Vitamin C

What it is

Vitamin C (ascorbic acid) is found in citrus fruit, peppers, berries, kiwi, broccoli and potatoes. Humans cannot make it, so we need it from food. Severe deficiency causes scurvy, which is now rare in the UK.

It is one of the best-known supplements, mostly taken to ward off colds. It is also used for skin health, iron absorption and as an antioxidant.

A normal varied diet with fruit and veg provides plenty, so most people do not need a supplement.

How it works

Vitamin C is needed to make collagen, helps absorb iron from plant foods, and acts as an antioxidant. Your body absorbs less as the dose goes up and excretes the extra in urine.

What the evidence says

• Strong Preventing and correcting deficiency (scurvy)
  Essential, but easily met by diet.
• Strong Helping iron absorption from plant foods
  Taking vitamin C with iron-rich meals or iron tablets helps absorption.
• Moderate Shortening the length of colds
  Regular use may shorten colds slightly (about a day in adults on average); starting it after symptoms begin shows little effect.
• Limited Preventing colds
  Regular supplements do not reduce how many colds most people get, except possibly in people under heavy physical stress such as marathon runners.
• Limited Skin health and wound healing
  Needed for collagen, but extra tablets in well-nourished people have shown little clear effect.

Evidence ratings are our plain-English summary of how much good human research exists, not a promise of results. How we rate evidence.

Dosage and timing

• The UK reference intake is about 40 mg a day for adults; NHS advice is that you should be able to get this from diet.
• Supplements are usually 250 to 1,000 mg. NHS guidance says taking 1,000 mg or less a day is unlikely to cause harm, though check current guidance.
• Splitting doses (for example 500 mg twice a day) may give better absorption than one large dose.
• Take with food if tablets upset your stomach.

Forms and what to look for

• Ascorbic acid is the standard, cheap and effective form.
• Buffered forms such as sodium or calcium ascorbate are gentler on the stomach.
• "Ester-C" and liposomal vitamin C are marketed as superior; evidence for any meaningful advantage is limited and they cost more.
• Gummies often contain sugar and may have lower doses than labelled.

What to expect, and when

• If you start from normal levels, you will probably notice nothing.
• Any effect on cold duration is modest and only seen with regular daily use over weeks.
• Deficiency symptoms such as bleeding gums improve over weeks once corrected.

Side effects

• Diarrhoea, stomach cramps and nausea at higher doses (typically above 1,000 mg).
• Heartburn or reflux, as it is acidic.
• Possible kidney stone risk with long-term high doses, especially in men with a history of stones.
• Can trigger iron overload problems in people with haemochromatosis.

Interactions

• High-dose antioxidants (including vitamin C) around chemotherapy or radiotherapy should only be taken with oncology team approval.
• It can increase iron absorption, relevant to haemochromatosis.
• May slightly affect some blood thinners at very high doses; check with your pharmacist.
• Can interfere with some lab tests (blood glucose meters and stool tests), so tell your clinician if taking high doses.

Who should avoid it

• People with haemochromatosis or iron overload.
• People with a history of kidney stones, especially oxalate stones, should be cautious with high doses.
• People with G6PD deficiency should speak to a clinician before high doses.
• Anyone having cancer treatment without oncologist approval.

Risks and safety

• The NHS suggests that more than 1,000 mg a day may cause diarrhoea and stomach pain; this is more a comfort limit than a danger.
• Not banned in sport.
• Mega-dose intravenous vitamin C is a separate clinical area and is not covered here.
• Some products labelled "natural" or "rose hip" contain less vitamin C than the headline claim.

Buying well: quality and testing

• Check mg of actual vitamin C per tablet, not just "rose hip extract" amounts.
• Plain ascorbic acid tablets or powder are cheap and fine.
• Third-party seals (NSF, USP, Informed Sport) add reassurance.
• Be wary of sky-high claims for "absorbable" or "liposomal" versions.

Bottom line

Vitamin C is safe and cheap but rarely necessary if you eat fruit and veg. If you do take it, a modest dose is plenty; megadoses mostly end up in the toilet and can upset your gut.