Aims: To evaluate the effectiveness of a French mass-media campaign in raising knowledge of both long-term alcohol-related harms (LTH) and low-risk drinking guidelines (LRDG), as well as in lowering alcohol consumption. Design: An 8-month longitudinal survey from February to October 2019, with three waves of on-line data collection (T0 before the campaign, T1 just after it ended and T2 6 months after it ended). Setting: France. Participants: A total of 2538 adult drinkers (aged 18-75 years). Measurements: The main outcomes' variables were LTH knowledge (cancer, hypertension, brain haemorrhage), LRDG knowledge (two guidelines: 'maximum of two drinks a day' and 'minimum of 2 days without alcohol per week'), intention to reduce alcohol consumption and self-declared consumption with respect to the French LRDG. At T1, exposure to the campaign was measured using self-reported campaign recall. Findings: In T1, we observed significant positive interactions between exposure group based on campaign recall and survey waves on knowledge of (i) the 'maximum two drinks a day' guideline [adjusted odds ratio (aOR) = 1.32, 95% confidence interval (CI) = 1.08-1.62, P = 0.008], (ii) brain haemorrhage (aOR = 1.80, 95% CI = 1.44-2.25, P < 0.001) and (iii) hypertension (aOR = 1.41, 95% CI = 1.09-1.81, P = 0.008) risks. Campaign exposure was also associated with a significant decrease in at-risk drinking in women (aOR = 0.67, 95% CI = 0.50-0.88, P = 0.004). No significant interaction was observed at T1 for the knowledge of the 'minimum of 2 days without alcohol a week' guideline, or of cancer risk. At T2, no significant interaction was observed for the main outcomes' variables. Conclusions: There appears to be an association between exposure to a 2019 French mass-media campaign to raise knowledge of long-term alcohol-related harms and low-risk drinking guidelines and reduce alcohol consumption and (i) improved knowledge of the 'maximum two drinks per day guideline', (ii) knowledge of the risks of hypertension and brain haemorrhage and (iii) a reduction in the proportion of people exceeding low-risk drinking guidelines (in the general population only). These associations were only observed over the short term and, in some cases, only for certain segments of the population.