Warning messages for electronic gaming machines (EGMs) have been mandated to increase consumers’ ability to make informed rational decisions and reduce excessive gambling consumption. However existing warnings have limited effectiveness. This paper presents the results of the first in situ trial of dynamic warnings for EGMs to evaluate the impact of message content. A series of dynamic warning messages was displayed across EGMs in venues. Gamblers (N = 667) completed surveys to assess message recall and impact on thoughts and behaviours. Participants recalled messages that encouraged self-appraisal to a greater extent than messages providing information. Both message types had a small impact on behaviour by reducing gambling consumption. Messages that specifically discuss money spent appear to have the greatest impact. Salient and effective warnings that interrupt continuous and excessive consumption can serve as the first line of defence in the prevention of harmful behaviours and impede consumers’ movement towards addiction.