BackgroundAgitation is a common, challenging symptom affecting large numbers of people with dementia and impacting on quality of life (QoL). There is an urgent need for evidence-based, cost-effective psychosocial interventions to improve these outcomes, particularly in the absence of safe, effective pharmacological therapies. This study aimed to evaluate the efficacy of a person-centred care and psychosocial intervention incorporating an antipsychotic review, WHELD, on QoL, agitation, and antipsychotic use in people with dementia living in nursing homes, and to determine its cost.Methods and findingsThis was a randomised controlled cluster trial conducted between 1 January 2013 and 30 September 2015 that compared the WHELD intervention with treatment as usual (TAU) in people with dementia living in 69 UK nursing homes, using an intention to treat analysis. All nursing homes allocated to the intervention received staff training in person-centred care and social interaction and education regarding antipsychotic medications (antipsychotic review), followed by ongoing delivery through a care staff champion model. The primary outcome measure was QoL (DEMQOL-Proxy). Secondary outcomes were agitation (Cohen-Mansfield Agitation Inventory [CMAI]), neuropsychiatric symptoms (Neuropsychiatric Inventory-Nursing Home Version [NPI-NH]), antipsychotic use, global deterioration (Clinical Dementia Rating), mood (Cornell Scale for Depression in Dementia), unmet needs (Camberwell Assessment of Need for the Elderly), mortality, quality of interactions (Quality of Interactions Scale [QUIS]), pain (Abbey Pain Scale), and cost. Costs were calculated using cost function figures compared with usual costs. In all, 847 people were randomised to WHELD or TAU, of whom 553 completed the 9-month randomised controlled trial. The intervention conferred a statistically significant improvement in QoL (DEMQOL-Proxy Z score 2.82, p = 0.0042; mean difference 2.54, SEM 0.88; 95% CI 0.81, 4.28; Cohen's D effect size 0.24). There were also statistically significant benefits in agitation (CMAI Z score 2.68, p = 0.0076; mean difference 4.27, SEM 1.59; 95% CI -7.39, -1.15; Cohen's D 0.23) and overall neuropsychiatric symptoms (NPI-NH Z score 3.52, p ConclusionsThese findings suggest that the WHELD intervention confers benefits in terms of QoL, agitation, and neuropsychiatric symptoms, albeit with relatively small effect sizes, as well as cost saving in a model that can readily be implemented in nursing homes. Future work should consider how to facilitate sustainability of the intervention in this setting.Trial registrationISRCTN Registry ISRCTN62237498.