The gambling industry tends to frame gambling harms as a matter of personal responsibility, which is implicit in their messages like ‘gamble responsibly’ or ‘take time to think’. Jurisdictions such as Australia and the UK are replacing industry messages with a range of independently designed ones, like in tobacco warning labels. Counter-industry messages have been proposed to challenge industry narratives, e.g., ‘gambling products are designed to be addictive’. Here we tested 10 potential counter-industry messages among UK gamblers (N = 4,094) using a mixed-methods approach. Results showed that the three best-performing messages came from existing counter-industry campaigns. Participants believed the messages and agreed that they were relevant to people experiencing gambling harm. Participants experiencing higher levels of harm tended to see the messages as more personally relevant to them. Free-text analysis showed that ‘gambling products are designed to be addictive’ received the most positive responses, and also that personal responsibility views were widespread among participants. Messages randomly shown later in the experiment were appraised more positively, suggesting that counter-industry messaging may become more effective when its core message is repeated in multiple ways. Continual design and testing will contribute to the development of best approaches and inform future implementation.