Purpose\ud Research into causality assessment tools enabling patients to assess suspected adverse drug reactions (ADRs) is limited. Supporting patients with tools could improve their confidence in discussions with health professionals and encourage reporting of suspected ADRs to regulators. This study describes development and preliminary validation of an instrument: Side Effect Patient ASsessment Tool (SE‐PAST).\ud \ud Methods\ud SE‐PAST was developed from survey and interview data involving patients experiencing suspected ADRs. It included 10 statements enabling causality assessment, covering timing, additional information sources, and experiences, with four options: yes/no/don't know/not applicable. Scoring and weighting resulted in four categories of causal association: highly probable, probable, possible, unlikely.\ud \ud Validation involved obtaining feedback from 31 individuals experiencing an ADR. Further validation involved online distribution through patient support groups and comparison of reported symptoms to known ADRs.\ud \ud Results\ud Validators found SE‐PAST easy to read (31), to understand (27), and to complete (29). A total of 294 respondents completed SE‐PAST online, with 98% completing eight or more causality assessment statements. Symptoms were categorised as highly probable (46; 16%), probable (80; 62%), possible (44; 15%), and unlikely (21; 7%). A total of 221 respondents identified one suspected medicine, with 95% of these reporting at least one symptom known to be an ADR.\ud \ud Of 227 providing feedback, 139 (61%) found SE‐PAST useful, 160 (71%) felt motivated to discuss their experience with a health professional, and 136 (60%) were encouraged to report to the regulator.\ud \ud Conclusion\ud SE‐PAST was easily completed and understood by people experiencing suspected ADRs and could be useful in encouraging patient reporting to health professionals and agencies.