Identifying with a group can bring benefits to physical and psychological health. These benefits can be found with both small‐scale and large‐scale social groups. However, groups can also be associated with health risks: A distinct branch of medicine (‘Mass Gathering Medicine’) has evolved to address the health risks posed by participating in events characterised by large crowds. We argue that emphasising either the positive or the negative health consequences of group life is one‐sided: Both positive and negative effects on health can occur (simultaneously). Moreover, both such effects can have their roots in the same social psychological transformations associated with a group‐based social identification. Reviewing evidence from across a range of mass gatherings, we offer a conceptual analysis of such mixed effects. Our account shows (i) how social identity analyses can enrich mass gathering medicine and (ii) how social identity analyses of health can be enriched by examining mass gatherings.