Public and private decision-making on health problems relies on scientific evidence. However, scientific knowledge includes uncertainty, as does knowledge about COVID-19. In an experimental study, we tested how the trustworthiness (on the three dimensions expertise, integrity, and benevolence) of a source of information (either a scientist or a politician), was affected when messages were either two-sided (including arguments pro and contra the effectiveness of mask-wearing) or one-sided (only pro arguments). Results showed that scientists were ascribed more expertise and integrity compared to politicians, and both sources were ascribed more expertise when they gave two-sided (instead of one-sided) information. Moreover, trustworthiness ratings on all three dimensions were affected by participants' prior topic attitudes and epistemic certainty beliefs. These findings underline that when a source provides two-sided information, this may increase people's willingness to trust that source. To use this strategy most effectively in health communication, more research should be done on how many and what types of counterarguments to include.