In communicating health risks to the public, it is essential that the message delivered is not misunderstood, as this can lead to risk behaviour. There has been much interest in risk communication and trust, but here we take this further by a cognitive and linguistic analysis of the intended and understood meanings of core words in drinking-water incidents. Based on a questionnaire study of 107 undergraduate UK students using the example of a ‘do not drink’ notice, we found that, although the majority (87.9%) would buy and drink bottled water, as many as 44% would still drink the polluted tapwater. Males and females would generally behave similarly; however, significantly more men would drink water straight from the tap. All in all, 78% of the population – males in particular – could potentially be at risk. We trace the risk behaviour to a misinterpretation of the words drink and not. Public health protection is generally perceived as the choosing of what is safe behaviour and informing the public of appropriate actions in the event of a crisis: however, communicators need to also address the words used to make up risk messages and the prior beliefs that they invoke.