“Take plenty of vitamins — they can’t do any harm!” — really?
I’ll start with the key point, because as a doctor I consider this the most important thing: a vitamin is neither a medicine nor fuel, but a component. The body needs it for its enzymes to work — in specific, small amounts. If someone is deficient, supplementation can make a major difference. If they are not deficient, extra vitamins add nothing — and with fat-soluble vitamins they can cause clear harm. The immune system is not a muscle you can “pump up”: it is a system of balance, harmed both by deficiency and by overload.
Key point
Your body is built like a wall: it needs bricks, mortar and a builder. The builder can lay only as much wall as there are both bricks AND mortar for. If the bricks have run out, it makes no difference that there is still an enormous amount of mortar and five builders are standing there: the wall will not rise by a single course. This is why supplementation makes sense where there is a genuine deficiency (a blood test shows this), and not where there is enough of every substance. To use the earlier example: bringing another 10 bags of mortar is pointless when the bricks are missing. The umpteenth (unnecessary) dose of vitamin C does not make you “stronger”; it simply leaves the body. Other vitamins are stored and can cause harm. Research consistently confirms this: regular, small-dose supplementation works in people who are deficient. In healthy people, even a high dose brings no improvement — quite the opposite. For some vitamins, high doses have been shown to increase the risk of disease.
How does this actually work? Three basic principles
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Most vitamins are coenzymes: a “key component” of a particular enzyme, without which the relevant biochemical reaction cannot take place. One enzyme needs one key — not ten. Once every enzyme in the body has received what it needs, the excess has nowhere to be incorporated: there is no mechanism that would turn extra vitamin into extra immune cells, more antibodies or “stronger” defence. At that point, immune-system performance is no longer limited by vitamins, but by sleep, stress, physical activity, nutritional status and age. This is why the formula “lots of vitamins = a strong immune system” fails in every large study involving participants who were not vitamin-deficient.
Think of maintaining your body as building a wall. Three things determine how high the wall can grow: the brick, the mortar and the builder — and the limiting factor is always whichever is in shortest supply. If the bricks have run out, carrying another ten bags of mortar over will not make the wall higher; if there is no builder, a mountain of bricks and mortar is equally useless. Applied to your body: if you are deficient in vitamin D, taking handfuls of vitamin C will not help — that is not what is missing. This has two practical consequences. First: supplement selectively, not by “carpet-bombing” — a blood test, not advertising, shows which “building material” is in short supply. Second: the most common limiting factor is not even a vitamin, but protein intake (in older people), sleep and physical activity — in other words, it is often not the material that is lacking, but the builder’s rest and working conditions, and there is no capsule for these. A balanced diet based on vegetables and fruit provides most building materials at once: What should be on my plate? →
Excess water-soluble vitamins (vitamin C and the B group) really does leave in the urine — which is why many people say they “can’t do any harm”. This is only partly true: persistently high doses of vitamin C may promote kidney stones in susceptible people, while high-dose B6 can cause nerve damage (numbness and unsteady walking) over several months — something that is recognised late partly because of its “harmless” reputation. Fat-soluble vitamins (A, D, E and K), however, do not leave the body: they are stored in the liver and fatty tissue, and excess builds up over months. Vitamin A overdose causes liver damage and can harm the foetus; extreme doses of vitamin D raise blood calcium levels; and I present the risks of high-dose vitamin E in the research section below — this is no longer a theoretical danger, but an outcome measured in 35 000 people.
What does the research say about each vitamin?
The table below condenses the findings of the largest summary analyses — you will find the details in the research section:
| Product | What does the advertising promise? | What do the studies show? |
|---|---|---|
| Vitamin C | “Prevents colds” | Does not prevent colds in the average person (the number of colds does not fall); with regular use, cold duration is 8–14% shorter; vitamin C started when symptoms begin no longer works1 |
| Vitamin D | “The more, the better” | A small daily or weekly dose measurably reduces the risk of respiratory infections — mainly in deficient people; infrequent, high “bolus” doses do not work2 |
| Zinc | “Immune booster” | Has little or no effect for prevention; when started at the beginning of a cold, it may shorten its duration (uncertain, low-certainty evidence), but side effects are more common (metallic taste, nausea)3 |
| Vitamin E | “Antioxidant cell protection” | At a high dose (400 IU/day), increased prostate-cancer risk by 17% in healthy men4 |
| Beta-carotene, antioxidant cocktails | “Slows ageing” | According to a Cochrane review combining data from 78 studies and almost 300 000 participants, beta-carotene and vitamin E supplementation slightly increased all-cause mortality5 |
| Multivitamin | “Daily insurance” | Has no measurable benefit in a person eating a balanced diet; with a deficient diet (older age, restricted diet), a low-dose combination product is more reasonable than a high dose of any one vitamin |
Notice the pattern: where study participants were deficient (vitamin D in winter, poorly nourished groups), supplementation worked. Where well-fed, healthy people were given high doses, either nothing happened or harm occurred. A vitamin is therefore not a “booster”, but a way to correct a deficiency — and this is not wordplay, but the difference between helping and causing harm.
Who genuinely needs supplementation?
Supplementation has a place in clearly defined situations — these are not advertising categories, but medical indications. Open the one that may apply to you:
In Hungary, from October to March, sunlight is not sufficient for the skin to produce vitamin D, and the requirement cannot be met through diet. The Hungarian professional recommendation suggests around 2000 IU (50 µg) daily for adults during the winter months — a regular, small dose, not a large “bolus” dose, and exactly the form for which favourable data have also emerged concerning respiratory infections.2 In people who are overweight, older or have malabsorption, a doctor may adjust the dose according to the measured blood level.
Vitamin B12 is found almost exclusively in food of animal origin, so supplementation is essential, not optional, with a vegan diet. In older age, reduced stomach-acid production and long-term use of acid-suppressing medicines or metformin impair absorption — deficiency causes anaemia and insidious, partly irreversible nerve damage. It is easy to check with a blood test; in these groups, annual testing is worthwhile.
It is one of the most thoroughly proven forms of supplementation in medicine: starting folic acid before conception significantly reduces the risk of neural-tube defects (spina bifida) in the foetus. For someone planning a pregnancy, this is not a “food supplement”, but part of antenatal care — the gynaecologist determines the dose.
Iron deficiency is the most common genuine deficiency (heavy menstruation, pregnancy, growing children, regular blood donors and malabsorption), and it needs to be treated — but only when confirmed by laboratory tests. Iron taken “as a precaution” causes constipation and stomach problems, while overloaded iron stores are particularly harmful; it is prohibited in hereditary iron-storage disease (haemochromatosis). First a full blood count and ferritin, then tablets — and the cause of the deficiency must also be investigated, because iron deficiency is often a symptom, not a diagnosis.
With coeliac disease, inflammatory bowel disease, after stomach or bowel surgery, illnesses involving persistent diarrhoea, alcohol dependence, strict weight-loss diets and exclusion diets, as well as with certain medicines (acid suppressants, diuretics and metformin), targeted supplementation may be needed — but here too, assessment determines what and how much. For someone in one of these groups, one comprehensive blood test a year is worth more than any product picked off a shelf.
If you buy one: how to choose a food supplement
Legally, a food supplement is not a medicine but a food. This is the most important sentence to understand before buying, because everything else follows from it: the manufacturer does not have to prove either effectiveness or that the box contains exactly what is stated on the label, in the stated amount — notification to the food authority is sufficient for placing it on the market. The mandatory clinical trials, purity checks and manufacturing controls required for medicines are absent here. There are reputable manufacturers — but you cannot distinguish them from the others by the label unless you know what to look for:
| What should you check? | Why? |
|---|---|
| Clearly stated active-ingredient amount per serving | The “proud big number” on the box often refers not to the active ingredient, but to the raw material. With a herbal extract, look for the amount of extract AND the drug:extract ratio (e.g. 5:1) — without this, the contents cannot be assessed. |
| A sensible dose — neither a homeopathic crumb nor a horse dose | Compare it with the daily recommended intake (NRV%): below 10% is window dressing, while there is no good reason for several-hundred-fold overdosing after considering the research findings above. |
| Availability of a batch-level (LOT) test certificate | A serious manufacturer can provide an independent laboratory certificate for a specific production batch, covering purity (heavy metals, microbiology and contaminants). If they cannot, that is information too. |
| For athletes: doping-tested certification | Food supplements are a recognised source of doping scandals: hormone and medicine residues can occur in them. For a competitive athlete, only a tested product (e.g. one with Informed Sport certification) should be considered. |
| For pregnant women and children: consult a doctor | “It’s herbal, so it’s harmless” is the most common mistake — several popular herbs should specifically be avoided during pregnancy. The same applies to herbal teas: Herbal tea — why doesn’t your doctor recommend it? → |
| Do not decide based on promised effects | A food supplement is not allowed to promise treatment of a disease — if a product promises “recovery” or reversal of a disease, that is unlawful in itself and says everything about the manufacturer’s reliability. |
And here is a common argument used to sell supplements: “today’s fruit and vegetables are empty, the soil is exhausted, so everyone needs supplementation”. The reality is far more modest: the nutritional content of cultivated varieties has fallen somewhat in some minerals over the decades, but measurements show nowhere near enough of a decline to prevent a varied diet from meeting requirements. If you eat vegetables, fruit, pulses and wholegrains every day, “exhausted soil” will not cause a deficiency — deficiency is usually caused by none of these foods reaching the plate.
When can vitamin supplementation specifically be harmful?
“It can’t do any harm” is not generally true of vitamins either. In the following situations, supplementation is risky or should only take place after medical advice:
- High-dose vitamin E and beta-carotene – in healthy people, high-dose vitamin E increases the risk of prostate cancer, while beta-carotene increases the risk of lung cancer in smokers; studies also found that both slightly increased all-cause mortality. Do not take them in capsules without a strong reason — it is not possible to overdose on them from food.
- Vitamin A during pregnancy – high-dose vitamin A (retinol) can harm the foetus; during pregnancy, do not consume vitamin-A-containing products or large amounts of liver-based food, and let your gynaecologist decide about supplementation.
- Anticoagulant treatment – vitamin K reduces the effect of coumarin-type anticoagulants (e.g. warfarin and acenocoumarol), while high-dose vitamin E and fish oil may increase bleeding tendency; discuss every product with your treating doctor and list them for the anaesthetist before surgery.
- Kidney stones and kidney failure – long-term, high-dose vitamin C may promote oxalate-stone formation in susceptible people; with impaired kidney function, dosing vitamin D, potassium and magnesium is also a medical responsibility.
- Long-term high-dose B6 – above 50–100 mg daily, it can cause sensory disturbance, numbness and unsteady walking over several months; symptoms begin insidiously and do not always disappear completely after stopping it.
- Haemochromatosis and cancer treatment – iron and vitamin C supplementation is prohibited in iron-storage disease; during chemotherapy or radiotherapy, antioxidant supplements could theoretically reduce the treatment’s effect — every supplement must be discussed with the treating doctor during oncology treatment.
- Herbal extracts alongside medicines – contrary to popular belief, they are not harmless: St John’s wort, for example, reduces the levels of numerous medicines (contraceptives, anticoagulants, antidepressants and transplant medicines). Tell your doctor and pharmacist about every supplement you take regularly.
The correct order
If you feel that “your immune system needs something”, the order is this: first your diet, sleep, physical activity and smoking — no capsule can replace these four “building materials”. Second, laboratory tests: vitamin D level, full blood count, ferritin and, if needed, B12 — these show whether there is anything to supplement at all. Third, targeted supplementation at a normal dose, discussed with your doctor. “A lot of everything, just in case” is not caution, but the most expensive way to achieve nothing.
What do the studies say? Based on readers’ questions
“Does vitamin C really prevent colds?”
This has been studied for 70 years, and the Cochrane review concluded, using data from 29 studies and more than 11 000 participants, that regular vitamin C use does not reduce the number of colds in the general population. What it does measurably achieve is a somewhat shorter cold in regular users — by 8% in adults and 14% in children — with milder symptoms. Vitamin C started when symptoms appear, however, had no detectable effect. One group was an exception: in people exposed to extreme physical exertion (marathon runners and soldiers on Arctic exercises), the number of colds was halved.1 The lesson: unless you are an Arctic soldier, a daily 1000-dose vitamin C product is not a shield — normal dietary intake (peppers, citrus fruit and sauerkraut) is more than enough. On the latter: The health effects of sauerkraut →
“And vitamin D? Everywhere I read that it’s necessary.”
Vitamin D is the exception that proves the rule — because there is a genuine, widespread deficiency during the winter half of the year. An analysis published in the BMJ combined individual data from nearly 11 000 participants in 25 randomised studies: vitamin D supplementation reduced the overall risk of acute respiratory infections by 12%. The details, however, argue against the “high-dose” myth: only small doses taken daily or weekly worked, while infrequent large bolus doses did not — and the benefit was greatest in participants who were clearly deficient (a 70% reduction in risk), but modest in those with adequate levels.2 In other words, it is not lots of vitamin D that protects you, but correcting a deficiency — with a regular, moderate dose from autumn to spring.
“I heard that zinc shortens a cold.”
The 2024 Cochrane review analysed data from 34 studies and more than 8 500 participants. Zinc has little or no effect for prevention: regular use did not meaningfully reduce the risk of colds. There is seemingly favourable evidence for treating a cold that has already started — an average duration about two days shorter — but the certainty of the evidence is low because the studies differ greatly and are partly of poor quality; meanwhile, the risk of side effects (metallic taste, nausea and stomach problems) increased measurably.3 In summary: zinc is not an “immune booster”, at most it is a symptomatic trial during the first days of a cold — and avoid zinc products sprayed into the nose, because they have caused loss of smell.
“What is wrong with antioxidants? They can only be good, can’t they?”
This is the biggest failure of “more = better” thinking. In the SELECT study, 35 533 healthy men received daily 400 IU of vitamin E, selenium, both or placebo for years: 17% more prostate cancers developed in the vitamin E group than in the placebo group.4 A Cochrane review of 78 randomised studies involving 296 707 participants also found that beta-carotene and vitamin E supplementation slightly but measurably increased all-cause mortality.5 The likely explanation is that free radicals are not simply “enemies”: they are also signalling molecules and weapons of immune defence, and artificially administered high doses of antioxidants interfere with this finely tuned system. From food — vegetables and fruit — these substances are harmless and useful; the difference is not the substance itself, but the dose and the packaging. I wrote about the body’s own “maintenance” system here: Autophagy — the cells’ self-cleaning mechanism →
Frequently asked questions
Not in the sense used in advertising — you take something and your immune system “moves up a gear”. The immune system is a system of balance: a response that is too weak means infection, while one that is too strong means allergy and autoimmune disease, so blind “boosting” would not even be desirable. What you can do is correct deficiencies (sleep, protein, vitamin D in winter, physical activity and not smoking), and then your immune system can do what it is genetically capable of. This may sound less attractive on a box, but it is the truth.
This applies only to water-soluble vitamins, and not even completely to them: long-term high-dose vitamin C may predispose you to kidney stones, while high-dose B6 can cause nerve damage. Fat-soluble vitamins A, D, E and K do not leave the body, but are stored — excess accumulates over months, and symptoms (nausea, headache, high calcium levels and liver problems) appear only late. Besides, you paid for whatever you pee out: excess vitamins produce the world’s most expensive urine.
Chemically, most vitamins are the same molecule regardless of where they come from — vitamin C is the same from rosehips and from a factory. The real difference is not in the molecule, but in the packaging: in food, the vitamin arrives with fibre, hundreds of accompanying plant substances and slower absorption; in a capsule, it arrives alone and in a sudden dose. This is partly why the benefits of a diet rich in vegetables and fruit are detectable in every study, while those of isolated vitamin capsules are barely detectable. If you can choose, choose food — not for mystical reasons, but because it is the form of dosing that works.
In a healthy adult who eats a balanced diet, large studies show no measurable health benefit — but a normal-dose product is not harmful either. It may make sense for an older person who eats little and has a restricted diet, during recovery, during a strict weight-loss diet or with exclusion diets. If you choose one in such a situation, a low-dose product with a broad range of ingredients is the sensible option — the exact opposite of a “high-dose immune bomb”. A multivitamin is never the solution to a poor diet, at most it is a crutch.
There is one product supported by good-quality evidence: a regular, moderate dose of vitamin D from autumn to spring. Everything else that really matters is not a capsule: enough sleep, regular physical activity, preferably outdoors, hand washing, not smoking, and recommended vaccinations (influenza and COVID) for the relevant age groups. There is no miracle cure for preventing colds — but a rested, well-nourished body copes measurably better with their course. About treating a cold at home: Colds and salt therapy →
Summary – Quick overview
Sources
- Hemilä H, Chalker E. Vitamin C for preventing and treating the common cold. Cochrane Database Syst Rev. 2013;(1):CD000980. DOI: 10.1002/14651858.CD000980.pub4
- Martineau AR, Jolliffe DA, Hooper RL, et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ. 2017;356:i6583. DOI: 10.1136/bmj.i6583
- 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: 10.1002/14651858.CD014914.pub2
- Klein EA, Thompson IM, Tangen CM, et al. Vitamin E and the risk of prostate cancer: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). JAMA. 2011;306(14):1549-1556. DOI: 10.1001/jama.2011.1437
- Bjelakovic G, Nikolova D, Gluud LL, Simonetti RG, Gluud C. Antioxidant supplements for prevention of mortality in healthy participants and patients with various diseases. Cochrane Database Syst Rev. 2012;(3):CD007176. DOI: 10.1002/14651858.CD007176.pub2
- Directive 2002/46/EC of the European Parliament and of the Council on the approximation of the laws of the Member States relating to food supplements. EUR-Lex. eur-lex.europa.eu
