Vitamin C is widely available around the world and was used in some settings for the treatment of COVID-19. Through harmonising two clinical trials – REMAP-CAP and LOVIT-COVID – over 2,500 patients in 20 countries took part, including both critically ill and non-critically ill patients with COVID-19 in hospital. It was shown that high-dose vitamin C did not improve outcomes for patients. This is the largest trial examining high-dose vitamin C in COVID-19 and provides evidence that high-dose vitamin C is not beneficial and suggests a high probability that it may be harmful.