Competing interests We have read and understood the BMJ policy on declaration of interests and declare the following interests: There was no specific funding for this article. JHB and RB receive support from the National Institute of Health Research. PA is funded by the UK Centre for Tobacco and Alcohol Studies and is supported by the NIHR Biomedical Research Centre, Oxford and the NIHR CLAHRC, Oxford. The views expressed are those of the authors and not necessarily those of the NHS, the NIHR or the Department of Health. JHB, PA, and RB have no other competing interests to declare.