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Limited evidenceImmunity

Vitamin C

An essential antioxidant. Contrary to popular belief, it does not prevent colds in the general population — but it may shorten them slightly.

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What the science says

  • Cold prevention Limited evidence

    A Cochrane review with over 11,000 participants: regular supplementation does NOT reduce cold incidence in the general population. Exception: people under extreme physical stress (marathon runners, soldiers), whose risk was cut in half.

  • Shorter cold duration Moderate evidence

    Regular use (not use started after symptoms appear) modestly reduced cold duration: ~8% in adults and ~14% in children.

Dosage & safety

Studied doseThe daily requirement (~90 mg) is easily met through diet. Studies use 200 mg to 1 g/day for the effect on cold duration.
SafetyVery safe; excess is excreted in urine. Doses above 2 g/day can cause diarrhea and increase kidney stone risk in predisposed people.

How to take it

TimingTiming does not matter; for cold-duration studies it was taken daily, before symptoms started.
With food?With or without food; food reduces the stomach upset some people get from gram doses.
Worth knowingAbsorption saturates around 200-400 mg per dose — splitting 1 g into two doses delivers more than one big tablet does.

Interactions

With medications

  • chemotherapy and radiotherapy

    Whether high-dose antioxidants blunt treatments that work partly through oxidative damage is unresolved, and oncology teams generally want to know. do not take gram doses during active cancer treatment without telling your oncologist

  • aluminium-containing antacids

    Vitamin C increases absorption of aluminium, which matters in kidney impairment. separate by at least 2 hours, and avoid the combination if you have kidney disease

With other supplements

  • non-heme iron

    Vitamin C substantially increases iron absorption — useful when correcting deficiency, unhelpful in iron overload conditions. take together deliberately for iron deficiency; avoid pairing them in haemochromatosis

  • copper

    High-dose vitamin C reduces copper absorption over time. separate by a few hours if you take both long-term

Well-established interactions only — this is not a complete list. Always tell your doctor and pharmacist what you take.

Frequently asked questions

Does vitamin C prevent colds?

Not for most people — the evidence is limited. A Cochrane review of over 11,000 participants found regular vitamin C does not reduce how often the general population catches colds; the exception was people under extreme physical stress, such as marathon runners.

Does vitamin C shorten a cold?

Modestly, with moderate evidence, if you were already taking vitamin C regularly: colds were about 8% shorter in adults and 14% in children. Starting vitamin C after symptoms appear is not what the trials tested.

How much vitamin C per day?

Diet easily covers the roughly 90 mg daily vitamin C requirement; cold studies used 200 mg to 1 g/day. Vitamin C absorption saturates at around 200–400 mg per dose, so splitting a gram into two doses delivers more.

Can you take too much vitamin C?

Vitamin C is very safe, but doses above 2 g/day can cause diarrhea and raise kidney stone risk in predisposed people. Avoid gram doses of vitamin C during active cancer treatment without telling your oncologist.

Scientific references

Where to buy

California Gold Nutrition, Gold C®, USP Grade Vitamin C, 1,000 mg, 60 Veggie Capsules

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