You probably know the routine. The first scratchy throat hits, and someone in the family reaches for vitamin C, zinc lozenges or echinacea drops.
Of the three, only zinc has clear meta-analytic support for actually shortening a cold. A 2024 Cochrane review pooled 8 treatment trials with 972 adults and found that zinc cut cold duration by about two days on average.
The authors rated the evidence as low certainty. They also found no clear benefit for prevention, and more side effects on zinc than on placebo, mainly nausea and a bad taste.
How zinc works best against a cold
Most of the trials used lozenges, often zinc gluconate or acetate, at high daily doses of 80 mg to 90 mg. Treatment ran for roughly a week.
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Timing is the catch. Guidance from the NIH consistently points to starting zinc within 24 hours of the first symptoms, which is before many people even think about taking something.
One form to skip entirely is intranasal zinc. The NIH Office of Dietary Supplements warns that zinc sprayed into the nose has been linked to severe, sometimes permanent, loss of smell.
What vitamin C does and does not do
Vitamin C is the supplement people most often reach for once a cold has started. That turns out to be the wrong moment.
A Cochrane review by Harri Hemilä and Elizabeth Chalker, covering more than 11,000 participants, found that regular daily vitamin C did not prevent colds in the general population. Taken only at the onset of symptoms, it also did not shorten the illness.
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Regular daily supplementation, taken before anyone catches a cold, did reduce the length of symptoms. The effect was modest. Adults who took vitamin C every day had colds roughly 8 percent shorter, and children about 14 percent shorter.
One group is an exception to the prevention rule. In trials of marathon runners, Canadian soldiers on winter exercise and Swiss schoolchildren at a ski camp, daily vitamin C roughly halved the risk of catching a cold.
Why echinacea is still the hardest to pin down
Echinacea is the most inconsistent of the three. The problem is not the plant itself. Every product on the shelf is different, with varying species, plant parts and extract methods.
The National Center for Complementary and Integrative Health, updated in November 2024, says echinacea may slightly reduce the chances of catching a cold. It also says it is still unclear whether the herb shortens an existing cold.
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Common side effects are digestive, mainly abdominal pain, nausea and stomach pain. Allergic reactions, occasionally severe, have also been reported.
Where vitamin D fits in winter
Vitamin D has drawn attention as an immune-supporting nutrient, especially in winter when sun exposure drops.
A 2017 meta-analysis in the BMJ by Adrian Martineau and colleagues pooled individual data from 25 trials and 11,321 participants. Daily or weekly vitamin D reduced the chance of having an acute respiratory infection by about 12 percent.
The larger effect was in people who started deeply deficient, with blood levels under 25 nmol/L. In that group, vitamin D roughly halved the risk of a respiratory infection.
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For a cold that is already starting, zinc lozenges begun within the first day have the strongest evidence. For vitamin C and vitamin D, the effect only shows up in people who were taking them regularly before symptoms appeared.
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This article is made and published by Mie Hermansen, who may have used AI in the preparation.