Does vitamin C prevent colds?

In the general population, no. The Cochrane review, with dozens of trials, does not find that taking it daily reduces how many colds you catch; what it finds is a somewhat shorter duration, around 8 % in adults. The exception is people under extreme physical stress (marathon runners, skiers, soldiers in the cold), where incidence does drop noticeably.

How much vitamin C should you take a day?

The EU labelling reference value is 80 mg, easily covered by food. Common supplements carry 500 to 1000 mg; since it is water-soluble, what is not absorbed is excreted in urine, so raising the dose does not multiply the effect.

Is taking vitamin C useful once I already have a cold?

The review finds no consistent benefit from starting it once symptoms have appeared. The small effect on duration is seen in people who were already taking it regularly before falling ill.

Does vitamin C help absorb iron?

Yes, and it is one of its authorised claims. Taking it with a meal containing plant iron (legumes, cereals, leafy greens) improves absorption. It is especially useful on vegetarian and vegan diets, where iron is absorbed less well.

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Vitamin C · Strong evidence

What vitamin C is for and how much to take

The short answer. Vitamin C has well-established functions: collagen, the immune system, iron absorption and tiredness. What it does not do is the thing that made it famous. The Cochrane review on colds, with dozens of trials, does not find that taking it daily stops the general population catching colds; it finds that it shortens them slightly. It is a cheap, safe supplement with badly calibrated expectations.

Several meta-analyses or position stands from scientific societies agree that there is an effect. No figure on this page is attributed to a specific product: the effect belongs to the ingredient and every number carries its source next to it.

01 What it does and how much

Each row is a measured effect, with the figure the study gives and the source number. Where the evidence gives no figure, the column says so instead of making up a percentage.

Effects of vitamin C with their source
Effect	How much	Detail
EU daily reference value	80 mg	The labelling NRV. 1000 mg tubs provide twelve times that reference, and most of it is then excreted in urine. [1]
Functions authorised in the EU	collagen, immunity, iron and tiredness	It contributes to normal collagen formation, normal immune system function, increased iron absorption and reduced tiredness. [2]
Preventing colds (general population)	no effect	The Cochrane review is clear: taking it daily does not reduce how many colds you catch. [3]
Cold duration	~8 % shorter in adults	One day less in twelve, in plain terms. It is a real, small effect and only with regular prior use, not starting when you are already ill. [3]
Extreme exercise and cold	incidence does drop	In marathon runners, skiers and soldiers in extreme cold the review does find fewer colds. That is the subgroup under severe physical stress, not people who go to the gym. [3]

02 How to take it

Dose. 80 mg covers the reference value; common presentations range from 500 to 1000 mg.
When. With food. At high doses, split up, because absorption saturates.
How long before you notice it. For tiredness due to low intake, days.

Details that change the result

It is water-soluble: the excess is urinated out. That is why megadoses are mostly money, not danger.
From 1000 mg at once some people notice digestive discomfort. Splitting it avoids that.
With a plant-based meal it improves iron absorption: its most underrated practical use.
A kiwi, a pepper or an orange provide a large part of the reference value. This is not a case of "impossible from diet".

03 When it makes sense and when it does not

Worth it if

A diet with little fruit and vegetables for stretches of time.
Alongside a meal rich in plant iron, to improve its absorption.
Extreme exercise in the cold or very high training loads, which is the subgroup where the review does see fewer colds.

Not worth it if

To avoid colds, if your life is like that of the general population in the study.
Starting it once you already have symptoms: there the review finds no benefit.
At 1000 mg a day indefinitely, expecting more than at 200.

04 Sources

Every figure above comes from here. They are position stands from scientific societies, meta-analyses and peer-reviewed reviews: if a claim cannot hang from one of these lines, it is not on the page. References are cited in their original form.

Reglamento (UE) 1169/2011, Anexo XIII: valores de referencia de nutrientes. Vitamina C, 80 mg. https://eur-lex.europa.eu/legal-content/ES/TXT/?uri=CELEX%3A32011R1169
Reglamento (UE) 432/2012: declaraciones autorizadas. La vitamina C contribuye a la formacion normal de colageno, al funcionamiento normal del sistema inmunitario, a aumentar la absorcion de hierro y a disminuir el cansancio y la fatiga. https://eur-lex.europa.eu/legal-content/ES/TXT/?uri=CELEX%3A32012R0432
Hemila H, Chalker E. Vitamin C for preventing and treating the common cold. Cochrane Database Syst Rev. 2013;(1):CD000980. Sin reduccion de la incidencia en poblacion general; menor duracion (~8 % en adultos); menor incidencia en personas con estres fisico extremo. doi:10.1002/14651858.CD000980.pub4

This is not medical advice. It is what some studies say about an ingredient, not a protocol for you. With an illness, medication or pregnancy involved, the decision belongs to a healthcare professional who knows you.

Questions about vitamin C

Answered with the data on this page

Does vitamin C prevent colds?: In the general population, no. The Cochrane review, with dozens of trials, does not find that taking it daily reduces how many colds you catch; what it finds is a somewhat shorter duration, around 8 % in adults. The exception is people under extreme physical stress (marathon runners, skiers, soldiers in the cold), where incidence does drop noticeably.
How much vitamin C should you take a day?: The EU labelling reference value is 80 mg, easily covered by food. Common supplements carry 500 to 1000 mg; since it is water-soluble, what is not absorbed is excreted in urine, so raising the dose does not multiply the effect.
Is taking vitamin C useful once I already have a cold?: The review finds no consistent benefit from starting it once symptoms have appeared. The small effect on duration is seen in people who were already taking it regularly before falling ill.
Does vitamin C help absorb iron?: Yes, and it is one of its authorised claims. Taking it with a meal containing plant iron (legumes, cereals, leafy greens) improves absorption. It is especially useful on vegetarian and vegan diets, where iron is absorbed less well.

And now, which to buy

Prices, kept apart from the evidence

This page explains what vitamin C does and deliberately names no brand. The comparison is the other half: 127 products from 11 stores from €0.041 per capsule, sorted by price and by how verifiable their certification is.

See the vitamin c comparison

Other guides

What each supplement does, with its sources