Increasing the amount of alcohol taxation is among the most effective measures for addressing the rising global burden of alcohol harm. However, less is known about the effect of changing alcohol tax structures. Substantial reforms to UK alcohol taxation structures enacted in August, 2023, mean that all alcohol is taxed based on its ethanol content, beers and ciders sold in on-trade premises (eg, public houses) are taxed at a reduced rate (hereafter called draught relief), and beer and particularly cider remain taxed at lower rates than other alcohol of equivalent strength. We aimed to model the effect of these reforms on alcohol consumption and health and economic outcomes, and the effects of hypothetical alternative scenarios. The Sheffield Tobacco and Alcohol Policy Model was used to estimate policy effects on alcohol consumption. The model is an individual-based microsimulation that uses data from the Health Survey for England, Living Costs and Food Survey, Hospital Episode Statistics, and the Office for National Statistics. Spending and revenues to retailers and the Government were estimated cumulatively for a 5-year period post-intervention. Policy effects on all-cause deaths, years of life lost, hospital admissions, and admissions costs were estimated cumulatively for a 20-year period post-intervention. The reform was estimated to decrease mean weekly alcohol consumption per drinker by less than 0·05 (-0·34%) units (1 unit=8 g/10 mL ethanol), and prevent 2307 deaths and 11 510 hospital admissions during 20 years compared with no policy change. Removing draught relief was estimated to prevent 1441 further deaths and 14 247 further admissions. Hypothetical scenarios showed that removing draught relief would only slightly improve public health outcomes, and increasing tax rates for beer and ciders to match other drinks of equivalent strength would reduce consumption by a further 2·5 units per week (-17%) and deaths by approximately 74 465. Alcohol tax structures based on alcohol strength enable tax policy to improve public health in a targeted way. However, the UK reforms are unlikely to substantially improve health outcomes as they do not raise taxes overall. Raising tax rates for the lowest taxed beer and ciders, which are favoured by those who consume harmful amounts of alcohol, could achieve substantially greater public health benefits and reduce health inequalities. National Institute for Health and Care Research and UK Prevention Research Partnership.