In the coronavirus "infodemic," people are exposed to official recommendations but also to potentially dangerous pseudoscientific advice claimed to protect against COVID-19. We examined whether irrational beliefs predict adherence to COVID-19 guidelines as well as susceptibility to such misinformation. Irrational beliefs were indexed by belief in COVID-19 conspiracy theories, COVID-19 knowledge overestimation, type I error cognitive biases, and cognitive intuition. Participants (N = 407) reported (1) how often they followed guidelines (e.g., handwashing, physical distancing), (2) how often they engaged in pseudoscientific practices (e.g., consuming garlic, colloidal silver), and (3) their intention to receive a COVID-19 vaccine. Conspiratorial beliefs predicted all three outcomes in line with our expectations. Cognitive intuition and knowledge overestimation predicted lesser adherence to guidelines, while cognitive biases predicted greater adherence, but also greater use of pseudoscientific practices. Our results suggest an important relation between irrational beliefs and health behaviors, with conspiracy theories being the most detrimental.