Objectives Prototypes (i.e., social images) predict health‐related behaviours and intentions within the context of the Theory of Planned Behaviour (TPB). This study tested the moderating role of temporal stability of drinker prototype perceptions on prototype–intentions and prototype–behaviour relationships, within an augmented TPB. The study examined abstainer, moderate drinker, heavy drinker, tipsy, and drunk prototypes. Design and Methods An online prospective study with 1‐month follow‐up was conducted among 410 young adults (18–25 years old, Mage = 21.0, SD = 2.14, 21.7% male). Assessed were prototype perceptions (favourability and similarity, T1, T2), stability of prototype perceptions, TPB variables (T1), intentions (T2), and drinking behaviour (T2). Intention analyses were corrected for baseline behaviour; drinking behaviour analyses were corrected for intentions and baseline behaviour. Results Hierarchical regressions showed that prototype stability moderated the relationships of drunk and abstainer prototype similarity with intentions. Similarity to the abstainer prototype explained intentions to drink sensibly more strongly among individuals with stable perceptions than among those with unstable perceptions. Conversely, intentions were explained stronger among individuals with stable perceptions of dissimilarity to the drunk prototype than among those with unstable perceptions. No moderation effects were found for stability of favourability or for relationships with behaviour. Conclusions Stable prototype similarity perceptions were more predictive of intentions than unstable perceptions. These perceptions were most relevant in enhancing the explanation of young adults' intended drinking behaviour. Specifically, young adults' health intentions seem to be guided by the dissociation from the drunk prototype and association with the abstainer prototype. Statement of contribution What is already known on this subject? Prototypes have augmented the Theory of Planned Behaviour in explaining risk behaviour. Temporal stability has been shown to successfully extend the TPB in explaining intentions. Temporal stability of TPB variables can moderate the relationships with behaviour and intentions. What does this study add? Stability of prototype perceptions moderates the prototype–intentions relationship. Stability of abstainer and drunk prototype similarity enhances the explanation of (intentional) drinking. Stable prototype perceptions are more explanatory than unstable perceptions.