AbstractBackgroundThere are concerns that COVID-19 mitigation measures, including “lockdown” may have unintended health consequences. We examined trends in mental health and health behaviours in the UK before and during the COVID-19 lockdown and differences across population subgroups.MethodsRepeat cross-sectional and longitudinal analysis of the UK Household Longitudinal Survey, including representative samples of adults (aged 18+) interviewed in four surveys between 2015 and 2020 (n=48,426). 9,748 adults had complete data for longitudinal analyses. Psychological distress was assessed using the General Health Questionnaire-12 (GHQ). Binary outcomes were created for loneliness, current smoking and use of e-cigarettes. Alcohol consumption was assessed using three binary measures assessing frequent, binge and heavy drinking. Cross-sectional weighted prevalence estimates were calculated and multilevel Poisson regression assessed associations between time period and the outcomes of interest, as well as differential effects by age, gender, education level and ethnicity.ResultsPsychological distress substantially increased one month into the COVID-19 lockdown, with the prevalence rising from 19.4% (95% CI 18.7%-20.0%) in 2017-19 to 30.3% (95% CI 29.1%-31.6%) in April 2020. Women’s mental health was particularly affected, as was that of the most educated and young adults. Loneliness remained stable overall. Smoking and e-cigarette use declined and the proportion of people drinking on four or more days per week increased, as did binge drinking.ConclusionsPsychological distress increased one month into lockdown, particularly among women and young adults. Smoking declined, but the frequency of alcohol consumption increased. Effective treatment and prevention are required to mitigate adverse impacts on health.What is already known on this topicCountries around the world have implemented radical COVID-19 lockdown measures, with concerns that these may have unintended consequences for a broad range of health outcomes.Evidence on the impacts of lockdown measures on mental health and health-related behaviours remains limited.What this study addsIn the UK, psychological distress markedly increased during the lockdown, with women particularly adversely affected.Smoking and e-cigarette use fell, but the frequency of alcohol consumption and binge drinking increased.