baseline, the day of medication sessions and longer term follow-up.

The use of a modified Delphi technique proved a successful method to generate content for the first structured tool designed to evaluate side effects specifically associated with MDMA-AP.

The M-SET is recommended for use in both research and clinical settings. Its implementation has the potential to improve the safety of delivering MDMA-AP as well as support the development of a more systematic and robust evidence base on its safety and tolerability.