The COVID-19 pandemic has seen a surge of health misinformation, which has had serious consequences including direct harm and opportunity costs. We investigated (N = 678) the impact of such misinformation on hypothetical demand (i.e., willingness-to-pay) for an unproven treatment, and propensity to promote (i.e., like or share) misinformation online. This is a novel approach, as previous research has used mainly questionnaire-based measures of reasoning. We also tested two interventions to counteract the misinformation, contrasting a tentative refutation based on materials used by health authorities with an enhanced refutation based on best-practice recommendations. We found prior exposure to misinformation increased misinformation promotion (by 18%). Both tentative and enhanced refutations reduced demand (by 18% and 25%, respectively) as well as misinformation promotion (by 29% and 55%). The fact that enhanced refutations were more effective at curbing promotion of misinformation highlights the need for debunking interventions to follow current best-practice guidelines.