Vitamins. Antioxidants
ATC code: A11GA01 (Ascorbic acid)
Brand names – supplements
Emergen-C, NOW Foods Vitamin C-1000, Thorne Buffered C Powder, Ester-C
Supplements are not tested in clinical trials and are not registered as medications.
Ascorbic acid (vitamin C) is a water-soluble cofactor for hydroxylases involved in collagen, carnitine, and neurotransmitter (noradrenaline, serotonin) synthesis. An antioxidant: it regenerates tocopherol and neutralises reactive oxygen species. Reduces Fe³⁺ to Fe²⁺ in the intestine, enhancing non-haem iron absorption. Not synthesised in humans and obtained only from diet. Body stores are small – 1,500–3,000 mg.
First line
First-line treatment for scurvy. Treatment dose is 200 mg daily for 5 days or 1,000 mg daily for 1–2 weeks, then maintenance at the physiological 75–90 mg daily. Symptoms (bleeding gums, petechiae, arthralgia) resolve over 2–4 weeks. BMJ 2018 and BNF support oral treatment; parenteral administration is reserved for severe malabsorption.
Individual decision
The evidence base is weak and mixed. Supplemental vitamin C 500–1,000 mg daily is considered in patients with nutritional deficiency, after major surgery, with burns, or in older adults with pressure ulcers – only as part of overall nutritional correction. NPUAP/EPUAP 2019 recommend vitamin C as part of nutritional support in pressure ulcers. The 2014 Cochrane review on vitamins and pressure ulcer healing (Langer 2014) is methodologically weak; large RCTs are absent. In patients without confirmed deficiency, isolated vitamin C does not affect wound healing time. The foundational approach is nutritional status assessment and correction of deficiencies as a whole, not mega-dose ascorbic acid.
The drug is promoted for these uses outside international guidelines. Each entry below is analyzed against AEMPS, FDA, EMA, Cochrane and major RCTs.
Not recommended
The 2013 Cochrane review (29 RCTs, more than 11,000 participants) showed that regular vitamin C 200 mg daily or higher in the general population does not reduce respiratory infection incidence. In athletes under extreme physical stress, the risk drops by 50% with daily intake. In most adults, routine vitamin C for cold prevention has no proven benefit. Symptom duration reduction when started after onset is about 8% in adults – minimal clinical significance.
Not recommended
The 2014 Cochrane review did not confirm an effect of vitamin C on asthma symptoms or prevention of exercise-induced bronchospasm. International pulmonology societies () do not include vitamin C in asthma management standards. Small positive RCTs have not been reproduced.
Not recommended
Vitamin C (ascorbic acid) is a water-soluble vitamin. Its clinical indication is prevention and treatment of scurvy in severe deficiency. Vitamin C as a means of slowing aging and extending life is not mentioned in international guidelines, and randomized trials (VITAL and predecessors) have not shown benefit. A normal diet (citrus, peppers, kiwi, broccoli) provides vitamin C in amounts that cover the requirement; clinical deficiency in developed countries is rare. High doses (over 1000 mg/day) can cause diarrhea, elevated urinary oxalate with kidney-stone risk, and iron overload in people with hemochromatosis. If vitamin C is being taken in high doses for longevity, consider seeking a second opinion.
Not recommended
High-dose vitamin C is not supported by international guidelines as cancer prevention or treatment. The mega-dose concept originates in Linus Pauling's 1970s publications and has not been confirmed in large subsequent RCTs. PDQ summary states clearly: RCTs have not confirmed an effect of high-dose ascorbic acid on cancer outcomes. Intravenous vitamin C drips at doses of 25–75 g offered in private clinics as «cancer treatment» or «prevention» fall outside NCCN, ESMO, and ASCO recommendations. Large meta-analyses (Zhang 2013, Luo 2014) showed no effect on all-cause or cardiovascular mortality. In patients with established cancer, high-dose vitamin C should be discussed with the treating oncologist because it may reduce the efficacy of some cytotoxic agents and radiotherapy.
Not recommended
Vitamin C (ascorbic acid) is a water-soluble vitamin. For chronic fatigue without confirmed vitamin C deficiency, ascorbic acid does not work: no clinical studies of prophylactic use for fatigue exist. For chronic-fatigue complaints, the workup should include anemia, iron deficiency, B12 deficiency, hypothyroidism, depression, obstructive sleep apnea, chronic infections, and drug side effects – the cause is often one of these and is treated per protocol. High-dose vitamin C (over 1000 mg/day) can cause diarrhea and elevated urinary oxalate with kidney-stone risk. If vitamin C is being taken for fatigue, consider seeking a second opinion and identifying the real cause.
Not recommended
International guidelines do not include vitamin C for primary cardiovascular prevention in healthy adults. Large RCTs (Physicians' Health Study II 2012, HOPE 2000, HOPE-TOO 2005) on antioxidant vitamins did not confirm effects on myocardial infarction, stroke, or cardiovascular mortality. 2021 and do not include vitamin C in CVD prevention recommendations. The antioxidant hypothesis of vascular protection has not translated clinically, although the effect is reproducible in laboratory models.
Ascorbic acid 100–200 mg increases ferrous iron absorption by 10–15%. The effect is stronger when taken concurrently with iron preparations rather than separated in time.
3 pairs found. Sorted from critical to minor.
Mechanism
Ascorbic acid does not affect cetirizine metabolism. The two are often combined in symptomatic preparations for acute respiratory infections.
Symptoms
The combination usually causes no specific symptoms. Each drug's individual side effects remain.
Management
No dose adjustment needed. The combination is standard in symptomatic formulations for acute respiratory infections and allergy.
Mechanism
Ascorbic acid raises gastric acidity and improves levothyroxine absorption in patients with atrophic gastritis or hypochlorhydria. In healthy patients the effect is minimal.
Symptoms
The combination usually causes no specific symptoms. Each drug's individual side effects remain.
Management
In patients with confirmed atrophic gastritis or PPI intolerance, ascorbic acid 500 mg with levothyroxine improves absorption. In healthy patients, no specific adjustment is needed. Monitor TSH on the usual schedule.
Mechanism
No direct pharmacokinetic interaction. Vitamin C increases iron absorption, not metformin's.
Symptoms
The combination usually causes no specific symptoms. Each drug's individual side effects remain.
Management
No dose adjustment needed. In diabetics on metformin, ascorbic acid may improve plant-source iron absorption in patients with deficiency.
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FDA category A at physiological doses. Pregnancy daily allowance is 85 mg. High doses (above 2,000 mg daily) in pregnancy are not recommended – there are reports of withdrawal syndrome in newborns.
Transfers into breast milk at physiological amounts. Supplementation at the RDA is safe. High-dose use during lactation is not justified.
Reference information, not a clinical decision. Discuss feeding pauses or changes with your physician or an IBCLC.
Ascorbic acid is evaluated for the following indications with varying evidence strength: Scurvy (vitamin C deficiency) (evidence tier A), Wound healing support (evidence tier C), Exercise-induced bronchoconstriction (evidence tier D). See the full indication matrix with dosing and citations above on this page.
Common side effects of Ascorbic acid (≥ 1 in 100): Diarrhoea, abdominal cramps at doses above 1,000 mg daily, Nausea, Heartburn. See the Safety section for uncommon and serious reactions.
FDA category A. FDA category A at physiological doses. Pregnancy daily allowance is 85 mg. High doses (above 2,000 mg daily) in pregnancy are not recommended – there are reports of withdrawal syndrome in newborns.
Transfers into breast milk at physiological amounts. Supplementation at the RDA is safe. High-dose use during lactation is not justified.
Ascorbic acid is contraindicated in: G6PD deficiency – haemolysis risk at high doses; Thrombophlebitis, thrombosis tendency (for high doses); Haemochromatosis, thalassaemia, sideroblastic anaemia; Oxalosis, calcium oxalate kidney stones; Hypersensitivity to vitamin C. Full list in the Safety section.
the mega-dose idea originates with Linus Pauling's 1970s publications. Subsequent large RCTs did not reproduce the effect. NCI PDQ summary states: high-dose vitamin C does not affect cancer outcomes. NCCN, ESMO, and ASCO do not include vitamin C in cancer prevention recommendations.
intravenous infusions of 25–75 g of vitamin C offered in private clinics as «cancer treatment» fall outside international guidelines. Large RCTs have not shown clinically meaningful effects on tumour progression or survival. In chemotherapy patients, high-dose vitamin C may reduce the efficacy of some drugs (bortezomib, cisplatin).
the subjective energising sensation from IV vitamins has not been reproduced in RCTs. «Vitamin drips» are a paid marketing service, not treatment. Chronic fatigue requires differential workup: anaemia, hypothyroidism, depression, iron or B12 deficiency.
systematic reviews show no effect on cold incidence in the general population. Symptom duration reduction during treatment is about 8 % – clinically insignificant.
large RCTs Physicians' Health Study II 2012, HOPE 2000, HOPE-TOO 2005 showed no effect of antioxidant vitamins on myocardial infarction or stroke. ESC and AHA do not include vitamin C in CVD prevention.
doses above 2,000 mg daily increase the risk of oxalate kidney stones, diarrhoea, and impaired B12 absorption. In G6PD deficiency, high doses cause haemolysis. IV mega-doses in patients with renal impairment have caused acute oxalate nephropathy.
the ascorbic acid molecule is identical regardless of origin. Bioavailability is comparable. Natural sources are preferred for accompanying phytonutrients and fibre – not for vitamin C itself.