Background Previous studies have reported that credibility and content (argument quality) are the most critical factors affecting the quality of health information and its acceptance and use; however, this causal relationship merits further investigation in the context of health education. Moreover, message recipients' prior knowledge may moderate these relationships. Objectives This study used the elaboration likelihood model to determine the main effects of argument quality, source credibility and the moderating effect of self‐reported diabetes knowledge on message attitudes. Methods A between‐subjects experimental design using an educational message concerning diabetes for manipulation was applied to validate the effects empirically. A total of 181 participants without diabetes were recruited from the Department of Health, Taipei City Government. Four group messages were manipulated in terms of argument quality (high and low) × source credibility (high and low). Results Argument quality and source credibility of health information significantly influenced the attitude of message recipients. The participants with high self‐reported knowledge participants exhibited significant disapproval for messages with low argument quality. Conclusion Effective health information should provide objective descriptions and cite reliable sources; in addition, it should provide accurate, customised messages for recipients who have high background knowledge level and ability to discern message quality.