Do vitamin D, zinc, and other supplements help prevent COVID-19 or hasten healing?

By: Robert H. Shmerling, MD, Senior Faculty Editor, Harvard Health Publishing

The appeal of safe, natural treatments is undeniable. It's true for age-old conditions such as the common cold, and for new diseases, especially if they have no known cure. So it makes sense that there would be a lot of interest in supplements for COVID-19, whether as prevention or treatment.

Indeed, zinc, melatonin, vitamin C, vitamin D, and other supplements have been commonly prescribed from the earliest days of the pandemic.

But do they work?

Why supplements might help prevent or treat COVID-19

While science can show whether a drug is effective, we may not always know why. When antibiotics were first discovered in the 1920s, there was limited understanding of the biology involved. But lacking an explanation for their benefit did not discourage doctors from recommending these highly effective treatments.

If it's less clear whether a drug works, biologic plausibility — that is, a logical and well-understood reason why the drug should work — increases the expectation that it might.

So, what suggests that vitamin C, vitamin D, zinc, and melatonin might help work against any virus?

• Vitamin C is an antioxidant that has long been promoted as a key player in healthy immune function.
• Zinc may have antiviral activity, whether by improving immune cell function that counters viral infections or by reducing the ability of viruses to multiply.
• Some evidence (see this study and this study) suggests that combining vitamin C and zinc may limit the duration and severity of cold symptoms.

The cases for vitamin D and melatonin differ. While there is also evidence that vitamin D and melatonin may have positive effects on immune function, a specific antiviral effect remains unproven.

What's the evidence that supplements are helpful for COVID-19?

Though COVID-19 is a new illness, a few clinical trials have explored the possibility that supplements may be effective. And, unfortunately, most of the evidence is unconvincing.

For example, a few observational studies link lower blood vitamin levels with a higher risk of testing positive for the virus that causes COVID-19 (see this study and this one). But studies like these cannot prove that vitamin D protects people against infection. Further, a randomized controlled study of people with moderate to severe COVID-19 who received a high dose of vitamin D showed no benefit. On the other hand, a small study of high-risk, elderly persons with COVID-19 found that rates of death at 14 days was lower among those receiving high dose vitamin D than those receiving a lower dose.

A 2021 study of zinc and vitamin C demonstrated no benefit for people with mild COVID-19. In this study, people whose symptoms did not require hospital admission were randomly assigned to receive

• only vitamin C, 8,000 mg/day (the recommended daily amount is 75 mg/day for women and 90 mg/day for men)
• only zinc, 50 mg/day (the recommended daily amount is 8 mg/day for women, 11 mg/day for men)
• both supplements at the doses above
• neither supplement.

The researchers found that people receiving the supplements, whether individually or combined, had no improvement in symptoms or a faster recovery when compared with otherwise similar patients receiving neither supplement.

Proponents of melatonin for COVID-19 have encouraged researchers to perform trials of this supplement, but so far convincing evidence of benefit is not yet available.

Even without convincing evidence, why not take them anyway?

Despite questions about the overall benefit of these supplements, many doctors began prescribing them routinely in the early days of the COVID-19 pandemic. The logic may have been that with so little known about how to best treat this new infection and a long track record of safety for these supplements, why not?

But there are significant hazards to consider. These include side effects, allergic reactions, interactions with other drugs, the cost of unnecessary supplements, and the dangers of taking too much. For example:

• High doses of vitamin C may cause diarrhea or stomach upset. There have also been concerns that high-dose vitamin C supplementation may interfere with blood thinners or cholesterol-lowering medications.
• High doses of vitamin D can cause severe symptoms, such as stomach upsets, kidney injury, and pancreatitis, and may even be life-threatening.

That said, people with nutritional deficiencies should receive supplements. Zinc or vitamin D deficiencies are not rare and may contribute to poor immune function. Therefore, even without specific evidence linking supplement use with improvement among people with COVID-19, these supplements may be appropriate for people in whom deficiency is suspected or confirmed. For example, a person with little sun exposure and a diet low in dairy products may be likely to have vitamin D deficiency. A simple blood test can confirm or rule out vitamin D or zinc deficiency.

If you do take supplements, it's safest to follow the daily recommended amounts your body needs unless your doctor advises otherwise (see this information for people 51 and older, and this information on a full range of supplements).

The bottom line

Based on the science, there is reason to be hopeful that supplements such as vitamin C or D, zinc, or melatonin might help in the fight against COVID-19. While there's no proof yet that they do, additional research could show a benefit in certain situations, or with a different dose or formulation of the supplement. So it's worth keeping an open mind.

In the meantime, we should not dismiss the findings of negative studies just because the results weren't what we'd hoped. When it comes to preventing or treating COVID-19, I'd rely more on the recommendations from the CDC than on unproven supplements.

Check with your doctor before starting a supplement. Ask about dosage, other medications you're taking, and other health conditions you have. The last thing you want to do is to take a supplement that causes more harm than good.