ABSTRACTBackgroundPrimary care clinicians are encouraged to intervene opportunistically, offering weight-loss advice to people living with obesity. The BWeL trial showed patients receiving brief weight-loss advice from their general practitioner lost weight at one year follow-up. We examined the behaviour change techniques (BCTs) clinicians used to identify which BCTs are associated with this weight loss.MethodsWe coded 224 audio recorded interventions from the BWeL trial using the behavioural change techniques version one taxonomy (BCTTv1) and the ‘refined taxonomy of behaviour change techniques to help people change their physical activity and healthy eating behaviours’ (CALOR-RE taxonomy). Linear and logistic regressions were performed to analyse associations between behaviour change techniques used in these taxonomies and patient weight loss.ResultsMean intervention length was 86 s.We identified 28 different BCTs BCTTv1 and 22 from CALOR-RE. No BCTs or BCT domains were associated with mean weight loss at 12 months, loss of 5% bodyweight, or action taken at 3 months. The BCT ‘Feedback on outcomes of behaviour (future)’ was associated with an increased likelihood that the patient reported taking action to lose weight by 12 months (OR = 6.10, 95%CI = 1.20, 31.0).ConclusionAlthough we found no evidence to support the use of particular BCTs, our results suggest that it is the brief intervention itself, rather than specific content, which may motivate weight loss. This can support clinicians to confidently intervene without needing complex training. Offering follow-up appointments can support positive changes to health behaviours, even if these are not associated with weight loss.