Vitamin C won't prevent a cold — and taking it after you're sick may not help either

Vitamin C has no proven effect on preventing colds; consistent use may shorten duration slightly

Starting after symptoms appear shows no clear benefit

Zinc lozenges show some effect — but the doses used exceed Korea's daily upper limit by more than double

This image is not directly related to the article. [Getty Images Bank]

'My throat feels scratchy — should I take two vitamin C tablets?'

Vitamin C and zinc are the first supplements most people reach for at the onset of a seasonal cold. Zinc is also a nutrient the Ministry of Food and Drug Safety has recognized as "necessary for normal immune function."

Yet decades of accumulated research show that the same supplement can produce very different results depending on when and how it is taken.

Vitamin C showed no clear benefit when people began taking it only after cold symptoms appeared. Zinc, meanwhile, has shown effects mainly in lozenge form — dissolved slowly in the mouth — rather than as a swallowed tablet.

This image is not directly related to the article. [123RF]

Too late for vitamin C once a cold strikes

A research team led by Harri Hemilä, a professor at the University of Helsinki in Finland, pooled data from 29 clinical trials involving 11,306 participants in a 2013 Cochrane Review. All trials had subjects take at least 200 milligrams of vitamin C daily on a consistent basis.

The Cochrane Review is widely regarded in the medical community as a highly reliable source of evidence, synthesizing multiple clinical trials under rigorous standards.

The pooled findings showed that consistent vitamin C intake did not reduce the rate at which the general population caught colds. However, it did shorten the duration of illness — by 8 percent in adults and 14 percent in children.

Among people who engaged in intense short-term physical activity — such as marathon runners or skiers — vitamin C cut the risk of catching a cold in half.

The problem is that most people turn to vitamin C only after they have already come down with a cold.

When the research team analyzed seven clinical trials in which participants began taking vitamin C only after symptoms appeared, they found no consistent effect on either the duration of illness or the severity of symptoms.

The same team published a follow-up analysis in 2023 in the international journal BMC Public Health.

Vitamin C had no effect on mild cold symptoms, but it did reduce the duration of severe cold symptoms by 26 percent.

This image is not directly related to the article. [Reuters]

Zinc works — but only when dissolved, not swallowed

Research into zinc as a potential cold remedy began with an accidental observation.

A three-year-old girl in the United States who had leukemia came down with a cold and, instead of swallowing a zinc tablet, let it dissolve slowly in her mouth. Her cold symptoms disappeared within hours.

Her father, George Eby, witnessed this and went on to conduct a clinical trial himself, publishing results in 1984 showing that people who dissolved zinc lozenges in their mouths recovered from colds faster than those who took a placebo.

Researchers theorized that the effect came not from zinc being absorbed into the body, but from the mineral acting locally as it dissolved in the mouth and throat. This suggests the mechanism may be fundamentally different from that of the swallowed zinc tablets commonly sold in South Korea.

A 2024 Cochrane Review, however, said "it is difficult to draw firm conclusions recommending zinc for the prevention or treatment of the common cold based on current evidence." The risk of mild side effects such as nausea and oral discomfort was 34 percent higher among zinc users.

Hemilä's research team, which authored the vitamin C Cochrane Review, argued in a paper published in October 2024 in the international journal Frontiers in Medicine that the earlier Cochrane Review on zinc had grouped zinc lozenges and zinc nasal sprays together in its analysis.

When the team reanalyzed clinical trials involving only zinc lozenges, they found that the duration of colds in adults shortened by 37 percent. However, no effect was confirmed in children.

Effective doses were more than double the recommended daily limit

There are important caveats. The zinc lozenge trials that showed positive results used more than 75 milligrams of zinc per day for a short period of one to two weeks.

The upper intake level for zinc set by the 2025 Korean Dietary Reference Intakes for adults is 35 milligrams per day — meaning the effective doses used in those trials were more than double the domestic limit. Prolonged high-dose zinc intake can cause side effects, including reduced copper absorption.

Hemilä's research team also found that zinc lozenges containing ingredients such as citric acid failed to release their active components properly in the mouth.

An earlier paper reported that of more than 40 zinc lozenge products sold in the United States in 2010, almost none released enough zinc to produce a therapeutic effect.

Most colds resolve on their own within a few days. Whether vitamin C and zinc are truly effective against colds remains a matter of ongoing debate in the scientific community.

References

DOI : 10.1002/14651858.CD000980.pub4

Hemilä H, Chalker E. Vitamin C for preventing and treating the common cold. Cochrane Database Syst Rev. 2013;(1):CD000980.

DOI : 10.1186/s12889-023-17229-8

Hemilä H, Chalker E. Vitamin C reduces the severity of common colds: a meta-analysis. BMC Public Health. 2023;23:2468.

DOI : 10.1128/AAC.25.1.20

Eby GA, Davis DR, Halcomb WW. Reduction in duration of common colds by zinc gluconate lozenges in a double-blind study. Antimicrob Agents Chemother. 1984;25(1):20-24.

DOI : 10.1002/14651858.CD014914.pub2

Nault D, Machingo TA, Shipper AG, et al. Zinc for prevention and treatment of the common cold. Cochrane Database Syst Rev. 2024;5:CD014914.

DOI : 10.3389/fmed.2024.1470004

Hemilä H, Chalker E. Shortcomings in the Cochrane review on zinc for the common cold (2024). Front Med (Lausanne). 2024;11:1470004.

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