supplements.

That said, here’s what to know about popular vitamins and botanicals often advertised as immune boosters:

Vitamin C

Long touted as a cold cure, vitamin C doesn’t appear to prevent respiratory illnesses. But it may help shorten colds, according to a review of studies in the Cochrane Database of Systematic Reviews. When study participants took 1 gram of vitamin C every day (not just when illness struck), colds that did occur were 8% shorter in length in adults and 14% shorter in children, says Harri Hemilä, MD, PhD, an adjunct professor in the Department of Public Health at the University of Helsinki in Finland. 

Could vitamin C work against COVID? There’s no evidence yet that it will help. But Hemilä says there’s little harm in giving it a try. Be careful to avoid overdoing, it though. Large doses of vitamin C above 500 milligrams daily can cause digestive upset and may increase the risk for cataracts and the risk of kidney stones in some people.

Vitamin D

Among vitamins that may help ward off illness, vitamin D is a sure bet. A Cochrane review showed that the “sunshine vitamin” cut the number of colds people caught if they took it every day or at least every week. And the incidence of colds was slashed in half among people who started out deficient in the vitamin, according to the study.

Recent research shows that people with low levels of vitamin D are more likely to have a severe case of COVID-19. But so far, no one has studied whether a supplement can help, although research is underway. Excess doses — taking more than 60,000 international units a day for several months — can be toxic. This causes calcium to build up in your blood, leading to nausea, vomiting, weakness, bone pain, and kidney problems.

Vitamin E

Vitamin E shows particular promise for improving immune function in seniors. A large study of nursing home residents in TheJournal of the American Medical Association found it could make a big difference in staying healthy this winter. “We showed that supplementation with 200 IU/day of vitamin E for a year resulted in significant reduction in respiratory infection –particularly upper respiratory infections, including common colds,” says Simin Nikbin Meydani, DVM, PhD, a senior scientist and director of the Nutritional Immunology Lab at the Human Nutrition Research Center on Aging at Tufts University.

Echinacea

This popular cold remedy has a mixed record. But the variation between studies could be due to the dose, the part of this flower used to make the supplement, and even the time of year it was grown, research suggests.

A study shows that echinacea is somewhat effective at preventing colds if taken three times a day during the cold season as a preventive measure and five times a day when you’re actually battling a cold. Using an extract of echinacea in drop form lowered the incidence of colds by 26% and also cut how long the colds lasted. And those taking it used less pain medication when they had colds.

But a newer study did not find it prevented the common cold or cut short how long a cold lasted.

Zinc

There isn’t good data to show that zinc prevents colds. “However, there are half a dozen randomized trials that found that zinc lozenges can shorten the duration of colds when started after the onset of symptoms,” Hemilä says.

Recovering from a cold was three times higher and colds were shortened by 30%-40% among people taking high-dose zinc lozenges, according to data in the Journal of the Royal Society of Medicine. You need 80-100 milligrams of the supplement daily to get the effect. Zinc gluconate and zinc acetate are equally effective.

One study showed that zinc reduced the risk of pneumonia in older people. People who are older may be deficient in the mineral. Possible side effects include a bad taste in your mouth, loss of smell if used as a spray, indigestion, diarrhea, headache, nausea, and vomiting.

Probiotics

The evidence is mixed for taking probiotics to prevent respiratory infections. A review of 33 studies found that while most research found some benefit, it needed further study. One study found that probiotics might help you cut respiratory infections short, compared to not taking the supplement.

When choosing a product, lead study author Tracey J. Smith, PhD, recommends looking for the strains Lactobacillus rhamnosus and Bifidobacterium animalis lactis. These probiotic microorganisms seem to reduce symptoms by muting the body’s inflammatory response, she says. (But if you have a serious chronic disease, check with a doctor first.)

Glutathione

The antioxidant glutathione, which is produced naturally by the body, boosted an important measure of immunity when given to people as a supplement in one study. These included increases in lymphocytes — white blood cells that are a major part of the immune system, including a subset of lymphocytes known as natural killer cells, says study author John P. Richie Jr., PhD, a professor of public health sciences and pharmacology at Penn State University College of Medicine in Hershey, PA. But the study was small — only 12 people — and more studies are needed to show that glutathione actually prevents or reduces illness.

Elderberry extract

A folk medicine favorite, elderberry extract was shown to shorten the flu in one study. People with the flu who got the syrup four times a day for 5 days saw their symptoms improve 4 days earlier that those who got a placebo. The dose was 15 milliliters — about 3 teaspoons.

But in a new study, in the Journal of General Internal Medicine, people who came to the emergency room with the flu had no better outcome when given elderberry syrup than those who got a placebo. More research is needed to see if it really makes a difference.

Elderberry has been suggested as a treatment for COVID-19, but no research has investigated how well it works. There is also some concern that this home remedy could increase the risk of a “cytokine storm,” according to one report. This overreaction by the body to the coronavirus can damage the lungs and other organs, research suggests.

Golden root extract

This supplement comes from the rhodiola plant. One strain of the species — R. rosea — was found to increase the percentage of memory T cells and B cells, important parts of the immune system, in a study of 15 men. But whether that gives protection from seasonal colds and the flu — or any other infection — remains to be seen.