The Covid-19 pandemic ushered in a new era of supplement sales. Between 2019 and the end of 2020, the immune-supplement market skyrocketed from $3.4 billion in annual sales to nearly $6 billion, according to one report. And things show no signs of slowing down. Another report clocked 2023 immune supplement sales at $9.4 billion.

Many people take so-called immune boosters because they believe the supplements help. Others are looking for an all-natural insurance policy. After all, an entire industry worth billions of dollars can’t be that far off the mark, right? Well, as we’ll see, it certainly could!

In one study, researchers bought and analyzed 30 supplements that were being marketed on Amazon for immune health. According to the labels, the supplements contained a total of roughly 300 ingredients, including botanicals like echinacea and elderberry, vitamins like C and D, and minerals like zinc. But only 13 of the supplements had accurate labels. The others had missing ingredients, extra ingredients that weren’t listed on the label, or both.

And those supplements didn’t come cheap. A 30-day supply ranged from roughly $12 to $90. That’s a lot to pay for something that may not even contain what the label claims…and that may not work.

But just because people buy and take those supplements to prevent or treat a respiratory infection doesn’t mean they work. Here’s the evidence from randomized trials testing whether some of the most popular supplements can ward off or treat a respiratory illness. (We only look at randomized trials that measure outcomes that matter, like whether a supplement prevents a cold or the flu or Covid or makes an illness shorter. The rise or fall of any single marker of immune function doesn’t tell you much.)

Vitamin C

Nothing screams “immunity boost!” quite like vitamin C. At least that’s what the companies that sell vitamin C supplements would have us believe. But the evidence that vitamin C actually helps prevent—or treat—a respiratory infection is pretty disappointing.

Across 24 trials with nearly 11,000 participants, people who took 200 to 2,000 milligrams a day of vitamin C for an average of three months were no less likely to catch a cold than those taking a placebo. When they did catch a cold, though, they were sick for slightly less time—about half a day less—than the placebo takers. (A typical cold lasted about a week.)

In another seven studies that tested vitamin C’s ability to treat a cold, taking roughly 3,000 mg at the first sniffle didn’t translate to a shorter or less severe cold.

Vitamin C may help one group avoid colds: intense exercisers. In five trials that enrolled roughly 600 people doing heavy exercise, like ultramarathoners or military recruits exercising in subarctic winter conditions, taking 250 to 1,000 mg a day of vitamin C for two to eight weeks reduced their risk of catching a cold by 50 percent.

Few trials have tested whether vitamin C supplements can resolve Covid symptoms faster. In one, researchers randomly assigned 66 adults with mild-to-moderate Covid symptoms to take either a placebo or 1,000 mg a day of vitamin C for two weeks. Symptoms didn’t clear up any faster in the vitamin C group than in the placebo group.

Bottom line

Unless you’re doing intense exercise, don’t waste your money on vitamin C to prevent or treat a cold or Covid.

Zinc

A zinc deficiency can impair your immune system, so it’s important to get enough zinc in your diet. Most people do. But don’t expect that taking a zinc supplement will help ward off a cold. In an analysis of nine trials that included a total of 1,449 volunteers, taking zinc didn’t lower the risk of getting a cold compared to taking a placebo. But if you already have a cold, sucking on zinc lozenges, which slowly release zinc, may help you fight it off by preventing viruses from replicating or attaching to cells in your throat and nose.

In a handful of trials, adults who used zinc lozenges every few hours during a cold shortened their illness by roughly a third compared to those taking placebo lozenges. The studies—many of them funded by zinc supplement makers or using lozenges the makers supplied—required participants to take at least 75 milligrams of zinc acetate or zinc gluconate a day at the first sign of symptoms. (Zinc lozenges containing citric acid, mannitol, sorbitol, or tartaric acid don’t appear to have the same effect.)

But one of the best-designed studies to test zinc lozenges came up empty. Researchers randomly assigned 87 adults with a cold to suck on either zinc acetate lozenges or placebo lozenges for five days. (If you’ve used zinc lozenges, you know how distinct their flavor can be. The researchers made sure the placebo lozenges roughly matched the flavor of the zinc lozenges.) On average, the volunteers used five lozenges each day. There was no difference in how long it took for each group to recover from their colds.

No randomized trials have tested zinc lozenges for preventing or treating Covid.

Bottom line

Zinc lozenges might help shorten a cold, but the evidence is iffy. Zinc lozenges can also cause nausea and may leave a lingering metallic taste in the mouth.

A vitamin C and zinc combo to treat Covid? 

What about zinc or vitamin C—taken together or separately—for treating Covid? Researchers randomly assigned 214 adults who tested positive for Covid to standard care or to take 8,000 mg of vitamin C, 50 mg of zinc gluconate (in a pill, not a lozenge), or both each day for 10 days. The study was stopped early due to “futility.” Translation: There was no benefit of either supplement—together or apart—compared to people taking nothing.

There is one caveat: The participants knew which treatment they were getting. That type of study design is frowned upon because participants may expect to feel better simply because they know they’re getting a treatment. In this study, though, knowing they were taking vitamin C, zinc, or both made no difference in how the participants felt compared to those who weren’t getting any supplements.

Elderberry

While vitamin C may have a long history as the king of immune supplements, elderberry has become the category’s darling. Despite the supplement’s popularity, few good studies have tested elderberry’s role in immune health.

In the only trial (which was company-funded) to look at whether elderberry can prevent a cold, researchers randomly assigned 312 adults to take elderberry for nine days leading up to airline travel and for six to seven days following travel. Twelve elderberry takers caught a cold compared to 17 placebo takers, though the difference wasn’t statistically significant. That said, because so few people caught a cold, the study may have been too small to find a difference.

Can elderberry help if you already have symptoms?

In two poor-quality company-funded studies, researchers randomly assigned a total of 124 people with the flu or flu-like symptoms to take elderberry or a placebo. In one, volunteers took elderberry as a lozenge for two days. In the other, they took the popular elderberry syrup Sambucol for five days. In both studies, flu-like symptoms resolved faster in the elderberry takers than the placebo takers.

But elderberry bombed in the largest and only studynot funded by an elderberry supplement maker. Researchers offered the antiviral drug Tamiflu to 87 adults and children who tested positive for the flu. (Tamiflu and other antiviral flu drugs can shorten the length of a flu by about a day, though most cases only last about a week.) The study volunteers were randomly assigned to take either one tablespoon of Sambucol or a placebo four times a day for five days. (Children took one tablespoon twice a day.) Flu symptoms lasted just as long—about five days—in the elderberry takers as in the placebo takers.

Bottom line

The research on elderberry’s immune benefits is sparse and mostly company-funded. The largest study (and the only one not company-funded) found no benefit.

Vitamin D

If you want to prevent a cold or flu, vitamin D might help a tiny bit. In one analysis that included 37 randomized trials that enrolled roughly 46,000 people, those who took vitamin D had an 8 percent lower chance of getting a cold or the flu than those who took a placebo. While that finding was statistically significant, an 8 percent drop probably doesn’t mean much for most people.

How about vitamin D for preventing Covid? Two huge trials (both were partly funded by vitamin companies) have looked.

In one, researchers randomly assigned roughly 6,000 volunteers to daily doses of either 800 IU of vitamin D, 3,200 IU of vitamin D, or no supplement. After six months, vitamin D takers (regardless of dose) were no less likely to catch Covid (or any other respiratory infection) than the no-supplement group. (The Recommended Dietary Allowance for vitamin D is 600 IU a day up to age 70 and 800 IU a day over 70.)

In the other trial, researchers randomly assigned 34,601 volunteers to take a placebo oil or cod liver oil (which contained 400 IU of vitamin D) every day for six months. The vitamin D made no difference: By the end of the study, 227 people in the cod liver oil group—vs. 228 people in the placebo group—had tested positive for Covid.

Can vitamin D keep Covid from getting worse in hospitalized patients? A handful of randomized trials have looked.

In two small pilot trials that enrolled a total of 126 people who were hospitalized with Covid, those who received vitamin D fared better than those who were either given a placebo or no vitamin D. (One of the studies was co-authored by employees of a company that makes vitamin D supplements.)

But in two largertrials on a total of 780 Covid patients, those who were given vitamin D when they were admitted to the hospital were in the hospital just as long and were just as likely to end up in the ICU or to die than those who received either a placebo or standard care.

Bottom line

Most evidence shows no meaningful benefit of taking vitamin D to prevent or colds or flu. As for vitamin D and Covid, the case isn’t closed yet. Two bigtrials that randomly assigned a total of more than 4,000 people to take vitamin D or a placebo wrapped up in 2024. Those results will help shed light on whether vitamin D can prevent or treat Covid. Stay tuned.

CSPI’s work on supplement safety and transparency

CSPI has been working for years to improve regulatory transparency, educate consumers about deceptive marketers of untested products, and make it harder for companies to use unsubstantiated claims to market those products.

Dietary supplements aren’t regulated as strictly as prescription drugs and over-the-counter medicines. They are treated more like foods and beverages. New York’s GRAS Transparency bill (S08615/A9295) would require companies to disclose to the state of New York when they use food chemicals that the companies have secretly self-determined are “generally regarded as safe” (GRAS) without notifying the FDA.

Everyone deserves to know what’s in their food and dietary supplements, to know when manufacturers are making unsubstantiated claims, and to feel secure that supplements are being advertised for exactly what they do. Without regulatory oversight, we never will. As long as the GRAS loophole exists as an option for food, beverage, and supplement manufacturers, it will be exploited…and so will consumers.