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 online data collection (T0 before the campaign, T1 just after it ended, and T2 6 months after it ended). SETTING: France PARTICIPANTS: 2,538 adult drinkers (18-75 years old) MEASUREMENTS: The main outcomes" variables were LTH knowledge (cancer, hypertension, brain haemorrhage), LRDG knowledge (two guidelines: "maximum of 2 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 2 drinks a day" guideline (adjusted odds ratio [aOR] 1.32 [95% confidence interval 1.08-1.62], p=0.008), ii) brain haemorrhage (aOR = 1.80 [1.44-2.25], p