You wake up with a stuffy nose and reach for the orange juice. Or maybe you grab a 1000 mg vitamin C tablet, convinced it’ll knock out your cold in record time. This ritual is so ingrained in popular culture that questioning it feels almost heretical.

Here’s the thing: the evidence doesn’t support mega-dosing vitamin C for colds. But that’s just one of several persistent myths surrounding this nutrient. Let me walk through four of the most common misconceptions about vitamin C “overdose” and what the research actually shows.

What is vitamin C and why do we need it?

Before tackling the myths, some background. Vitamin C (ascorbic acid) is a water-soluble vitamin that humans cannot synthesise internally, unlike most other mammals. We must get it from food or supplements.

It serves several functions in the body:

Antioxidant activity: Neutralises free radicals that can damage cells
Collagen synthesis: Required for producing collagen, which maintains skin, blood vessels, and connective tissue
Immune function: Supports various cellular functions of the immune system
Iron absorption: Enhances uptake of non-haem iron from plant foods
Neurotransmitter production: Needed for synthesising noradrenaline and other neurotransmitters

The UK recommended daily intake is 40 mg for adults, which you can easily get from a single orange or a serving of broccoli [1]. Most people eating a varied diet won’t become deficient. Scurvy, the disease caused by severe vitamin C deficiency, is rare in developed countries.

Natural sources of vitamin C

Most fruits and vegetables contain vitamin C. Good sources include citrus fruits (oranges, lemons, grapefruit), berries (strawberries, blackcurrants, raspberries), kiwi fruit, capsicum (bell peppers), tomatoes, broccoli, Brussels sprouts, and kale.

One practical consideration: vitamin C is relatively unstable. Heat, light, and air exposure reduce its activity. So if you’re trying to maximise vitamin C intake from food:

Eat produce fresh rather than storing it for weeks
Prefer steaming or quick cooking methods over boiling for extended periods
Raw vegetables and fruits retain the most vitamin C

That said, if you’re eating a reasonable amount of fresh produce, you’re probably getting enough regardless of cooking method. Don’t overthink it.

Myth 1: Vitamin C causes kidney stones

This is perhaps the most persistent concern about high-dose vitamin C supplementation, and there’s a kernel of truth buried in here that makes it worth understanding properly.

The theory

When your body processes vitamin C, some of it gets converted to oxalate, which is then excreted in urine. Calcium oxalate stones are the most common type of kidney stones, accounting for about 80% of cases. So the logic goes: more vitamin C → more oxalate → higher stone risk.

What the research shows

A large prospective study following nearly 200,000 people for 11 years found that men taking vitamin C supplements at doses above 1000 mg daily had a 19% increased risk of kidney stones [2]. That sounds concerning until you look closer.

First, the increased risk only appeared in men. Women taking the same doses showed no elevated risk whatsoever. Researchers aren’t entirely sure why, but it likely relates to differences in how men and women metabolise vitamin C.

Second, and this is important: vitamin C from food sources showed no association with kidney stone risk in either sex.

Third, the absolute risk increase is modest. Going from a baseline risk of roughly 1 in 10 (the approximate lifetime risk for kidney stones) to about 1 in 8.4 isn’t trivial, but it’s not catastrophic either.

Practical implications

If you’re a man with a history of calcium oxalate kidney stones or known risk factors, I’d be cautious about taking high-dose vitamin C supplements (above 1000 mg daily). For women, the data doesn’t suggest much concern even at higher doses.

For everyone, getting vitamin C from food rather than supplements seems to carry no kidney stone risk at all. This makes sense when you consider that even vitamin-C-rich foods contain modest amounts compared to supplement pills. A large orange has about 70 mg. To match a 1000 mg supplement, you’d need to eat 14 oranges.

Myth 2: Vitamin C causes diarrhoea and stomach upset

This one isn’t really a myth. High-dose vitamin C can cause gastrointestinal symptoms. But “can cause” and “will definitely cause” are different things, and the dose-response relationship is more individual than most people realise.

What happens

When vitamin C intake exceeds what the gut can absorb, the unabsorbed portion draws water into the intestines through osmotic effects. This can lead to loose stools, cramping, or outright diarrhoea.

The threshold varies

Here’s the interesting part: the dose that triggers GI symptoms varies enormously between individuals and even within the same person depending on health status.

Most people can tolerate 2-6 grams daily without issues. Some people get symptoms at 1 gram; others can take 10 grams with nothing more than slightly loose stools.

When you’re unwell (fighting an infection, for instance), intestinal tolerance often increases substantially. Some researchers claim tolerance can rise to 30-50 grams during acute illness, though I’d take such figures with scepticism. What’s clear is that the threshold isn’t fixed.

So what?

If you do experience GI upset from vitamin C, simply reduce the dose. This isn’t a dangerous side effect; it’s your body telling you it can’t absorb any more. Stop taking it and symptoms resolve within hours.

The NHS advises that taking more than 1000 mg daily may cause stomach pain, diarrhoea, and flatulence in some people [1]. That’s reasonable guidance for most situations.

Myth 3: Natural vitamin C is better than synthetic

This belief has spawned an entire industry of “natural” vitamin C supplements extracted from acerola cherries, camu camu, rosehips, or other sources. These products typically cost 3-10 times more than synthetic ascorbic acid. Are they worth it?

The chemistry

Synthetic and natural vitamin C are chemically identical. Both are L-ascorbic acid with the same molecular structure. Your body cannot distinguish between them.

Bioavailability studies

Multiple studies have compared absorption of natural versus synthetic vitamin C. The consistent finding: no significant difference in bioavailability [3].

A systematic review examining this question concluded that natural-source vitamin C and synthetic ascorbic acid are absorbed and utilised equivalently [4].

Where natural sources might have an edge

That said, there’s a reasonable argument for getting vitamin C from food rather than supplements.

Fruits and vegetables containing vitamin C also provide fibre, potassium, magnesium, various polyphenols, and other compounds that may enhance overall health effects. These co-occurring nutrients can have synergistic effects that isolated supplements don’t replicate.

One systematic analysis found that vitamin C from dietary sources showed better associations with disease prevention than supplemental vitamin C [4]. Whether this reflects synergy with other food components, healthier overall diets among people who eat more produce, or some other factor isn’t entirely clear.

The verdict

If you’re choosing between a “natural source” vitamin C supplement and a cheap synthetic one, save your money. They’re biochemically identical.

If you’re choosing between supplements and actual food, food probably wins for most people, but not because the vitamin C itself is different. It’s the whole package of nutrients that matters.

Myth 4: Mega-doses of vitamin C cure or prevent colds

This is the big one. The belief that vitamin C prevents or treats the common cold is so widespread that many people consider it established fact. It isn’t.

The history

Linus Pauling, a Nobel Prize-winning chemist, popularised this idea in the 1970s. He claimed that gram-dose vitamin C could prevent colds and even treat cancer. His scientific credentials lent credibility to these claims, which were based on limited evidence and personal conviction rather than rigorous trials.

What does the evidence actually show?

The most comprehensive analysis comes from Cochrane systematic reviews, which pool data from multiple randomised controlled trials.

Prevention: Taking vitamin C supplements regularly does not reduce how often the average person catches colds [5]. If you take 200 mg or more daily as prevention, you’ll still catch roughly the same number of colds as someone taking nothing.

However, there’s a notable exception. In people under extreme physical stress, such as marathon runners, soldiers in subarctic conditions, or skiers, regular vitamin C supplementation cut cold incidence roughly in half [5]. For these specific populations, there’s real benefit.

Duration: Regular vitamin C supplementation modestly reduced cold duration. For adults, colds were about 8% shorter. For children, about 14% shorter [5]. In practical terms, if your cold normally lasts 10 days, it might last 9 days instead. Not nothing, but hardly dramatic.

Treatment: Taking vitamin C after cold symptoms start doesn’t appear to help much [5]. If you’re already sick, popping vitamin C tablets probably won’t make the cold go away faster.

Why does the myth persist?

Several factors keep this belief alive:

Confirmation bias: If you take vitamin C when you feel a cold coming on and the cold turns out mild, you might credit the vitamin. You don’t consider all the times colds were mild without taking anything, or times when vitamin C didn’t seem to help.

Regression to the mean: When you feel most unwell is often when you reach for remedies. Illness naturally improves over time regardless of treatment, so anything you take at the worst point gets credited with the improvement.

The Pauling halo: Linus Pauling was genuinely brilliant in chemistry. His recommendations on vitamin C weren’t evidence-based, but his Nobel Prize gives them an air of authority.

How much vitamin C should you actually take?

Given all this, what’s a reasonable approach?

For most healthy adults, the 40 mg recommended daily intake covers basic needs. You can easily achieve this through diet.

If you want some buffer or don’t eat much produce, 100-200 mg daily from a supplement is reasonable. At these doses, absorption is nearly complete and side effects are essentially nil.

Higher doses (500-1000 mg) are probably safe for most people but offer diminishing returns. Absorption efficiency drops as dose increases. At 200 mg, nearly 100% is absorbed. At 500 mg, about 75% is absorbed. At 1250 mg, only about 40% makes it into your bloodstream [6].

Above 1000 mg daily, you’re largely paying for expensive urine. The excess is simply excreted. Men with kidney stone risk should be particularly cautious at these doses.

For therapeutic purposes during acute illness, some clinicians recommend higher doses (2-6 grams) during infections. The evidence supporting this is weak, but short-term high-dose use is unlikely to cause harm in most people.

Who should be careful with vitamin C supplements?

While vitamin C is generally safe, certain groups should exercise caution:

People with kidney stones or renal disease: High doses may increase oxalate excretion and stone risk, particularly in men
Those with haemochromatosis: Vitamin C enhances iron absorption, which could worsen iron overload
People taking certain medications: High-dose vitamin C can interfere with some chemotherapy drugs, blood thinners, and other medications. Check with your doctor.
Those with G6PD deficiency: Very high intravenous doses (not typical oral supplements) can cause haemolysis

For the general population, moderate supplementation (under 1000 mg daily) has an excellent safety record.

References

NHS. Vitamin C. Available at: https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-c/
Thomas LDK, et al. Ascorbic acid supplements and kidney stone incidence among men: a prospective study. JAMA Intern Med. 2013;173(5):386-388. PMID: 23381591
Carr AC, Vissers MC. Synthetic or food-derived vitamin C: are they equally bioavailable? Nutrients. 2013;5(11):4284-4304. PMID: 24169506
Michels AJ, et al. Human genetic variation influences vitamin C homeostasis by altering vitamin C transport and antioxidant enzyme function. Annu Rev Nutr. 2013;33:45-70. PMID: 23642198
Hemilä H, Chalker E. Vitamin C for preventing and treating the common cold. Cochrane Database Syst Rev. 2013;2013(1):CD000980. PMID: 23440782
Levine M, et al. Vitamin C pharmacokinetics in healthy volunteers: evidence for a recommended dietary allowance. Proc Natl Acad Sci USA. 1996;93(8):3704-3709. PMID: 8623000