Background: The outbreak of coronavirus disease 19 (COVID-19) has led to measures of social distancing and quarantine worldwide. This stressful period may lead to psychological problems, including increases in substance use.

Objective: To investigate changes in alcohol, tobacco, and cannabis consumption before and during COVID-19 lockdown and motives for these changes in substance use.

Method: A web-based survey was filled out by an unselected population during the social distancing measures of the COVID-19 pandemic in Belgium that assessed changes in alcohol, tobacco, and cannabis consumption in the period before and during the COVID-19 lockdown and also asked about reasons for change.

Results: A total of 3,632 respondents (mean age 42.1 ± 14.6 years; 70% female) filled out the survey. Overall, respondents reported consuming more alcohol (d = 0.21) and smoking more cigarettes (d = 0.13) than before the COVID-19 pandemic (both p < 0.001), while no significant changes in the consumption of cannabis were noted. The odds of consuming more alcohol during the lockdown were associated with younger age (OR = 0.981, p < 0.001), more children at home (OR = 1.220, p < 0.001), non-healthcare workers (p < 0.001), and being technically unemployed related to COVID-19 (p = 0.037). The odds of smoking more cigarettes during the lockdown were associated with younger age (OR = 0.988, p = 0.027), current living situation (p < 0.001), lower education (p = 0.015), and working situation related to COVID-19 (p = 0.018). Boredom, lack of social contacts, loss of daily structure, reward after a hard-working day, loneliness, and conviviality were the main reasons for consuming more of the various substances.

Conclusions: During the lockdown, individuals consumed slightly more alcohol and smoked marginally more cigarettes compared to the period before the lockdown. Further research focussing on follow-up of individuals at risk may be useful to provide appropriate care in post-COVID times.