OBJECTIVES: As obesity becomes a global epidemic, the use of weight loss medications (WLM) continues to increase. The impact of WLM on functioning and well-being is not well understood, cultural differences have been largely ignored and discrepancies between clinician and patient perceptions have been noted. The purpose of the study was to develop a measure of the impact of WLM applicable for use in clinical trials and clinical practice. METHODS: Qualitative data were collected from the literature, patients (n=82) and experts (n=9) in three countries (US, France, UK) were interviewed regarding the impact of WLM on functioning, well-being and health. Interview transcripts were thematically coded. Based on these data, a conceptual model of the impact of WLM on functioning, well-being and symptom experience was developed and a patient reported outcomes (PRO) measure generated. RESULTS: The impact of WLM on psychological health, interference in daily life, treatment burden, efficacy and side effects was considered significant by patients. Key modifiers to this impact (i.e., age, occupation, stress) and consequences (i.e., compliance, reduced productivity) were noted. Based on a conceptual model, a WLM treatment related impact measure (TRIM-Weight) was generated with five discrete domains. Discrepancies between perceived patient and clinician impacts were identified, most notably for the impact on sleep and daily life as well as perceived importance of side effects. Gender and cultural differences were also identified. For example, men tended to be more goal-oriented in their weight loss goals than women and side effect profiles differed by country as a result of prescription patterns.  CONCLUSIONS: A measure of the impact of WLM on functioning, well-being and health was developed. The instrument development process, full conceptual model, and cultural, gender and clinician/patient differences will be presented. This information should help clinicians to identify key PRO issues for WLM, facilitate targeted treatments and allow for meaningful measurement of treatment effect.