Due to rapid technological advances and the increasing diffusion of smart devices, public health applications (apps) have become an integral aspect of public health management. Yet, as governments introduce innovative public health apps (e.g., contact tracing apps, data donation apps, ehealth apps), they have to confront controversial debates that fuel conspiracy theories and face the fact that app adoption rates are often disappointing. This study explores how conspiracy theories affect the adoption of innovative public health apps as well as how policymakers can fight harmful conspiracy beliefs. Acknowledging the importance of word of mouth (WOM) in the context of conspiracy beliefs, the study focuses on the interplay between WOM and conspiracy beliefs and their effects on app adoption. Based on theories of social influence and conspiracy beliefs, substantiated by data derived from a multi-wave field study and confirmed by a controlled experiment, the results show that (1) changes in WOM concerning public health apps change conspiracy beliefs, (2) the effects of WOM on changes in conspiracy beliefs depend on both the sender (peer vs. expert) and the receiver's initial conspiracy beliefs, and (3) increases in conspiracy beliefs reduce public health app adoption and trigger more negative WOM regarding such apps. These results should inform health agencies about how to market innovative public health apps. For consumers with initially low levels of conspiracy beliefs, the distribution of expert WOM supporting the efficacy of public health apps effectively prevents the development of conspiracy beliefs and increases app adoption. However, expert WOM is ineffective in reducing conspiracy beliefs among firm conspiracy believers. These consumers should instead be targeted by campaigns distributing peer WOM that highlights an app's benefits and contradicts conspiracy theories.