Vitamin C (Ascorbic Acid)
Vitamin C is a vitamin your body can't make, so it has to come from food or supplements. Your body needs it to make collagen and to absorb iron from plant foods. Taking it regularly doesn't prevent colds for most people, but it may make them slightly shorter.
Vitamin C is a water-soluble vitamin that humans can't make, so it has to come from food or supplements. The body needs it to make collagen, L-carnitine and some neurotransmitters. It acts as an antioxidant, supports immune function, and helps the body absorb nonheme iron, the form found in plant foods.
Research & evidence
Strong support Some support Weak or no supportColours show how well the sources back each point. The reason is next to each one.
Regular vitamin C supplementation may modestly reduce the duration of common colds, though it does not consistently prevent colds from occurring in the general population.
Moderate evidenceSome support. The source hedges (“may” or “associated with”)
Study details
Regular vitamin C supplementation reduced cold duration by 8% (95% CI 3-12%) in adults and 14% (7-21%) in children (31 comparisons, 9,745 episodes); at 1-2 g/day, duration in children was reduced by 18%. No effect on cold incidence in the general community (RR 0.97, 95% CI 0.94-1.00; 10,708 participants), but a reduction in incidence (RR 0.48, 95% CI 0.35-0.64) in a separate subgroup of 598 people under extreme physical/cold stress (marathon runners, skiers, soldiers on subarctic exercises). Therapeutic (post-symptom-onset) vitamin C showed "no consistent effect" (7 comparisons, 3,249 episodes) — the duration benefit applies to regular/prophylactic use only, not treatment started after symptoms begin. No formal GRADE certainty rating appears in the review text; quality was addressed via sensitivity analyses excluding lower-quality trials, which did not change the conclusions.
Source: Hemilä H, Chalker E. "Vitamin C for preventing and treating the common cold." Cochrane Database Syst Rev. 2013 Jan 31;2013(1):CD000980. doi: 10.1002/14651858.CD000980.pub4. PMCID: PMC8078152. (Cochrane's plain-language evidence summary of the same review is also available via their public evidence portal.) View source
Vitamin C is required for normal collagen synthesis, and deficiency directly impairs wound healing and skin/connective tissue integrity.
Strong evidenceStrong support. Stated plainly by NIH or an FDA label
Study details
NIH ODS states vitamin C is "required for the biosynthesis of collagen" and that deficiency "impairs collagen synthesis, weakening connective tissues," causing petechiae, ecchymoses, purpura, joint pain, poor wound healing, hyperkeratosis, and corkscrew hairs (named dermatological/skin findings, not just general "connective tissue" by inference). These effects come from scurvy (severe deficiency, below about 10 mg a day for about a month), not from mild insufficiency.
Source: NIH Office of Dietary Supplements, Vitamin C — Health Professional Fact Sheet, Introduction and "Vitamin C Deficiency" sections. View source
Taking vitamin C alongside a meal that includes plant-based (nonheme) iron sources significantly improves iron absorption from that specific meal; however, evidence from complete-diet studies suggests this effect does not reliably translate into improved iron status when vitamin C is consumed as part of one's regular, varied diet over time.
Mixed evidenceSome support. Results are mixed
Study details
NIH ODS confirms vitamin C enhances nonheme-iron bioavailability as settled mechanism, though the 14-18% vs. 5-12% bioavailability figures reflect whole dietary patterns, not vitamin C in isolation. A controlled feeding study (Cook & Reddy 2001) found the enhancing effect is "pronounced" only when "feeding single meals to fasting subjects" — when the same researchers tested a complete diet varied in vitamin C over 5 days, iron absorption did not differ significantly across intake levels, which the authors say likely explains why long-term vitamin C supplementation trials generally fail to improve iron status (e.g., serum ferritin).
Source: NIH Office of Dietary Supplements, Iron — Health Professional Fact Sheet; and Cook JD, Reddy MB. "Effect of ascorbic acid intake on nonheme-iron absorption from a complete diet." Am J Clin Nutr. 2001 Jan;73(1):93-8. doi: 10.1093/ajcn/73.1.93. View source
Dosage
| Form | Amount |
|---|---|
| Any form | Recommended daily amounts for adults: 90 mg for men and 75 mg for women; people who smoke need 35 mg a day more. The upper limit for adults is 2,000 mg a day. |
Side effects
High doses can cause diarrhea, nausea and stomach cramps. The upper limit for adults is 2,000 mg a day.
Well documented Reported (case reports, anecdotal or one study) None reportedThe circle shows how well documented an effect is, not how serious it is. A rarely reported effect can still be serious, so always read ‘When to stop’.
| Effect | Frequency | When to stop |
|---|---|---|
| Diarrhea, nausea or stomach crampsWell documentedListed plainly by NIH, MSK or LiverTox | Most common with high doses | Lower the dose; talk to a doctor if it persists. |
| Low blood sugar or low blood pressureWell documentedListed plainly by NIH, MSK or LiverTox | Not stated | Stop and talk to a doctor if this happens. |
| Kidney stonesReportedCase reports only | Case reports; the best evidence is in people who already have hyperoxaluria | See a doctor if you think you have a kidney stone. |
Safety & interactions
Safety
High doses most commonly cause diarrhea, nausea, stomach cramps and other digestive upset, because unabsorbed vitamin C draws water into the gut. The upper limit for adults is 2,000 mg a day. High intakes could raise oxalate and uric acid in urine, which could contribute to kidney stones, especially in people with kidney problems. Studies have had conflicting results, so it isn't clear whether vitamin C actually plays a role in kidney stones; the best evidence is in people who already have hyperoxaluria. In people with hereditary hemochromatosis, long-term high doses could worsen iron overload and damage tissues.
Contraindications
MSK lists four contraindications for vitamin C supplements: recurrent kidney stones, kidney impairment or long-term dialysis, hemochromatosis, and G6PD deficiency (MSK says hemolytic anemia may occur at high doses in people with it). MSK also says to avoid vitamin C if you're having radiation therapy or chemotherapy, or taking bortezomib (Velcade) as part of cancer treatment. See Drug interactions for these and for acetaminophen.
Drug interactions
Antioxidants — including vitamin C — may protect tumor cells from chemotherapy and radiation therapy by interfering with the reactive-oxygen-species damage these treatments rely on for their cytotoxic effect. NIH ODS raises this concern for antioxidants generally, naming cyclophosphamide, chlorambucil, carmustine, busulfan, thiotepa, and doxorubicin; MSK describes the same concern and names vincristine, doxorubicin, methotrexate, cisplatin, and imatinib. Both sources note the evidence is contested: other data suggest antioxidants may instead protect healthy tissue or reduce treatment side effects, and it is unclear whether oral vitamin C raises blood levels enough to matter either way. A distinct, more specific case: vitamin C is reported to directly reduce the activity of bortezomib (Velcade) — a different mechanism than the general antioxidant concern — and MSK's guidance is to avoid the combination, while noting a clinical trial minimized the interaction by dosing bortezomib in the morning and ascorbic acid in the evening. Anyone undergoing chemotherapy or radiation should consult their oncologist before taking vitamin C or other antioxidant supplements, especially at high doses.
Vitamin C enhances absorption of non-heme (plant-based) iron. NIH states this does not appear to be a concern in healthy individuals, but in people with hereditary hemochromatosis (an iron-overload condition), chronic consumption of high doses of vitamin C could exacerbate iron overload and result in tissue damage. MSK lists hemochromatosis as a contraindication for vitamin C supplements. Anyone with hemochromatosis should talk to their healthcare provider before taking vitamin C.
In healthy volunteers, taking vitamin C with acetaminophen (paracetamol) increased how much acetaminophen was absorbed, which may raise the risk of its side effects. MSK advises talking to your healthcare provider if you take acetaminophen.
Vitamin C taken with other antioxidants may blunt the rise in HDL ("good") cholesterol from niacin plus simvastatin therapy. NIH says clinicians should monitor lipid levels in people taking statins with antioxidant supplements.
Established: stated plainly by an authoritative source. Documented: supported by case reports or trial data. Theoretical: based on mechanism, animal data, or a cautious listing. Always check with your pharmacist or prescriber.
Which form to buy
Ascorbic acid
Good choiceAll common forms work about the same (per NIH)
Absorbed as well as vitamin C from food, and relatively low in cost; the researchers NIH cites called plain ascorbic acid the preferred form.
Ester-C, mineral ascorbates, ascorbic acid with bioflavonoids
Works, with a catchStudies disagree on whether it's better
Plasma vitamin C levels were the same as with plain ascorbic acid in two studies; one found higher levels in white blood cells with Ester-C.