Nudge-based misinformation interventions are presented as cheap and effective ways to reduce the spread of misinformation online. However, despite online information environments typically containing relatively low volumes of misinformation, most studies testing the effectiveness of nudge interventions present equal proportions of true and false information. As the effectiveness of nudges can be highly context-dependent, it is imperative to validate the effectiveness of nudge-based interventions in environments with more realistic proportions of misinformation. The current study (N = 1387) assessed the effectiveness of a combined accuracy and social-norm nudge in simulated social-media environments with varying proportions of misinformation (50%, 20%, and 12.5%) relative to true and non-news-based (i.e., "social") information. The nudge intervention was effective at improving sharing discernment in conditions with lower proportions of misinformation, providing ecologically valid support for the use of nudge-based interventions to counter misinformation propagation on social media.