Zinc for colds:
what the studies show and what the label is not allowed to say
Zinc does not seem to protect you from catching a cold, and the most recent review of the evidence says it might shorten one, with a certainty the authors themselves rate as low. The part nobody reads is something else: the doses that produced the study results are several times higher than a supplement sold in the European Union is allowed to contain.
The short answer,
before you reach the shelf
Zinc taken as a preventive does not seem to protect you from catching a cold. Zinc started after you have caught one might shorten the episode, but the evidence is weak and comes from studies that used lozenges at doses far above what a supplement in the European Union is allowed to contain. Vitamin C does not prevent colds in ordinary people, but in those who already take it every day it shortens the episode by 8% in adults. Eight per cent of a seven-day cold is a little over thirteen hours.
That is it. The rest of the article explains where these figures come from, why the shelf tells you something different and why no manufacturer, ourselves included, is allowed to print the word cold next to the name of an ingredient on the packaging.
The gap between what the studies show and what a label can promise is not marketing hypocrisy. It is the law, and it is a law that exists precisely because the evidence is so thin.
If you have landed here because it is September and you are getting ready for the cold season, the article on which vitamins to take in autumn covers the big picture. This one goes deep into a single shelf.
What was actually measured,
in 8,526 people
The idea that zinc does something for colds is not a marketing invention. It comes from the laboratory, back in the eighties, and has since been tested in dozens of studies. The problem is not that nobody checked. The problem is that the checks have not added up to a clear answer.
The most recent systematic review published in the Cochrane Library pooled 34 studies, 15 on prevention and 19 on treatment, with 8,526 participants in total. Twenty-two were in adults, twelve in children. It is the best overall picture we have, and it is worth reading piece by piece, not as a headline.
The question: if you take zinc before you catch a cold, do you catch colds less often?
The result: the relative risk of catching a cold was 0.93, with a confidence interval from 0.85 to 1.01. The interval includes 1, which means it also includes the scenario in which zinc changes nothing at all. The authors’ wording is literally this: there may be a small reduction, or none, in the risk of developing a cold.
Certainty of the evidence: low.
The question: if you take zinc after you have caught a cold, do you get over it faster?
The result: the average length of the cold was 2.37 days shorter. That sounds like a lot. But the confidence interval runs from 0.53 days to 4.21 days, that is, from half a day to more than four. When one end of the interval is eight times the other, the average in the middle is not a promise.
Certainty of the evidence: also low.
The question: what do you pay for it?
The result: the authors write that there is probably an increased risk of non-serious side effects when zinc is used as a treatment. The most common were taste disturbances and stomach upset.
Worth remembering: this is the only one of the three where the certainty is better than low. In other words, the best proven thing about zinc for colds is that it spoils your sense of taste.
It is also worth saying that the review has not gone unchallenged. A research group later published a methodological critique of it, which is a sign of a healthy field, not of chaos: when the evidence is borderline, serious people argue about how to count, not about what sells better.
The dose in the studies
is not the dose in the jar
This is the part we did not find in any of the results on the first page of search results, even though it is the one that changes everything.
Of the 34 studies, 17 used lozenges. Not swallowed capsules: lozenges that dissolve slowly in the mouth. And the doses of zinc gluconate ranged from 45 to 276 mg a day, given for anywhere between four and a half and twenty-one days.
Compare that with the tolerable upper intake level for zinc from all sources combined, set at European level: 25 mg a day for an adult. The lowest dose in the treatment studies is almost double that limit. The highest is more than eleven times the limit.
| What you are comparing | In the treatment studies | In an EU supplement |
|---|---|---|
| Amount of zinc per day | 45 to 276 mg zinc gluconate | under 25 mg in total, from all sources |
| How it gets into the body | Lozenges, dissolved slowly in the mouth, in most of the studies | Swallowed, as a gummy, capsule or tablet |
| For how long | Between 4.5 and 21 days, only for the length of the episode | Every day, as a steady intake |
| What it aims for | An effect measured over the course of an illness | Meeting an intake, not treating anything |
Two interventions with the same name and very little in common. When someone sells you a 10 or 20 mg zinc supplement by citing a study in which people sucked 100 mg a day for a week, they are not lying to you about the study. They are lying to you about what is in your hand.
Why we do not recommend seeking out the doses used in the studies: they exceed the upper limit, which was set precisely because excess zinc has known effects, including interfering with copper absorption. Those studies were short, supervised and had a stated research purpose. A daily intake above 25 mg, taken for months because that is what an abstract said, is something else.
Vitamin C:
the two figures that always get mixed up
Vitamin C has a longer history and, paradoxically, cleaner evidence. The benchmark Cochrane review pooled 29 comparisons with 11,306 participants for the question of getting ill, and 31 comparisons with 9,745 episodes for the question of duration. The studies included used at least 0.2 g a day.
The first figure, on prevention: in studies of the general population, with 10,708 participants, the relative risk was 0.97, with a range from 0.94 to 1.00. Put simply, daily vitamin C does not protect you from colds. The number of colds a year stays essentially the same.
There is one well-documented exception, and it is specific. In five studies of 598 marathon runners, skiers and soldiers training in subarctic conditions, the relative risk was 0.48, with a range from 0.35 to 0.64. Almost halved. If you run marathons or train outdoors in freezing weather, that figure applies to you. If you go to the office, it does not.
The second figure, on duration, is the one most often misquoted: in adults who were taking vitamin C every day, colds were 8% shorter, with a range from 3% to 12%. In children, 14% shorter, with a range from 7% to 21%, and at 1 to 2 g a day, 18% shorter.
The figure applies to people who were already taking vitamin C every day before they caught the cold.
It is worth looking at without enthusiasm and without scorn. Half a day less of a cold is not nothing, but it is not a reason for anyone to buy anything either. It is the kind of effect you get for free by eating a varied diet, because vitamin C also comes from food, and the nutrient reference value on the label is 80 mg.
What happens if you start
when your throat already hurts
This is what almost everyone actually does. Nobody takes vitamin C in August for a cold in November. People buy effervescent tablets on the day they first feel a scratch in their throat.
The review tested exactly this scenario separately, in seven comparisons covering 3,249 episodes, with vitamin C given therapeutically, that is, after symptoms appeared. The authors’ conclusion is short: no consistent effect of vitamin C was seen on either the duration or the severity of colds in the therapeutic trials.
The 8% from the previous chapter does not apply to the sachet you open on the first day of a dry throat. It applies to the daily habit you already had beforehand. These are two different things, and advertising mixes them up on purpose.
For zinc, the situation is the reverse, which is exactly why it is confusing: there, the signal, such as it is, appeared in the treatment studies, not the prevention studies. But, as we showed above, those studies used lozenges at doses you cannot legally buy as a supplement here.
What the label is allowed to say
and what nobody is
Let us finish with the part that none of the articles in the top search results mentions, even though it is the one that explains why the shelf looks the way it does.
European Regulation 1169/2011, Article 7, says that information about a food must not attribute to it the property of preventing, treating or curing a human disease, nor refer to such properties. A cold is a disease. So nobody, absolutely nobody, is allowed to write on a supplement that it treats colds.
There is one more category, defined in the health claims regulation: the reduction of disease risk claim, meaning any claim that states, suggests or implies that a food or one of its constituents significantly reduces a risk factor in the development of a disease. Such claims are not banned, but they are authorised one by one, at European level, and may only be used if they appear in the Community register. For zinc, vitamin C and colds, no such authorisation exists.
What remains are the functional claims, the flat ones. For vitamin C, the authorised wording is that it contributes to the normal function of the immune system. For zinc, the European scientific panel established a cause and effect relationship between dietary zinc intake and the normal function of the immune system, and from the authorised list we normally use the wording that it contributes to normal cognitive function. For vitamin D, that it contributes to the normal function of the immune system and the maintenance of normal muscle function.
Normal function of the immune system means that without that nutrient the system does not work normally. It does not mean that taking more makes it work better than normal, and even less that it protects you from any particular virus.
And one more condition that often gets skipped: a claim about a nutrient may only be used if the product contains a significant amount of it, that is, at least 15% of the reference value per 100 g or per serving, or 7.5% per 100 ml in the case of drinks. These rules do not apply only to the packaging. They apply to any commercial communication, including product pages and adverts.
We have explained this mechanism in detail, with examples of wordings that pass and wordings that do not, in the article on which vitamins to take in autumn.
The upper limit
and why it matters when you take two products
Zinc has a tolerable upper intake level, that is, a daily amount from all sources combined below which no adverse effects are expected. Unlike vitamin C, for which there was not enough data to set such a limit, for zinc the figures exist and they are fairly low.
| Age | Upper limit of zinc per day |
|---|---|
| Adults | 25 mg |
| 11 to 17 years | 25 mg |
| 7 to 10 years | 22 mg |
| 4 to 6 years | 18 mg |
| 1 to 3 years | 13 mg |
| 7 to 11 months | 10 mg |
The figure that matters for an adult, 25 mg, is the total. It includes food, it includes your multivitamin, it includes the hair and nails formula and it includes the seasonal product you bought in November. Nobody adds it up, because each box tells its own story in percentages of the reference value, and the reference value for zinc is 10 mg. A product with 227% of the reference value sounds generous. In milligrams, that is 22.74, or more than 90% of your daily limit, taken up by a single product.
The German Federal Institute for Risk Assessment drew exactly this conclusion and proposed a solution: a maximum of 6.5 mg of zinc per recommended daily dose of a single supplement, plus a label warning above 3.5 mg telling people not to take other zinc supplements as well. It is not law in Romania. But it is the clearest signal that the problem is not the product taken on its own, but the total.
Check before you add anything: if you already take a multivitamin or a hair, skin and nails formula, read the amount of zinc in milligrams on each one, not the percentage. If the total goes over 25 mg a day, drop something. Children should not take adult supplements, and during pregnancy and breastfeeding any supplement should be discussed with your doctor first. We have written at length about what to take together and in what order in the complete supplement timetable, and about why adult doses cannot simply be divided down for children in the article on vitamins for children at the start of the school year.
Four things people say
about zinc and vitamin C
We picked the four we actually came across in the top Romanian search results, in the order in which they appear.
The signal in the review comes from treatment studies, so the idea of starting at the first sign is not absurd. But those studies used lozenges with 45 to 276 mg of zinc a day, and the certainty of the evidence is rated as low. A swallowed 10 or 20 mg gummy reproduces neither the dose nor the way it is taken.
The relative risk measured in the general population, across 10,708 participants, was 0.97, with the upper end of the range at exactly 1.00. The documented exception, with the risk almost halved, is in marathon runners, skiers and soldiers training in subarctic conditions. It is a real exception, but it is not your everyday life.
For vitamin C, an upper limit genuinely could not be set, for lack of sufficient data. For zinc it could, and it is 25 mg a day for an adult, from all sources. Zinc is a mineral, not a water-soluble vitamin, and too much of it has known effects, including on copper absorption. The difference between the two is exactly why they cannot be lumped together.
This percentage circulates across the whole Romanian shelf, sometimes as 30%, sometimes as 33%, always without a named study. We could not identify its source. The figure in the current review is a difference of 2.37 days, with a range from 0.53 to 4.21 and low certainty, which is something else.
What is in our gummies
and what they are not allowed to promise
We have a product with vitamin C, zinc and vitamin D3, and the cold season is when it sells. So let us put the figures on the table, including the awkward ones.
The daily dose of two gummies contains 500 mg of vitamin C, 22.74 mg of zinc, 1,600 IU of vitamin D3, that is, 40 µg, plus 50 mg of rosehip powder. As percentages of the reference values, that is 625%, 227% and 800% respectively.
The 22.74 mg of zinc is more than 90% of an adult’s upper limit and goes over the limit set for a child aged 7 to 10. It is not an unusual dose on the Romanian shelf, but it is a dose that leaves no room for a second zinc product on the same day. If you are taking something else, do not take this as well.
What this product is allowed to say, in short: vitamin C contributes to the reduction of tiredness and fatigue and to the normal function of the immune system, zinc contributes to normal cognitive function, and vitamin D contributes to the normal function of the immune system and the maintenance of normal muscle function. That is all.
What it is not allowed to say, and we do not say: that it prevents colds, that it shortens them, that it stimulates anything, that it boosts your immunity. Not only is it not allowed, but for the first two the evidence would not support the claim even if the law permitted it.
If the part you are really interested in is vitamin D for the cold season, we have written separately about how many units a day make sense, about what time of day to take it and about what the number in your blood test means. Our 4,000 IU vitamin D3 gummies contain a much larger amount than the 1,600 IU here, and those articles explain why the number is not chosen by how big it is.
What to actually do
in the cold season
If you have read this far hoping for a shopping list, sorry. The list looks like this.
First, eat your vitamin C instead of buying it. The reference value is 80 mg a day, and ordinary food covers that without any effort. A supplement makes sense if your diet genuinely does not cover it, not as an insurance policy against viruses.
Second, if you take zinc, add it up. One zinc product a day, under 25 mg in total, and that is it. There is no extra benefit above that limit, and that is where the risks begin.
Third, if you want to take something for the cold season on a sound basis, vitamin D is the conversation with the most solid figures on the whole shelf, and it is a conversation about latitude and light, not about viruses. We wrote the dose guide for exactly that.
Fourth, the rest is boring and it works: sleep, varied food, washing your hands. If the arrival of autumn has thrown your routine off course, the article on the post-holiday reset has the practical side.
When it is no longer about supplements: a high fever that will not come down, symptoms that get worse after seeming to improve, difficulty breathing, chest pain, or feeling unwell for more than ten days with no improvement. The same applies if you have a chronic illness, take regular medication, are pregnant or are breastfeeding. In all these cases, see your GP, not the shelf.
And one last thing, the one worth repeating. The fact that no label is allowed to say cold is not a loophole for marketing to exploit. It is a protective measure. When you do see the word, on packaging or in an advert, you have learned something about the seller, not about the ingredient.
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