Health information about COVID-19 has been circulating in social networking sites, including unproven claims that smoking and alcohol drinking could protect against COVID-19. We examined if exposure to such claims was associated with changes in tobacco and alcohol consumption. We conducted a population-based, landline and mobile phone survey of 1501 randomly sampled adults aged 18 years or older (47.5% male) in Hong Kong in April 2020. Respondents reported if they had ever seen claims that 'smoking/alcohol drinking can protect against COVID-19' from popular social networking platforms. Current tobacco and alcohol users reported if they had increased or reduced their consumption since the outbreak. Prevalence data were weighted by sex, age and education of the general adult population. 19.0% (95% CI 16.8% to 21.4%) of all respondents reported having seen claims that 'smoking/alcohol drinking can protect against COVID-19' from social networking sites. Multinomial logistic regression showed that exposure to the claims was significantly associated with increased tobacco use (OR 2.37, 95% CI 1.08 to 5.20) in current tobacco users (N=280) and increased alcohol use (OR 4.16, 95% CI 2.00 to 8.67) in current drinkers (N=722), adjusting for sex, age, education level, alcohol/tobacco use status, home isolation, anxiety and depressive symptoms, and survey method. Our results first showed that exposure to health misinformation that smoking/alcohol drinking can protect against COVID-19 was associated with self-reported increases in tobacco and alcohol consumption in Chinese during the pandemic. © Author(s) (or their employer(s)) 2021. No commercial re-use. See rights and permissions. Published by BMJ.