Introduction The assessment of online health-related information presents a significant challenge for today's youth. Using the Processing of Commercial Media Content (PCMC) model, we investigated the impact of advertising on the perceived credibility of informational content concerning fitness (in boys) and dietary supplements (in girls). Methods In a survey-based preregistered experiment, 681 Czech adolescents (aged 13-18, 52% girls) were randomly assigned to three groups and exposed to websites with a banner ad, a native ad, or simple informational text without an ad. Results The presence of ads on websites diminishes the perceived credibility of informational content. While we did not observe the difference between banner and native advertising, we found one gender-specific difference. Specifically, boys perceive informational content linked with native ads as more credible compared to girls. Additionally, no difference was found between younger and older adolescents. Adolescents demonstrated success in identifying both types of advertisements, irrespective of their age. Conclusions The findings are discussed in light of individual differences and various approaches to processing online content. It appears that adolescents are accustomed to native advertising, which seamlessly integrates with traditional banner ads. Moreover, it is plausible that boys assess advertising texts more heuristically and less systematically than girls. Contrary to PCMC expectations, even younger adolescents seem to possess sufficient skills to identify the advertisements.