Vitamin C (ascorbic acid) is a water-soluble vitamin that humans cannot synthesise internally. Unlike most mammals, we lost this ability due to a genetic mutation millions of years ago, which means we depend entirely on dietary intake.

The vitamin performs several critical functions: it acts as an antioxidant, supports collagen production, helps absorb iron from plant foods, and contributes to immune function [1]. These aren’t fringe claims. Vitamin C deficiency causes scurvy, a disease that killed thousands of sailors before anyone understood why citrus fruits prevented it.

The modern interest in high-dose vitamin C largely traces back to Linus Pauling, a two-time Nobel laureate who published “Vitamin C and the Common Cold” in 1970. His claims were controversial then and remain debated now, but they sparked decades of research into whether vitamin C does more than simply prevent deficiency.

So what does the clinical evidence actually show? Below I’ve reviewed the research on 19 proposed benefits, along with the side effects and contraindications you should know about.

When is vitamin C best absorbed?

Water-soluble vitamins don’t accumulate in body fat, so you need regular intake from food or supplements. Absorption is generally best on an empty stomach, either 30 minutes before eating or two hours after.

That said, if you have sensitive digestion, taking acidic ascorbic acid on an empty stomach can cause discomfort. In that case, consider buffered forms like sodium ascorbate or calcium ascorbate, or take your supplement with food and accept slightly reduced absorption.

Natural versus synthetic: does it matter?

The supplement industry often implies that “natural” vitamin C is superior. Chemically speaking, natural and synthetic L-ascorbic acid are identical, and at least two human trials found no clinically meaningful difference in bioavailability [2, 3].

What might matter is the context. Vitamin C from whole foods comes packaged with bioflavonoids and other compounds that could theoretically enhance absorption or provide complementary benefits. Whether this translates to meaningful differences for people taking supplements remains uncertain.

Evidence-based benefits

1. Reducing post-operative pain

Pain after surgery is common. About 80% of patients undergoing outpatient procedures experience it, and poorly managed pain can delay recovery and lead to chronic issues.

A meta-analysis of seven randomised controlled trials (519 participants) found that intravenous vitamin C reduced pain scores and opioid consumption both immediately after surgery (1-2 hours) and at 24 hours [4]. Oral vitamin C didn’t show the same effect.

The caveat: this applies specifically to intravenous administration in clinical settings. Taking vitamin C tablets before dental work probably won’t replicate these results.

2. Reducing bleeding in gingivitis

Periodontitis affects roughly 20-50% of people worldwide and has links to cardiovascular disease and type 2 diabetes.

A systematic review of six randomised trials found vitamin C supplementation improved bleeding index in people with gingivitis, though it didn’t significantly improve probing depth or clinical attachment in established periodontitis [5].

Interpretation: vitamin C might help with mild gum inflammation but isn’t a substitute for professional periodontal treatment.

3. Potential benefits in sepsis

Sepsis kills an estimated 11 million people annually. A meta-analysis of 23 randomised trials (2,712 patients) found that treatment regimens containing vitamin C reduced mortality, organ dysfunction scores, and duration of vasopressor therapy compared with controls [6].

These results are interesting but come with important caveats. The studies were heterogeneous, doses varied widely, and vitamin C was typically given alongside other treatments. I wouldn’t call this settled science, but there’s enough signal to justify continued research.

4. Lowering uric acid levels

Elevated uric acid increases risk of gout and is associated with cardiovascular disease and kidney problems.

A meta-analysis of 16 randomised trials (1,013 participants) found that vitamin C supplementation (average: 500 mg daily for 30 days) reduced serum uric acid levels [7]. The effect was most pronounced in people under 65.

The mechanism appears to involve increased renal excretion of uric acid. Whether this translates to reduced gout attacks hasn’t been conclusively demonstrated.

5. Modest benefits for COVID-19 hospitalisation

One randomised trial of 150 COVID-19 patients found those receiving intravenous vitamin C (50 mg/kg/day) plus standard care became asymptomatic earlier (7.1 versus 9.6 days) and had shorter hospital stays (8.1 versus 10.7 days) compared with standard care alone [8].

No difference in mortality or need for mechanical ventilation was observed. Given the small sample size, I’d treat these findings as preliminary rather than definitive.

6. Reducing C-reactive protein

C-reactive protein (CRP) is an inflammatory marker linked to cardiovascular disease risk.

A meta-analysis of 12 randomised trials (446 participants) found vitamin C supplementation significantly reduced CRP levels [9]. The effect was strongest in people with baseline CRP levels above 3 mg/L and when vitamin C was given intravenously.

7. Exercise-induced bronchospasm

Some people experience airway narrowing during or after exercise, even without underlying asthma.

A meta-analysis of three double-blind trials (40 participants) found that taking 0.5-2 g of vitamin C before exercise reduced the post-exercise drop in lung function by about 50% [10].

The sample size is tiny, so treat this cautiously. But if you experience exercise-induced breathing difficulties, vitamin C is cheap and low-risk to try.

8. Blood pressure reduction in hypertension

Essential hypertension (high blood pressure without an identifiable cause) affects most people with elevated blood pressure.

A meta-analysis of eight randomised trials (614 participants with essential hypertension) found oral vitamin C (300-1,000 mg daily for 4-24 weeks) reduced both systolic and diastolic blood pressure [11]. The effect was most significant in people over 60 taking more than 500 mg daily for longer than 6 weeks.

The reductions were modest. Don’t expect vitamin C to replace blood pressure medication.

9. Shortening colds (with regular use)

This is probably what most people associate vitamin C with, thanks to Linus Pauling.

A meta-analysis of nine trials found that taking higher doses (3-4 g daily) at cold onset shortened illness duration and reduced symptoms, but only in people who already supplemented regularly with lower doses (1 g or less daily) [12]. Starting vitamin C only when you’re already sick showed no significant benefit.

Separately, a meta-analysis of nine trials in children (3,135 participants) found vitamin C didn’t prevent upper respiratory infections but reduced their duration by about 1.6 days [13].

My take: if you’re going to use vitamin C for colds, consistency matters more than loading up once you’re ill.

10. Blood sugar control in diabetes

A meta-analysis of 24 randomised trials (937 participants) found that, overall, oral vitamin C didn’t alter glucose, glycated haemoglobin, or insulin concentrations [14].

However, stratified analysis showed vitamin C did lower blood glucose in older participants with type 2 diabetes when taken for more than 30 days.

In other words: vitamin C isn’t a treatment for diabetes, but some diabetic individuals might see modest benefits.

11. Lipid improvements in specific populations

A meta-analysis of 40 randomised trials (1,981 participants) found vitamin C supplementation had no overall effect on blood lipids [15].

But subgroup analysis told a different story. In people with dyslipidaemia or low vitamin C levels, supplementation did improve total cholesterol, LDL cholesterol, triglycerides, and HDL cholesterol.

Translation: if your lipids are already normal and your vitamin C status is adequate, don’t expect supplements to change anything.

12. Anxiety reduction

A randomised, double-blind trial of 142 students found that 1,000 mg of vitamin C had short-term anxiolytic effects in people with high trait anxiety [16].

This is a single study with a specific population. I wouldn’t generalise too broadly, but it suggests vitamin C’s role in neurotransmitter synthesis might have some psychological relevance.

13. Association with cognitive function

A cohort study found that people with the highest blood vitamin C concentrations had lower rates of cognitive impairment, even after adjusting for other factors [17].

Association isn’t causation. People with higher vitamin C levels might simply have healthier diets and lifestyles overall. But the relationship between antioxidants and brain health continues to be researched.

14. Cardiovascular health markers

A prospective study following 20,299 people for nearly 13 years found those with the highest blood vitamin C concentrations had 24% lower risk of heart failure [18]. Each 20 μmol/L increase in blood vitamin C reduced risk by 9%.

Another meta-analysis of 44 trials found vitamin C improved endothelial function [19], which matters because endothelial dysfunction precedes cardiovascular disease.

These are observational and surrogate endpoint findings. They suggest cardiovascular relevance but don’t prove vitamin C prevents heart attacks.

15. Reduced atrial fibrillation after cardiac surgery

Atrial fibrillation is a common complication after heart surgery, occurring in 25-65% of patients depending on the procedure.

A meta-analysis of 15 studies (2,050 participants) found vitamin C reduced post-operative atrial fibrillation incidence by 27% and shortened hospital stays by 10% [20].

This is a fairly specific application, most relevant to surgical patients rather than the general public.

16. Lower hip fracture risk

Hip fractures in people over 65 carry significant mortality risk.

A meta-analysis of six studies (more than 10,000 participants) found those consuming the most vitamin C had 27% lower hip fracture risk compared with the lowest consumers [21]. Every 50 mg increase in daily vitamin C intake reduced risk by 5%.

The mechanism likely involves vitamin C’s essential role in collagen synthesis. Bones need collagen for structural integrity.

17. Reduced cancer incidence (from dietary sources)

Several meta-analyses have found associations between higher dietary vitamin C intake and lower risk of various cancers, including stomach, pancreatic, oesophageal, prostate, lung, and breast cancers [22-26].

High-dose intravenous vitamin C has been proposed as a cancer treatment, with some case reports of tumour regression [27]. However, robust evidence from controlled trials is lacking, and the scientific consensus remains that intravenous vitamin C is experimental [28].

I’d be cautious about over-interpreting the cancer data. Dietary studies can’t isolate vitamin C from other aspects of fruit and vegetable consumption.

18. Lower cervical precancer risk

Cervical intraepithelial neoplasia (CIN) can progress to cervical cancer if untreated.

A meta-analysis of 12 population studies found vitamin C intake was inversely related to CIN incidence, with every 50 mg daily increase reducing risk by 8% [29].

19. Reduced cataract risk

A prospective cohort study of 2,054 twins found dietary vitamin C reduced nuclear cataract incidence by 33% and slowed progression over 10 years [30].

The proposed mechanism involves vitamin C protecting the lens from oxidative damage.

Side effects

At normal doses (up to 2,000 mg daily), vitamin C is very safe. Being water-soluble, excess amounts are excreted through urine rather than accumulating.

Higher doses can cause gastrointestinal issues: diarrhoea, bloating, heartburn, and abdominal cramps. If you experience these, reduce your dose.

Contraindications and precautions

Hydration: Vitamin C has mild diuretic effects, so increase water intake when supplementing.
Haemochromatosis: Vitamin C increases iron absorption. People with iron overload disorders should avoid high-dose supplementation.
Blood disorders: Those with thalassaemia, sickle cell anaemia, or G6PD deficiency should avoid high doses due to risk of serious complications.
Kidney stone history: High-dose vitamin C can increase urinary oxalate. People with kidney stone history or oxalate metabolism issues should limit supplementation to the recommended dietary allowance or no more than 250 mg daily [31].
Aluminium-containing antacids: Vitamin C increases aluminium absorption. Separate doses by 2-4 hours.
Oestrogen medications: Vitamin C may slow oestrogen clearance, potentially increasing both effects and side effects.
Fluphenazine: Vitamin C may reduce the effectiveness of this psychiatric medication.
Chemotherapy drugs: Vitamin C might interfere with certain cancer treatments. Always consult your oncologist.
HIV/AIDS protease inhibitors: Vitamin C may reduce effectiveness of amprenavir, nelfinavir, ritonavir, and saquinavir.
Statins: Vitamin C might reduce effectiveness of cholesterol-lowering medications including atorvastatin, fluvastatin, lovastatin, and pravastatin.
Warfarin: High-dose vitamin C may reduce anticoagulant effectiveness and affect clotting.
Anti-inflammatory drugs: Vitamin C may slow metabolism of paracetamol, aspirin, and related medications, potentially increasing adverse reactions.
Calcium channel blockers: Nicardipine and nifedipine may reduce cellular vitamin C uptake when taken together.

References

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