Introduction The US Food and Drug Administration requires e-cigarettes to carry a nicotine addiction warning. This research compared the effects of messages communicating comparative risk of electronic and combusted cigarettes (CR messages) with and without the mandated warning and tested the effects of showing a nicotine fact sheet (NFS) before exposure to CR messages with warning.

Method In an online experiment, 1528 US adult smokers were randomised to one of four conditions: (1) three CR messages, (2) three CR messages in condition one with an addiction warning, (3) an NFS followed by the three messages in condition 2 and (4) control messages. Outcomes included message reactions and perceived effectiveness, e-cigarette-related and cigarette-related beliefs and behavioural intentions and nicotine-related beliefs.

Results CR messages with and without an addiction warning did not differ. The NFS condition produced higher odds of correctly understanding the risk of nicotine and stronger beliefs that switching to e-cigarettes could reduce health risks (response efficacy) than other treatments. Compared with control, all messages made it more likely for people to report e-cigarettes are less harmful than cigarettes and increased response efficacy and switch intentions to e-cigarettes. Only NFS condition increased correct beliefs about the risk of nicotine and self-efficacy about switching to e-cigarettes.

Conclusion Including an addiction warning on CR messages did not reduce intentions to switch to e-cigarettes. Communicating accurate risk of nicotine together with CR messages and addiction warning increased smokers’ self-efficacy beliefs about switching completely to e-cigarettes, making it a potentially promising antitobacco communication strategy.