Introduction Tobacco manufacturers’ aggressive promotion of new smokeless tobacco products such as snus warrants a timely and effective public health response. This study tested potential countermarketing messages to discourage current and former smokers from becoming dual users of smokeless tobacco and cigarettes.

Methods In a pretest-post-test experiment, 1836 adult current and recently former smokers from a national sample were randomised to view one of six antismokeless tobacco ads followed by a snus ad, to view a control ad followed by a snus ad; or to view two control ads. Perceived effectiveness of ads and actual changes in attitudes and openness to snus were compared across groups using analyses of variance.

Results Some ads that were perceived as most effective did not change attitudes or openness to trying snus, and conversely, some ads not perceived as effective changed attitudes and openness to snus. Ads portraying the negative health effects of smokeless tobacco were perceived as most effective, but ads with antitobacco industry themes significantly decreased favourable attitudes toward snus. Responses to ads were different for smokers who had ever used smokeless tobacco: for this group health effects and humorous/testimonial ads were effective.

Conclusions Measures of perceived effectiveness of antitobacco ads need to be augmented with measures of actual effectiveness to assess countermarketing messages. Some of the developed ads, such as ads with anti-industry themes, were effective for the overall population of smokers whereas humorous/testimonial and health effects ads were particularly effective in changing attitudes of past users of smokeless tobacco.