BackgroundTwo-thirds of people with dementia live at home, receiving most care from family carers, about 40% of whom have clinically significant depression or anxiety. This impacts on the person with dementia, families and society, predicting care breakdown. There are currently no clinically effective and cost-effective NHS family carer interventions.ObjectivesTo assess the STrAtegies for RelaTives (START) intervention in the short (4 and 8 months) and long term (1 and 2 years) compared with treatment as usual (TAU).DesignRandomised, parallel-group, superiority trial with blinded assessment recruiting participants 2 : 1 (intervention to TAU) to allow for therapist clustering.SettingThree UK mental health services and one neurological service.ParticipantsFamily carers of people with dementia.InterventionEight-session manual-based coping intervention delivered by supervised psychology graduates to individuals.Main outcome measuresAffective symptoms [Hospital Anxiety and Depression Scale-total (HADS-T)] and cost-effectiveness. Secondary measures: anxiety and depression symptoms and caseness, quality of life (QoL), abusive behaviour and long-term care home admission.ResultsTwo hundred and sixty participants were randomised (173 intervention, 87 TAU). We used intention-to-treat analysis in the short term (152 intervention, 77 TAU) and in the long term (140 intervention, 69 TAU).In the short term, the intervention group had lower HADS-T [mean difference –1.80, 95% confidence interval (CI) –3.29 to –0.31;p = 0.02] and higher quality-adjusted life-years (QALYs) (mean difference 0.03, 95% CI –0.01 to 0.08). Costs were no different between groups [mean £252 (95% CI –£28 to £565) for intervention group]. The cost-effectiveness acceptability curve showed a greater than 99% chance of being cost-effectiveness at a £30,000/QALY willingness-to-pay threshold and a high probability of cost-effectiveness based on the HADS-T score. Carers in the intervention group had less case-level depression [odds ratio (OR) 0.24, 95% CI 0.07 to 0.76], a trend towards reduced case-level anxiety (OR 0.30, 95% CI 0.08 to 1.05), lower Hospital Anxiety and Depression Scale-anxiety (HADS-A) (–0.91, 95% CI –1.76 to –0.07;p = 0.03) and Hospital Anxiety and Depression Scale-depression (HADS-D) (–0.91, 95% CI –1.71 to –0.10;p = 0.03) and higher Health Status Questionnaire (HSQ) QoL (mean difference 4.09, 95% CI 0.34 to 7.83). Group differences in abusive behaviour (OR 0.48, 95% CI 0.18 to 1.27) and the person with dementia’s quality of life-Alzheimer’s disease (QoL-AD) (mean increase 0.59, 95% CI –0.72 to 1.89) were not significant.In the long term, the intervention group had lower HADS-T (mean difference –2.58, 95% CI –4.26 to –0.90;p = 0.03) and higher QALYs (mean difference 0.03, 95% CI –0.01 to 0.06). Carers in the intervention group had less case-level depression (OR 0.14, 95% CI 0.04 to 0.53), a trend towards reduced case-level anxiety (OR 0.57, 95% CI 0.26 to 1.24), lower HADS-A (–1.16, 95% CI –2.15 to –0.18) and HADS-D (1.45, 95% CI –2.32 to –0.57), and higher HSQ (mean difference 7.47, 95% CI 2.87 to 12.08). Thirty-two (18.7%) people with dementia in the intervention group and 17 (20.2%) in TAU were admitted to a care home (hazard ratio 0.83, 95% CI 0.44 to 1.56;p = 0.56). There were no significant differences between groups in abusive behaviour (OR 0.83, 95% CI 0.36 to 1.94), the person with dementia’s QoL-AD (0.17, 95% CI –1.37 to 1.70) or costs (£336, 95% CI –£223 to £895) for intervention group. The probability that the intervention would be seen as cost-effective at £30,000/QALY threshold and cost-effectiveness on the HADS-T remained high.ConclusionsThe START intervention was clinically effective and cost-effective in the short and longer term. The results are robust to the sensitivity analyses performed. Future work is needed to consider mechanism of action; the effects on people with dementia in clinical terms (cognition, neuropsychiatric symptoms, longer-term care home admission); and on health and social care costs. In addition, we will explore the effects of carer abusive behaviour on the care recipient’s care home admission and if this then reduces abusive behaviour. We would also like to implement START and evaluate this implementation in clinical practice.Trial registrationCurrent Controlled Trials ISCTRN70017938.FundingThe National Institute for Health Research Health Technology Assessment programme.