COVID-19: A Skeptical Look at Supplements with Immune-Boosting Claims

Supplements used as a “cure” for coronavirus range from ineffective to dangerous

This is not a substitute for information from the CDC or WHO. For access and resources on COVID-19, scroll to the bottom of this article.

There is currently no treatment for COVID-19. Actually, it is unlawful to claim a product can cure, mitigate, or prevent coronavirus. However, that doesn’t mean that supplement companies aren’t side-stepping the red tape by advertising their product as “health-promoting” or “immune-boosting.”

Most of the supplements on this list are backed back by a few studies that suggest they help cure the common cold or have some sort of antimicrobial benefit (not to be confused with antiviral). They are depending on lack of consumer knowledge to sell their products, and it seems to be working.

While none of these supplements can do anything against COVID-19, the safety and research behind them varies from supplement to supplement. Some are ineffective but largely safe to consume, while others are ineffective at treating the coronavirus and dangerous to ingest.

Below is a deep-dive into some of the supplements fraudulently being sold to prevent or treat COVID-19.

Colloidal silver and silver products

Warning: neither safe nor effective can cause argyria

Colloidal silver is one of the supplements on this list that shows no convincing health benefits and may cause serious health problems. Colloidal silver is made of tiny silver particles and is usually in a liquid solution. It’s been sold as an “alternative medicine” for a while, but recently, it’s been marketed as a cure or prevention against COVID-19.

Silver may be listed as

  • Colloidal silver
  • Nanosilver
  • Silver ion
  • Silver chloride
  • Silver iodide
  • Silver oxide
  • Silver phosphate

The buzz around colloidal silver (sometimes called nanosilver) can be explained by some research in nanotechnology that suggests silver may be antimicrobial, meaning it might destroy some bacterial (and maybe fungal) material as well as preliminary in vitro evidence that silver could have antiviral properties. Some research shows nanosilver to have an effect on HIV-1, human immunodeficiency virus, hepatitis B virus, herpes simplex virus, respiratory syncytial virus, and monkey pox virus. (Lara, et al.) (Galdiero, et al.)

This early research is exciting to the broader scientific community, but it should not be misconstrued to mean that silver is either safe or effective in humans, especially with respect to COVID-19. A handful of in vitro studies on its efficacy against a specific strain of virus does not mean it will broadly react the same way against all viruses.

The FDA argues that we just don’t know enough about its safety or efficacy at this time, claiming “there is a lack of adequate data to establish general recognition of the safety and effectiveness of colloidal silver ingredients or silver salts for OTC use in the treatment or prevention of any disease.” While it’s possible there may be a time in the future where nanosilver is an ingredient in common vaccines, this in no way should make you feel comfortable or encouraged to self medicate. Researchers note that “many of the interactions of [silver nanoparticles] with the human body are still poorly understood.” (Dos Santos, et al.) (FDA)

But possible complications from ingesting colloidal silver are understood, and they are far from inconsequential.

According to the National Center for Complementary and Integrative Health (NIH), the most common side effect of ingesting colloidal silver is a condition called argyria, which is caused by a buildup of silver in the body’s tissue. Symptoms of argyria include “ a bluish-gray discoloration of the skin,” and by the way, it’s usually permanent. Various case studies have biopsied discolored skin which verified the presence of silver, of which researchers claim “verified that colloidal silver rather than a prescribed medication was the source of the patient’s dyspigmentation.” (NIH) (Chang, et al.)

One review looked at the potential benefits and hazards of using silver in nanotechnology. The reviewers weigh the benefit of using silver as an antimicrobial against pathogenic microbes (not viruses) against the health cost it poses to scientists. The reviewers summarize that “silver nanoparticles are showing unacceptable toxic effects on human health and the environment,” noting that exposure to silver causes discoloration of the skin and eyes, and state that it may even have greater effects, like damage to the liver, kidneys, and respiratory and intestinal tracts. (Panyala, et al.)

Colloidal silver can also cause negative interactions with other drugs, including antibiotics and thyroxine, a medication used to treat thyroid deficiency. (NIH)

Bottom line? Self-medicating with colloidal silver is not recommended.

Grapefruit seed extract

Warning: neither safe nor effective; products likely adulterated

Grapefruit seed extract is neither safe, natural, or effective at anything and should not be consumed. Despite it’s wholesome, homeopathic-sounding name, it should not be regarded as a safe or natural remedy.

Grapefruit seed extract is commonly used as a disinfectant. That fact alone should cause you to hesitate to supplement your diet with it. You wouldn’t drink bleach to cure a cold, would you? Nor would you down a bottle of Everclear to treat the flu. In case it isn’t obvious, you can’t “disinfect” your body like you can disinfect your countertops. However, this hasn’t stopped manufacturers from promoting grapefruit seed extract as a remedy for just about every ailment (and even a fake one. See: chronic candida).

Little is known about how these companies create the product. Some say they grind the seeds and then process it chemically to yield an extract. But as Science-based Medicine notes, “commercial grapefruit seed extract products may be adulterated with synthetic disinfectants and antibacterials.” Yikes. In the last 20 years, product testing has found triclosan, benzethonium chloride, and methylparaben. (Gavura)

A study examined the contents of six commercially available grapefruit seed extracts. Five out of the six products did indeed show antimicrobial properties, but researchers did not attribute this to any compound found in grapefruit seeds. Instead, they concluded that “the potent as well as nearly universal antimicrobial activity being attributed to grapefruit seed extract is merely due to the synthetic preservative agents contained within.” They continue to claim that any products that do not have added ingredients have no antimicrobial activity. (Woedtke, et al.)

Common cold remedies: some efficacy against the common cold, zero proven efficacy against COVID-19

Some ingredients in common cold remedies, including vitamin C, vitamin D, and zinc, have been shown to decrease the duration or severity of the common cold or influenza. Because of these studies, supplement companies market common cold remedies as “antiviral.” However, these studies do not suggest these remedies will do the same for all viruses. As Kamal Patel of Examine notes, it’s “invalid to extrapolate from efficacy against the common cold or respiratory tract infections broadly to the novel coronavirus in particular.” (Patel)

Before considering the efficacy of these products, it’s important to think about the following points:

  • Minor or moderate evidence that a drug is effective against a cold or flu virus does not mean it will be effective against SARS-CoV-2 (the virus that causes COVID-19).
  • These remedies cannot prevent or cure the common cold, flu, coronavirus, or any other virus. There is only minor evidence that they may shorten the duration and severity of the common cold and flu.
  • These remedies are subject to price gouging as consumers panic buy, which may prompt consumers to overpay for ineffective remedies.
  • Misleading marketing of these products may give consumers false hope in efficacy, of which none has been proven for SARS-VoV-2.

Vitamin C

Vitamin C is a popular common cold remedy. But unlike most cold remedies, this one is supported by science. Research has shown that athletes supplementing with vitamin C can cut the risk of getting a cold in half. High doses of vitamin C has been shown to decrease the duration and severity of the common cold and influenza. But because the coronavirus is a novel virus, there is no evidence to suggest that it could treat, mitigate, or cure COVID-19.

Researchers note that vitamin C might affect the immune system in a positive way, and that research already exists showing that vitamin C can affect viral respiratory infections. For example, there are currently three trials with human subjects that reported a lower incidence of pneumonia in vitamin C-supplemented groups. Animal studies show that high doses of vitamin C may help protect broiler chickens from contracting avian coronavirus. (Hemila, et al.) (Atherton, et al.) (Leibovitz, et al.)

Clearly, more research is warranted on the efficacy of vitamin C supplementation in lieu of the pandemic. Currently, clinical trials are underway to study whether vitamin C can improve the prognosis of patients with COVID-19.

A team in Wuhan, China, hypothesizes that high doses of vitamin C could improve the prognosis of patients with COVID-19. USA Today reports they began the research on February 14 and that no findings have been published. The randomized, clinical trial includes 140 participants and evaluates the efficacy with respect to ventilation-free days, mortality, ICU length of stay, and demand for first aid measurements, among other measurements. (ZhiYong Peng)

At this point, the efficacy of vitamin C is only theoretical. There is no peer-reviewed published research to suggest that high doses of vitamin C will do anything to reduce the severity or duration of COVID-19.

Vitamin c supplementation is largely considered safe, especially when derived from diet. The recommended daily dose of vitamin C for adults is between 65 to 90 mg/day, and the upper limit is 2,000 mg/day. Megadoses of vitamin C may cause GI distress (like diarrhea and nausea), heartburn, abdominal cramps, headaches, and insomnia. (Zeratsky)

It might be tempting to view vitamin C supplementation as a low risk theoretical preventative measure against COVID-19. Certainly, eating foods rich in vitamin C, like citrus fruits and leafy greens, is a very good idea. There is a lot of research to suggest that a healthy diet leads to a better prognosis with just about any disease.

While vitamin C supplementation may be largely safe, the danger might be in assuming that because clinical trials are underway, that means that it will protect from COVID-19. There is absolutely no reason to believe that vitamin C will prevent, mitigate, or cure the disease. Further, the very best outcome from clinical trials is the possibility that it could reduce the duration or severity of illness. Supplementing with vitamin C may offer false hope in the wake of a pandemic that currently lacks a vaccine.


Zinc is an essential trace element, meaning your body needs to ingest very small amounts to function optimally. Most people get enough zinc in their diet. Foods rich in zinc include meat, egg, legumes, and oysters. Zinc is also found in dietary supplements, cold lozenges, and some over the counter cold remedies. (Patel, Kamal)

Zinc is involved in a lot of processes in the body, including immune function. This is the main reason supplement companies are eager to market it as an “immune booster” during the COVID-19 pandemic.

And research around zinc supplements is actually pretty interesting. In limited studies, zinc has done the following:

  • Zinc has been shown to decrease the duration of the common cold.

A meta-analysis including seven trials using zinc lozenges for naturally acquired common colds using a dose of over 75 mg daily found a reduction in the duration of illness. However, trials used doses over the recommended daily allowance of 40 g/day. (Hemila)

  • Zinc has been shown to make a very small reduction in severity of the common cold.

Studies suggest that zinc could reduce sneezing and coughing in those with the common cold. In one study, 65 participants with the common cold took zinc gluconate lozenges with 23 g of zinc for seven days. Each received two at once, then one every two hours, but not exceeding six per day. After seven days, the zinc group was more asymptomatic than the control group. (Eby, et al.)

  • Zinc has been shown to have an intrinsic antiviral activity that extends to the coronavirus family (but not SARS-Cov-2, specifically) in in vitro studies.

One study shows that zinc may have an antiviral property that could extend to the corona family. However, this does not prove efficacy. This has only been studied in vitro, or in the cell. No human studies have been conducted, and no studies on SARS-CoV-2 have been conducted at this time. Further, it’s unknown at what dose this could be replicable in humans, or whether it would work at all. (Velthuis, et al.)

Too much zinc does have side effects. Doses over the recommended daily allowance of 40 mg/day can cause nausea and a metallic taste.

Large doses of zinc can cause a copper deficiency. It’s not recommended to be taken at all, but if it is taken, it should be for a short amount of time. (Patel)

Vitamin D

Warning: Toxic at megadoses

Vitamin D supplementation can help prevent respiratory infections. A meta-analysis looked at 25 randomized controlled trials and found that vitamin d reduced the risk of acute respiratory infections among all patients, especially in those with deficient levels of vitamin D. (Martineau, et al.)

However, a higher dose might not translate into increased efficacy. A randomized trial of 703 participants found that among healthy children, daily administration of 2000 UI or 400 IU of vitamin D supplementation did not reduce overall wintertime upper respiratory tract infections. (Aqlipay, et al.) (Patel)

What’s more, too much vitamin D can be dangerous. Vitamin D toxicity can cause a buildup of calcium in the blood (hypercalcemia). Hypercalcemia can have milder effects, like nausea and frequent urination, but can progress to very serious problems, like the formation of calcium stones, bone pain, and kidney damage. (DeNoon)

Sometimes vitamin D supplements contain magnesium, which assists with activation. While many people are thought to have a magnesium deficiency, magnesium toxicity is possible with supplementation. Too much magnesium can cause difficulty breathing, irregular heartbeat, and even cardiac arrest. (Uwitonze, et al.) (Reddy, et al.) (NIH) (Mayo Clinic)

Instead of supplementing vitamin D, consider eating foods like salmon and tuna, dairy products, and cereals fortified with vitamin D.

Elderberries and Echinacea

There have been three studies on the efficacy of elderberry extract on upper respiratory tract infections. All three claimed some benefit of elderberry, but only 77 participants received the treatment. (Holst, et al.)

Elderberries could be safe and could have some efficacy against upper respiratory tract infections, but the bottom line is that there isn’t enough research available to say with any certainty that they are effective or safe to supplement. Varying doses could have varying results. And like any homeopathic remedy, the benign-sounding name of “elderberry extract” may lull people into a false sense of security when evaluating who can receive supplementation and at what dose.

Further, homemade elderberry supplements could be especially dangerous. Improperly cooked berries can cause cyanide toxicity, and the rest of the plant is poisonous under any preparation. (Patel)

Echinacea is a popular herb used as a cold remedy. There have been 20 clinical trials involving echinacea, but researchers conclude that the “efficacy of Echinacea sp. is dubious” due to widely varying results among studies. Echinacea can interact with immunosuppressive drugs and other medications. (Holst, et al.)

Echinacea is generally well-tolerated, but there have been case studies that document liver damage from use. Toxic hepatitis is thought to be caused by products that are mislabeled. Some contain zero echinacea, and worse, selenium, arsenic, and lead have been found in some echinacea products.

Echinacea may interact with medication, especially those changed by the body and those changed by the liver. It also interacts with immunosuppressants.

Researchers suggest that because of lack of evidence of safety and efficacy, elderberries and echinacea should not be supplemented, especially by pregnant women.


Two studies suggest that garlic can reduce the duration of the common cold as well as the frequency. Of course, like other common cold remedies discussed here, this does NOT mean garlic will have any efficacy against COVID-19.

In one study, 146 participants received a placebo or an allicin (the active compound in garlic) containing supplement for 12 weeks. Researchers found that the treatment group had fewer colds vs the control group (24 vs 65). The placebo group reported more days of illness with longer duration of symptoms (5.01 vs 1.52 days). (Josling, et al.)

There is some toxicity associated with garlic supplementation. For example, garlic oil is lethal to rats at a dose of 100 mg/kg (rats). It’s unlikely that you could eat enough garlic to experience toxicity, but it is possible to overdose with supplementation through garlic oil, garlic extract, or any kind of pill.

Oregano oil, grapefruit seed extract, oregon grape, olive leaf extract, and other antimicrobial supplements

Any supplement that claims antimicrobial properties will not treat, prevent, or lessen the symptoms of COVID-19. Supplements with antibiotic or antimicrobial properties will do nothing to prevent, mitigate, or cure a viral infection.

Oregano oil has been studied as a way to slow down the rate at which meat goes bad. It’s been hypothesized that oregano may have some antimicrobial properties that would function in humans, but only one study has ever been conducted, and this study was funded by a producer of oregano oil. While more research is needed, with respect to COVID-19, oregano products are wholly ineffective and a waste of money.

The root and stem of oregon grape is applied to the skin to treat some skin conditions, and is possibly safe for that application. It’s also ingested to treat stomach ulcers, heartburn, upset stomach, and for some reason, COVID-19. There is no evidence that oregon grape can do anything other than reducing the skin condition psoriasis when applied directly to the skin. There is no reason to believe that oregon grape is safe for consumption, especially for children and pregnant women. Further, it does interact with some medications. (WebMD)

There is some evidence that olive leaf extract can reduce the duration and severity of upper respiratory illness in high school athletes. (Somerville, et al.) However, the amount of the active compound, calcium elenolate, is hard to detect in effective levels in olive leaf extract. (Patel)

We don’t know much about it’s efficacy as an antibacterial or antiviral, but supplementation of up to 1000 mg daily for 8 weeks may be safe, assuming there are no allergic reactions. (Patel) However, there is little to no evidence that olive oil leaf extract will have any effect on illness.

What should I do to prevent coronavirus instead of taking supplements?

Consumers are looking to supplements to give themselves an edge against coronavirus that goes beyond common sense solutions, like social distancing and personal hygiene. The truth is no supplement can do anything to prevent or treat COVID-19.

For now, instead of spending your money on unproven, expensive, and potentially dangerous supplements, invest your time and energy into following CDC guidelines.

If you’d like to do more to stay healthy, continue to get regular exercise (in a safe environment) and eat healthy. If it makes you feel better, you can be more intentional about cooking with ingredients with compounds that are mildly effective against the common cold, like vitamin C, vitamin D, zinc, and garlic.

Additional resources for COVID-19 information

About the author:

Matt Mosman (MS, CISSN, CSCS) is a research scientist, endurance athlete, and the founder and Chief Endurance Officer at EndurElite. Matt holds his B.S. in Exercise Science from Creighton University and his M.S. in Exercise Physiology from the University of California. Matt and his family reside in Spearfish South Dakota, where they enjoy running, mountain biking, camping, and all the outdoor adventures Spearfish has to offer.