The pandemic has led to a surge in supplements as people hope to ward off COVID-19. Of these, vitamins C and D, along with zinc and melatonin, have gained particular attention.

Should we use supplements to help prevent or treat COVID-19 ? What is the evidence?

Vitamin C

Vitamin C is a water-soluble antioxidant needed to increase the production and function of immune cells. Long touted as a ‘cure’ for the common cold, this has never been realized. At most, regular vitamin C may shorten the duration of a cold, but only by about 8% (about 10 hours over a five-day cold).

The recommended intake is between 75 mg to 90 mg per day (a large orange, one cup of broccoli, a small red pepper). While excess amounts are excreted in the urine, mega-doses can result in intestinal discomfort.

Pre-pandemic studies have suggested vitamin C may benefit people suffering from sepsis (an extreme reaction to infection that can lead to organ damage and death). In these studies, vitamin C is delivered intravenously at doses around fifty times that of dietary recommendations. However, a study on hospitalized COVID-19 patients in China one year ago found no difference in need for ventilation and mortality while it did demonstrate a potential benefit for critically ill patients. Although a number of studies are underway, vitamin C isn’t recommended for treatment.

Vitamin D

Vitamin D is a building block for bones and many steroids in the body. The main source is from the sun. However, with concerns of skin cancer, food sources (oily fish and fortified milk) have taken over. One cup of fortified milk provides approximately 20% of the daily recommended 600 IU intake. Having too much vitamin D is unlikely through natural sources, but taking mega-doses through supplements can result in toxic effects.

People with darker skin and higher amounts of body fat tend to have lower levels of vitamin D, and hospitalizations and deaths from COVID-19 have been greater in these two populations. Subsequent cross-sectional studies have reported a higher risk for COVID-19 in people with vitamin D deficiency. Small randomized trials in hospitals have reported vitamin D supplementation associated with faster clearing of infection and severity, but a larger study in preprint found no difference. Dozens of trials are currently underway to investigate its role in the prevention and treatment of COVID-19.

As a result of the lack of robust evidence thus far, vitamin D is not recommended. However, many health agencies recommend vitamin D supplementation during the winter months and in people with darker skin living away from the equator.


Zinc helps fight off infections and keeps the immune response from getting out of control. Zinc may also reduce the severity and duration of the common cold. People who took zinc lozenges (twice daily recommended amount) reduced the duration of their colds by 40%.

Dr. Scott Lear, Simon Fraser University professor

The daily recommended zinc intake is between 8 to 11 mg. Most people get sufficient amounts from meat, dairy products, beans, and peas. While overt zinc deficiency is rare in Canada, as many as one-third of adults over 60 years may not be meeting these recommendations.

It’s unknown whether zinc has any effect on the COVID-19 virus. Lab-based studies have suggested mechanisms by which zinc may reduce the risk for infection (reviewed here). A retrospective study in preprint found people with low zinc levels had worse outcomes from COVID-19.

However, a non-randomized study found no effect of zinc supplementation on mortality in people hospitalized for COVID-19 and a number of studies are in process. A just-published, much-anticipated study by the Cleveland Clinic investigating supplementation with ascorbic acid (vitamin C) and zinc found no benefit when it came to reducing the duration of COVID-19 symptoms.


Melatonin is commonly known as the sleep hormone. Its release is suppressed by exposure to light resulting in lower levels during the day. Besides promoting the onset of sleep, melatonin is involved in immune function and has anti-inflammatory and antioxidant properties.

Melatonin levels decline with age. As a hormone that your body naturally produces, there is no recommend dietary amount of melatonin. However, most foods contain trace amounts while cherries, nuts, eggs, milk and fish contain higher amounts.

Early on in the pandemic, melatonin was identified as a potential treatment to blunt the effects of COVID-19. A study in 26,779 tested for COVID-19 infection found people taking melatonin had a 52% lower chance of being positive and an observational study currently in preprint suggests treatment with melatonin is associated with positive outcomes in COVID-19 patients requiring intubation. However, given these studies are observational, more evidence is warranted and randomized trials are currently investigating its potential.

The bottom line

The limited studies to date have investigated the use of mega doses of these supplements in the treatment of COVID-19 and shouldn’t be translated to prevention with over the counter doses. In societies where healthy food is readily available, overt nutrient deficiencies are rare. A balanced diet, regular activity, and sufficient sleep are recommended to keep your immune system functioning. Wearing a mask, frequent handwashing and physical distancing remain the best strategies for reducing your chances of getting COVID-19.

Scott Lear is a professor in the Faculty of Health Sciences at Simon Fraser University. He holds the Pfizer/Heart & Stroke Chair in Cardiovascular Prevention Research at St. Paul’s Hospital.

He has been an outspoken advocate for mandatory masks during the pandemic. He writes the weekly blog Feeling Health with Dr. Scott Lear.


Melatonin to prevent and treat COVID-19? Perhaps