Question & Answer

Can large doses of vitamin C prevent COVID-19?

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  • Vitamin C (ascorbic acid) is an essential vitamin. Adults need about 45mg of vitamin C a day, and you can get it by eating fruit and vegetables (World Health Organisation). If you don’t get enough of this vitamin, that can lead to scurvy, a disease resulting from a deficiency of vitamin C.
  • In March 2020, claims circulated in social and mainstream media suggesting that vitamin C could be used as a treatment for COVID-19. These claims were made by a number of people and included stories of doctors using vitamin C to treat patients with COVID-19 in hospital. Some claims recommended taking large doses of vitamin C to ward off the coronavirus.
  • Using vitamin C for preventing and treating colds and flu has long been controversial. Although vitamin C supports the immune system (European Food Safety Authority), there’s little evidence that it can prevent or treat colds, let alone the new coronavirus disease.
  • A recent Cochrane systematic review found that the evidence is too limited and low in quality to draw any firm conclusions on the role of vitamin C supplementation in the prevention or treatment of pneumonia.
  • In most people, vitamin C supplements do not prevent the common cold. The common cold is a mild infection that causes little harm on its own. By contrast, COVID-19 is a much more serious and harmful disease. There is no evidence vitamin C protects against SARS-CoV-2, the virus that causes COVID-19.
  • Taking vitamin C at less than 1000mg a day is unlikely to cause any harm, but high doses of vitamin C can cause stomach upset and diarrhoea.
  • There is an ongoing study looking at the potential of treating COVID-19 infections using high doses of vitamin C. This trial is not yet finished, and results are expected in September 2020.


  • Currently, there is no high-quality evidence to support the use of Vitamin C in the prevention or treatment of COVID-19.

Things to Remember


  • Drafted by:  Dr Nikita Burke, College of Medicine, Nursing and Health Sciences, School of Nursing and Midwifery, Evidence Synthesis Ireland NUI Galway.
  • Reviewed by: Dr Colette Kelly, Director, Health Promotion Research Centre, NUI Galway
  • Reviewed by: Prof Declan Devane, College of Medicine, Nursing and Health Sciences, HRB-Trials Methodology Research Network, Evidence Synthesis Ireland & Cochrane Ireland, NUI Galway.
  • Evidence Advisor: Dr Frank Moriarty, Pharmacist & Lecturer, School of Pharmacy and Biomolecular Sciences, RCSI & visiting research fellow at The Irish Longitudinal Study on Ageing (TILDA).
  • Evidence Advisor (Public): Anne Daly, PPI Ignite, NUI Galway.
  • Evidence Advisor (Journalistic): Dr Claire O’Connell, Journalist, Contributor, The Irish Times.