Background: To examine whether (1) the 'What Do You Drink' (WDYD) intervention resulted in drinking refusal self-efficacy (DRSE) changes directly after the intervention, and if so, whether these changes sustained at six-months follow-up and (2) DRSE was related to alcohol use over time, and if so, whether the strength of these relationships differed across conditions. Insight herein can help explain the sustained preventive effects of the WDYD intervention on alcohol use, as reported previously. Methods: Alcohol use and DRSE data were collected from 907 participants (60.3% male; M = 20.8 (SD = 1.7) in a two-arm parallel group randomized controlled trial applying ecological momentary assessment with 30 time-points. Participants were randomized to the experimental (n = 456: WDYD intervention) or control condition (n = 451: no intervention). Results: Latent Growth Curve (LGC) analyses that modeled individual change in DRSE over time by condition revealed that participants in the experimental condition experienced a higher social pressure DRSE compared to participants in the control condition at six-months follow-up. Moreover, LGC analyses with time-varying covariates revealed that DRSE was negatively related to weekly alcohol consumption and social pressure DRSE to frequency of binge drinking. The WDYD intervention did not affect the strength of these relationships. Conclusions: The WDYD intervention increased the level of social pressure DRSE directly after the intervention that sustained at six-months follow-up. This change is likely to be responsible for the sustained preventive effects of the WDYD intervention on alcohol use, as reported previously.