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Vitamin C Supplements: Colds, Iron, Skin, Kidney Stones and How Much Is Too Much

Vitamin C taken every day doesn't stop most people catching colds, but it made adults' colds about 8% shorter in community trials (grade A); starting it once a cold begins showed no consistent effect (grade B). With iron pills it adds little (grade B). Adults should stay under 2,000 mg a day, the upper limit, and men taking 1,000 mg or more had more kidney stones (grade C). Ask first with iron overload or kidney disease.

Published How we grade evidenceEditorial policy

Vitamin C (ascorbic acid) is a water-soluble vitamin your body can't make. It builds collagen, helps immune cells and helps you absorb iron from plant foods. Adults need 90 mg a day (men) or 75 mg (women), and people who smoke need 35 mg more; three-quarters of a cup of orange juice or half a cup of raw red pepper gives about 93 to 95 mg (NIH ODS). This guide covers adults.

The short version

  • Colds, taken every day (general community trials, against placebo): no fewer colds, but adults' colds ran about 8% shorter. Grade A (Hemilä 2013).
  • Colds under heavy physical stress (marathon runners, soldiers on winter exercises, schoolchildren at a ski camp): about half as many colds. Grade B (Hemilä 2013).
  • Iron pills for iron-deficiency anemia (the same iron with vs without vitamin C, 440 adults): hemoglobin rose about the same. Grade B (Li 2020).
  • Blood pressure (adults, pooled trials): about 3.7 mmHg lower systolic pressure, low certainty. Grade C (Kosari 2026).
  • Kidney stones (US cohorts): men taking 1,000 mg or more a day from pills had 19% more stones; women did not. Grade C for harm (Ferraro 2016).

The grades in this guide for iron, blood pressure, blood sugar, gout, CRPS, wounds, heart surgery, sepsis, skin, kidney stones and blood thinning are its own; our vitamin C page doesn't grade these outcomes yet.

Question (who, compared with what)What trials foundSize of effectGrade
Catching colds (10,708 children and adults in community trials, daily vs placebo)No real differenceRisk ratio 0.97A
Length of colds (adults, daily vs placebo)Shorter8%A
Hemoglobin (440 adults with iron-deficiency anemia, iron plus C vs the same iron alone)Rose about the same2.00 vs 1.84 g/dL in 2 weeksB
Systolic blood pressure (adults, 17 trials)LowerAbout 3.7 mmHgC
Kidney stones (40,536 men, pills of 1,000 mg+ a day vs none)More stones19% higher riskC

How much vitamin C do you need, and is 1,000 or 2,000 mg safe?

Most adults who eat fruit and vegetables reach the 75 to 90 mg target. People more likely to fall short include smokers and those breathing secondhand smoke, people who eat a very limited range of foods, and people with malabsorption or some chronic diseases (NIH ODS). Severe lack causes scurvy: tiredness, swollen or bleeding gums, easy bruising and slow healing.

Many pills hold 500 or 1,000 mg. Upper limit for adults: 2,000 mg daily, set because higher intakes bring diarrhea, nausea and stomach cramps; go above it only on a clinician's advice, with blood tests (NIH ODS; add up your stack). Below the limit, the main caution is kidney stones in men (see below). Cold remedies, "immune" blends and multivitamins can all contain vitamin C, so add up the labels.

Does vitamin C help colds?

Grade A for slightly shorter colds with daily use; it does not stop most people catching them. In community trials of 10,708 children and adults, daily vitamin C left the chance of catching a cold unchanged (risk ratio 0.97), though adults' colds ran 8% shorter and were somewhat milder. In 5 trials of 598 people under heavy physical stress, such as marathon runners, soldiers on subarctic exercises and schoolchildren at a ski camp, colds were about halved (grade B). Starting vitamin C only after a cold began showed no consistent effect (grade B) (Hemilä 2013). In 10 placebo-controlled trials of children and adults taking 1 g a day or more (half the adult upper limit), colds were about 15% less severe (Hemilä 2023). The cold and flu guide compares vitamin C with zinc and the rest; see also common cold.

Does vitamin C help you absorb iron?

Grade B: in people with iron-deficiency anemia, it adds very little to iron pills. In 440 adults in China, mostly women, hemoglobin rose 2.00 g/dL in 2 weeks on iron plus vitamin C and 1.84 g/dL on iron alone, which met the trial's test for being equivalent (Li 2020). A pooled analysis of 10 studies found hemoglobin only 0.14 g/dL higher with vitamin C, but most of those studies were in young children or pregnant women and compared different iron products, such as ferrous ascorbate against ferrous sulfate, rather than the same pill with and without vitamin C (Deng 2024). Vitamin C does help the body absorb nonheme iron, the kind in plant foods (NIH ODS), so it is a concern for people with too much iron (see Safety). The iron guide covers timing and forms.

Does vitamin C lower blood pressure or blood sugar?

Grade C for blood pressure: a small drop, on low-certainty evidence. An umbrella review that pulled together 17 trials found vitamin C may lower systolic pressure by about 3.7 mmHg; diastolic pressure did not fall overall (Kosari 2026). In people with type 2 diabetes, a review of mostly short, small trials found systolic pressure 6.27 mmHg lower, rated moderate certainty, and HbA1c 0.54 percentage points lower, rated very low certainty (grade C for blood sugar) (Mason 2021). Don't skip, stop or swap a blood pressure or diabetes medicine for it. See high blood pressure and supplements for blood pressure.

Does vitamin C help gout?

Grade B for a small drop in uric acid, too small to treat gout. Across 13 trials with 556 adults (typically 500 mg a day for about 30 days), uric acid fell by 0.35 mg/dL, with wide differences between trials. That total included 4 trials that gave vitamin C mixed with vitamin E or other antioxidants, and 2 that compared it with vitamin E rather than a dummy pill; where vitamin C was the only thing given, uric acid fell by 0.54 mg/dL (Juraschek 2011). It stays at B, not C, because the drop held, and was larger, in the trials where vitamin C was the only thing given. In 40 people who already had gout, 8 weeks of 500 mg daily lowered uric acid by just 0.23 mg/dL, while starting or increasing allopurinol lowered it by 1.9 mg/dL (Stamp 2013). For new gout, among 14,641 male doctors in a long trial, those assigned vitamin C were diagnosed 12% less often than those on placebo (grade C, one trial) (Juraschek 2022). Don't stop or swap a prescribed gout medicine such as allopurinol for vitamin C without your prescriber. See supplements for gout and high uric acid.

Does vitamin C help after a wrist fracture?

Grade C, mixed. Complex regional pain syndrome (CRPS) is long-lasting pain and swelling that can follow an injury. In 416 people with wrist fractures, 50 days of vitamin C cut CRPS to 2.4% against 10.1% on placebo, with 500 mg and 1,500 mg a day (below the 2,000 mg adult upper limit) working and 200 mg falling short (Zollinger 2007). A later trial found no difference at one year in function, healing or CRPS, and at some points the vitamin C group did worse: more CRPS at 6 weeks among those with undisplaced fractures, and more complications and pain at 26 weeks among those with displaced fractures (Ekrol 2014). Two poolings of the same 3 trials disagree: one found no clear benefit and rated the evidence low quality (Evaniew 2015), the other found a lower risk at 500 mg a day (Aïm 2017). Ask your surgeon; see CRPS.

Does vitamin C help wounds heal?

Grade D, from one tiny trial. In 16 people with chronic foot ulcers, half of whom were low in vitamin C at the start, 500 mg of slow-release vitamin C healed ulcers better at 8 weeks than placebo, though people found to be low were then given vitamin C plus glucosamine after 4 weeks, so the 8-week result isn't vitamin C alone (Gunton 2021). See wound healing.

Does vitamin C help after heart surgery or in sepsis?

These are hospital uses, often with vitamin C given into a vein. After heart surgery, 9 trials outside the US found about 44% fewer cases of atrial fibrillation (an irregular heartbeat), while 5 US trials found no effect. Of those 9, 5 gave vitamin C into a vein and 4 had no dummy pill, just usual care (grade C, results split by country) (Hemilä 2017). Among 872 intensive-care adults with sepsis who needed drugs to hold up their blood pressure, high-dose vitamin C into a vein raised the risk of death or lasting organ failure at 28 days, 44.5% against 38.5% (grade C for harm) (Lamontagne 2022). See does vitamin C help sepsis?

Does vitamin C help skin, dark spots or acne?

Grade C for skin, and only from diet. Among middle-aged American women, higher vitamin C intake from food went with a lower chance of a wrinkled look (odds ratio 0.89), an association, not proof (Cosgrove 2007). Pill trials for skin have tested blends, such as collagen with vitamin C, so they can't show what vitamin C alone does (Žmitek 2024). Vitamin C serums for dark spots and acne are a different route: put on the skin, not swallowed, and their results don't carry over to pills.

Is vitamin C safe for your kidneys?

Grade C: the risk showed up in men on high-dose pills. Following 40,536 US men and 156,735 US women for about 11 years, men taking 1,000 mg or more a day from supplements had a 19% higher stone risk; there was no link in women, or for vitamin C eaten in food (Ferraro 2016). Pooling 4 observational studies, another review also linked vitamin C pills to stones in men but not women, though the link showed at some doses and not others (Jiang 2019). The body turns some vitamin C into oxalate, a stone ingredient. The kidneys guide and kidney stones protocol have more.

Does vitamin C thin your blood?

We found no trial showing that vitamin C pills thin blood or cause bleeding. The closest finding is a lab measure: in healthy men, a single 2 g dose (2,000 mg, the adult upper limit for a whole day; don't take this much without a clinician's advice) made platelets (the blood cells that start clots) clump 35% less in a test tube 6 hours later (grade D) (Wilkinson 1999). On warfarin, the worry runs the other way: in one case report, a man's INR stayed low on vitamin C even as his warfarin dose was raised, then jumped to 15.4 within two days of stopping it, so keep any vitamin C steady and tell your anticoagulation clinic before changing it (grade D) (Sattar 2013). See supplements and blood thinners.

What we'd skip

  • Big doses only once a cold starts: no consistent effect in 7 comparisons (Hemilä 2013).
  • Vitamin C to avoid heart attacks, strokes or cataracts: in 14,641 male doctors (11,545 in the cataract analysis), 500 mg a day for about 8 years did not change heart events, deaths or cataracts (Sesso 2008; Christen 2010).
  • Vitamin C instead of gout medicine: the drop in uric acid was a fraction of allopurinol's (Stamp 2013).
  • Vitamin C drips outside hospital: high doses into a vein were linked to harm in sepsis and to hemolysis (see Safety) (Lamontagne 2022).

Safety and interactions

  • Stomach upset: diarrhea, nausea and cramps are the common complaints at high intakes (NIH ODS).
  • Kidney stones and kidney disease: men taking 1,000 mg or more, and anyone who has had stones, should ask before taking it daily (Ferraro 2016). Case reports describe kidney damage from oxalate crystals at 2 g a day or more in people who had bowel surgery or whose pancreas makes too few digestive enzymes (Lin 2019; Fijen 2019). With kidney disease, ask your kidney team first.
  • Hemochromatosis (iron overload): NIH ODS warns that long-term high doses could worsen iron overload and damage tissues (NIH ODS). Avoid supplements unless your specialist agrees.
  • G6PD deficiency (an inherited red blood cell enzyme problem): a systematic review found no evidence of harm from vitamin C at usual doses (Lee 2017), but hemolysis (red cells bursting) has been reported with high-dose vitamin C drips, grade D (Quinn 2017). Ask before high doses if you have it.
  • Glucose meters: very high vitamin C levels, as after hospital drips, made some finger-stick meters read falsely high (Katzman 2021); tell your care team if you get infusions.
  • Medicines: warfarin (see above). With cancer chemotherapy or radiation, ask your oncologist before taking vitamin C or other antioxidants; with a statin plus niacin, antioxidants including vitamin C may blunt the rise in HDL cholesterol (NIH ODS). For zinc, see the zinc guide.
  • Surgery: tell your surgical team about every supplement; see supplements to stop before surgery.
  • Pregnancy and breastfeeding: use the amount your midwife or obstetric team advises. For children, ask your child's doctor.

When to see a doctor

  • Urgently for severe pain in your side or back, blood in your urine, or very little urine (possible kidney stone or kidney injury).
  • Soon for dark urine, yellow skin or eyes, or breathlessness after high doses, especially with G6PD deficiency (possible hemolysis).
  • For a cold that lasts more than 10 days, a high fever, chest pain or trouble breathing (possible pneumonia).
  • For bleeding gums, easy bruising and tiredness on a very limited diet (possible scurvy), or an INR out of range on warfarin.

Don't stop a prescribed medicine on your own; ask the prescriber or pharmacist. Pregnant, breastfeeding or under 18? A clinician should advise you.

FAQ

How much vitamin C should I take a day? Adults need 90 mg (men) or 75 mg (women), plus 35 mg if you smoke, and most get it from food. For adults the upper limit is 2,000 mg daily; more often causes diarrhea and stomach upset.

Is 1,000 mg of vitamin C a day safe? It is below the 2,000 mg upper limit, but in US cohorts men taking 1,000 mg or more from pills had 19% more kidney stones (grade C). Skip it if you have had stones, kidney disease or iron overload.

Does vitamin C help you absorb iron? It helps absorb iron from plant foods, but in a trial of 440 adults with iron-deficiency anemia, adding it to iron pills made almost no difference to hemoglobin (grade B).

Does vitamin C thin your blood? We found no study showing that it does. A single 2 g dose (the adult daily upper limit) lowered a test-tube platelet measure in healthy men (grade D); don't go that high without a clinician. On warfarin, keep vitamin C steady and tell your clinic before changing it.

Does vitamin C stop colds? Not for most people: daily use left the number of colds unchanged in community trials but made adults' colds about 8% shorter (grade A). Starting it once a cold begins showed no consistent effect (grade B).

Sources

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