Water-soluble vitamin (C)
Vitamin C (ascorbic acid) is a water-soluble vitamin the body cannot make on its own: it must come from food or supplements. It is essential for collagen formation (the "glue" of tissues), wound healing, iron absorption and immune defence. Severe deficiency causes scurvy, now rare, but mild deficiencies can occur in smokers, older people and those with a diet poor in fruit and vegetables.
Also known as: vitamina C, vitamin C, ascorbic acid, ascorbato
Iron + vitamin C: vitamin C increases oral iron absorption.
Ascorbic acid (vitamin C) reduces ferric iron (Fe³⁺) to ferrous (Fe²⁺) in the intestinal lumen and keeps it soluble, increasing absorption of non-haem iron — the Prontuário Terapêutico records that 30 mg of vitamin C enhance the absorption of 200 mg of iron. This is a clinically favourable interaction, often used to improve the response to iron supplementation in iron-deficiency anaemia; it poses no risk at usual doses. Take them together (or with citrus juice) to gain the benefit, while avoiding simultaneous intake with calcium, tea or coffee, which reduce iron absorption.
Iron + vitamin C: vitamin C increases oral iron absorption — a useful, not harmful, association.
Ascorbic acid reduces ferric to ferrous iron and forms soluble complexes, improving intestinal absorption.
Ferritin and haemoglobin.
Signs of iron overload (rare at usual doses).
Favourable combination: may allow a lower iron dose; watch for overload in at-risk patients.
DailyMed (FDA) — approved Iron label: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=4e88a847-397b-4ddb-83d7-42c9b963a0b7 ; approved Ascorbic acid (vitamin C) label: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=1606e29f-0e80-4069-b688-daab9cbff20a — additional reference: Prontuário Terapêutico do INFARMED (11th ed., 2012)
Warfarin + vitamin C (high doses): possible INR change.
High-dose vitamin C (several grams per day) has been associated with changes in INR in patients anticoagulated with warfarin — generally a reduced anticoagulant effect, although reports vary. The mechanism is not fully clarified (possible interference with warfarin absorption or metabolism). With usual doses (up to ~500 mg/day) the risk is minimal, but in patients taking megadoses of vitamin C, monitor INR after starting, changing or stopping the supplement, and adjust the warfarin dose as needed.
Warfarin + vitamin C: high-dose vitamin C may alter INR. Monitor INR.
Very high vitamin C doses may interfere with anticoagulation (mechanism not fully clarified).
Periodic INR.
Signs of bleeding or thrombosis.
Avoid high doses (>1 g/day) without monitoring; check INR.
DailyMed (FDA) — approved Warfarin label: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=541c9a70-adaf-4ef3-94ba-ad4e70dfa057 ; approved Ascorbic acid (vitamin C) label: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=1606e29f-0e80-4069-b688-daab9cbff20a — additional reference: Prontuário Terapêutico do INFARMED (11th ed., 2012)
Taking with food reduces vitamin C gastric discomfort.
May take with or without food, according to tolerance.
EMC-UK (MHRA) — approved Ascorbic acid (vitamin C) SmPC: https://www.medicines.org.uk/emc/product/102185/smpc
Vitamin C increases iron absorption, which may worsen iron overload.
Use with caution in patients with iron overload.
EMC-UK (MHRA) — approved Ascorbic acid (vitamin C) SmPC: https://www.medicines.org.uk/emc/product/102185/smpc
High vitamin C doses increase oxalate excretion and stone risk.
Avoid high doses in patients with recurrent oxalate stones.
EMC-UK (MHRA) — approved Ascorbic acid (vitamin C) SmPC: https://www.medicines.org.uk/emc/product/102185/smpc
Vitamin C at recommended doses is safe in pregnancy.
Use recommended doses; avoid megadoses.
Compatible with breastfeeding.
No specific additional contraception.
EMC-UK (MHRA) — approved Ascorbic acid (vitamin C) SmPC: https://www.medicines.org.uk/emc/product/102185/smpc
Clinical and educational support tool. The information does not replace a medical prescription or the opinion of a qualified healthcare professional.
Essential vitamin with antioxidant activity and cofactor of hydroxylases. Functions depend on tissue concentration: collagen, carnitine and catecholamine synthesis; enhances non-haem iron absorption. There is no evidence that high daily doses prevent colds or shorten their duration.
Electron donor in oxidation-reduction reactions; cofactor of prolyl and lysyl hydroxylases (proline and lysine hydroxylation in collagen synthesis), of dopamine beta-hydroxylase (catecholamines) and of carnitine synthesis enzymes. Reduces ferric to ferrous iron in the duodenum, favouring absorption.
Intestinal absorption is active and saturable in the small intestine (sodium-dependent transporter); the absorbed fraction decreases with dose (nearly complete up to ~200 mg, lower above 1 g). Vitamin C is water-soluble; unabsorbed excess stays in the lumen and causes osmotic diarrhoea.
Partially metabolised in the liver (oxidation to dehydroascorbic acid and then to oxalate, threonic acid and ascorbate-2-sulfate). Oxalate is eliminated in urine — hence the stone risk at high doses in predisposed patients.
Half-life depends on dose: about 8–12 days at low body levels, decreasing to 1–2 hours at very high concentrations (renal excretion of the excess). Excretion is renal, unchanged and as metabolites (oxalate).
Medicines from the same therapeutic group (ATC classification) or the same pharmacological class.
