Key takeaways
- In a 1993 trial, about 33% of ultramarathon runners who took 600 mg of vitamin C a day reported cold symptoms after a 90-kilometer race, compared with about 68% of runners on a placebo345.
- In general-population trials with 10,708 people, a 2013 Cochrane review found that regular vitamin C left the risk of catching a cold almost unchanged2.
- Your gut absorbs 70% to 90% of moderate doses but less than half of doses above 1 gram, and five servings of fruits and vegetables can supply more than 200 mg a day1.
- In the LOVIT trial of 872 adults with sepsis, 44.5% of those given high-dose IV vitamin C died or had lasting organ failure by day 28, compared with 38.5% on placebo6.
- The adult upper limit is 2,000 mg a day from food and supplements combined, and Swedish men who took vitamin C pills were about twice as likely to develop kidney stones111.
In 1993, a research team led by E.M. Peters tested vitamin C on people who push their bodies to the limit. Runners entered in a 90-kilometer ultramarathon took either 600 mg of vitamin C a day or a dummy pill for three weeks before the race5.
In the two weeks after the race, about 68% of the runners on the placebo reported cold symptoms. Among runners on vitamin C, about 33% did, according to a later study’s account of the trial4. By our math, that’s 35 fewer sick runners out of every 100. The Cochrane reviewers’ own figure for this trial agrees: cold risk cut about in half3.
It was a small trial. Symptoms were reported by phone, and letters about the study later noted that its company sponsor had gone out of business5. Still, a few other small trials in people under heavy strain pointed the same way23.
For people at a desk, though, the same body of research shows almost nothing. And in hospitals, a 2022 trial of high-dose IV vitamin C for sepsis ended with more harm than benefit6.
That split is the real vitamin C story, and it’s more specific than either fans or skeptics admit.
Your gut puts a ceiling on vitamin C pills
Your body can’t make vitamin C, and how much of a dose reaches your blood depends heavily on its size.
At everyday amounts, 30 to 180 mg a day, your gut takes in most of it: 70% to 90%, according to the NIH Office of Dietary Supplements (ODS). Push past 1 gram a day and you absorb less than half, and your kidneys flush away much of the surplus1.
Your tissues fill up quickly. About 100 mg a day seems to be enough to top off your cells, and blood levels climb very little beyond roughly 200 mg a day, according to ODS1.
Big pills hit the same wall. A single 1,250 mg dose raises blood levels to only about twice what 200 to 300 mg of vitamin C from food produces. Researchers estimate that 3 grams every four hours would push blood levels to about 220 micromoles per liter1.
A drip into a vein skips the gut. IV vitamin C can reach about 26,000 micromoles per liter, ODS reports1. By our math, that’s more than 100 times the highest level pills can reach, which is why trials of pills can’t settle questions about drips.
The cold trials sorted people into two groups
Linus Pauling made vitamin C famous as a cold remedy, as our article on judging experts recounts. The most thorough look at what came next is a 2013 Cochrane review by Harri Hemilä of the University of Helsinki and Elizabeth Chalker2. Cochrane reviews pool every qualifying trial on a question, using strict rules.
For the general community, the answer was close to a shrug. In trials covering 10,708 people, regular vitamin C cut the risk of catching a cold by about 3%, a difference too small to tell apart from no effect2.
Five trials with 598 people told a different story. They included marathon and ultramarathon runners, students at a ski school in the Swiss Alps and Canadian troops on subarctic operations23. In this group, the risk of a cold fell by about half, on doses of 250 mg to 1 gram a day23.
No one knows why strain matters. The trials were small and short, so the fair reading is narrow: the benefit showed up during brief, intense stress.
Taken daily, vitamin C also made colds a bit shorter. Our article on immune “boosting” gives the numbers for adults and children. In children who got 1 to 2 grams a day, colds were 18% shorter2.
Starting vitamin C once you’re sick hasn’t held up
Many people reach for vitamin C at the first sniffle. The Cochrane review found seven trial comparisons, covering 3,249 colds, in which vitamin C started after symptoms began. They showed no consistent effect on how long colds lasted or how bad they got2.
Still, one large adult trial reported a benefit from a single 8-gram dose on the first day of a cold2. An older trial found that 6 grams a day worked about twice as well as 3 grams3.
The review’s authors called for new trials using at least 8 grams a day in adults3. Because vitamin C is cheap and generally safe, they wrote that people with colds could reasonably test it on themselves2.
If you try that, know the trade-offs. By our math, 8 grams is four times the adult upper limit of 2,000 mg, and ODS notes that large doses can cause diarrhea, nausea and stomach cramps1. In the cold trials that tracked side effects, though, complaints were about as common on vitamin C (5.8%) as on placebo (6.0%)3.
IV vitamin C in sepsis: a hopeful idea that failed its big test
Sepsis is the body’s runaway response to an infection, and it can shut down organs.
In 2017, a study at one U.S. hospital gave patients IV vitamin C with steroids and thiamine (vitamin B1), without a randomized comparison group. It reported far fewer deaths67. The leaders of the next big trial later said they found themselves between enthusiasts and doubters6.
The test came from LOVIT, a randomized trial, meaning chance decided who got the treatment. From 2018 to 2021, it enrolled 872 adults with sepsis at 35 intensive care units in Canada, France and New Zealand6. All needed drugs to hold up their blood pressure7.
Half got 50 mg of vitamin C per kilogram of body weight every six hours for four days. The rest got a placebo drip7.
By day 28, 44.5% of the vitamin C group had died or still had organ failure, compared with 38.5% on placebo6. By our math, that’s about 6 more bad outcomes for every 100 patients treated. The trial, led by François Lamontagne and Neill Adhikari, was funded by the Lotte and John Hecht Memorial Foundation6.
In 2023, a panel of 21 experts from 16 countries pooled the trials. Six trials with 2,148 patients showed no drop in deaths at 90 days. If anything, the estimate leaned toward about 2 more deaths per 100 patients, a gap that could be chance8. The panel issued a conditional recommendation against IV vitamin C for adults with sepsis, beyond ordinary nutrition8.
Researchers still argue over the details. In a 2024 commentary, Hemilä and Chalker pointed out that 18 extra deaths in LOVIT’s vitamin C group came on days 5 to 7, after the infusions had ended9. They also noted that a quarter of LOVIT’s patients started with vitamin C levels below half the usual cutoff for deficiency9.
That is their reading of the data, and it may shape future trials. For now, the guideline panel’s advice stands8.
In cancer care, IV vitamin C is still an experiment
IV vitamin C is popular in integrative cancer care. At complementary medicine conferences in 2006 and 2008, 172 of 199 practitioners surveyed said they had given patients vitamin C10.
The National Cancer Institute’s (NCI) summary, updated in May 2025, says the FDA has not approved high-dose vitamin C to treat cancer10. The human trials are small and point in different directions10:
- Pancreatic cancer, 2024: 34 patients got two chemotherapy drugs, with or without 75 grams of IV vitamin C three times a week. The vitamin C group lived longer, with no added side effects.
- Prostate cancer, 2024: 47 men got the chemotherapy drug docetaxel with IV vitamin C (1 gram per kilogram) or a placebo. Vitamin C didn’t improve results.
- Ovarian cancer, 2014: In a pilot trial of 27 women, adding IV vitamin C to chemotherapy reduced chemotherapy side effects.
- Colorectal cancer: A larger phase III trial is under way.
Lab work adds a caution. In test tubes, an oxidized form of vitamin C cut the effect of three chemotherapy drugs by 30% to 70%10. That isn’t proof of harm in people, but it’s a reason to involve your oncologist1.
NCI notes that IV vitamin C studies have screened out people with G6PD deficiency, an inherited enzyme shortage, and people with kidney disease or kidney stones, because they face a higher risk of harm10. Our megadose article describes the rare but serious kidney and blood-cell injuries behind that rule.
Food gives you all the vitamin C most people can use
Here the natural-health instinct to start with the plate is on solid ground. A few servings of produce can fill your tissues without a single pill. ODS lists these amounts1:
- Orange juice, 3/4 cup: 93 mg
- Kiwifruit, 1 medium: 64 mg
- Broccoli, cooked, 1/2 cup: 51 mg
- Strawberries, sliced, 1/2 cup: 49 mg
- Brussels sprouts, cooked, 1/2 cup: 48 mg
- Cantaloupe, 1/2 cup: 29 mg
- Baked potato, 1 medium: 17 mg
Five varied servings of fruits and vegetables can supply more than 200 mg a day, ODS says1. That’s past the point where blood levels flatten out.
Here are the National Academies’ daily targets and upper limits, as listed by ODS1:
| Age or stage | Recommended amount (mg a day) | Upper limit (mg a day) |
|---|---|---|
| 1 to 3 years | 15 | 400 |
| 4 to 8 years | 25 | 650 |
| 9 to 13 years | 45 | 1,200 |
| 14 to 18 years | 75 (boys), 65 (girls) | 1,800 |
| Adults 19 and older | 90 (men), 75 (women) | 2,000 |
| Pregnancy, 19 and older | 85 | 2,000 |
| Breastfeeding, 19 and older | 120 | 2,000 |
The upper limits count food and supplements together. They don’t apply when a clinician gives vitamin C as treatment1. Smokers need extra, and our vitamins and minerals guide gives the amount.
A few groups run low more often, ODS says: people with very limited diets, including some older adults and people with alcohol or drug use disorders; people with severe gut absorption problems; people on kidney dialysis; and babies fed evaporated or boiled cow’s milk1. Our deficiency-signs article covers how scurvy is spotted.
Where big doses cause trouble
Kidney stones. Researchers at Sweden’s Karolinska Institutet followed 23,355 men with no history of kidney stones for 11 years11. Men who took vitamin C pills, usually 1,000 mg tablets in Sweden, were about twice as likely to develop a stone that needed medical care11.
A medical news report gave rates of 310 per 100,000 among users and 163 per 100,000 among nonusers but didn’t say over what period12. By our math, the study’s totals (436 stones among 23,355 men over 11 years) come to about 170 per 100,000 men a year, so the reported rates are most likely yearly: roughly 3 versus 1.6 men per 1,000 developing a stone each year1112. Multivitamins showed no link, and the study included only men11.
Study leader Agneta Åkesson said the finding needs confirming. She added that because high-dose vitamin C pills lack well-documented benefits, skipping them might be wisest, especially for anyone who has had a stone11. ODS explains that high intakes can raise oxalate, a stone-forming compound, in urine1.
Iron overload. Vitamin C helps your gut absorb iron from plant foods. That’s useful for most people, but it could worsen hereditary hemochromatosis, an inherited iron-overload disease1.
Medicines. ODS flags two concerns. The first involves cancer care: limited data hint that antioxidants might blunt radiation or some chemotherapy drugs, so ODS tells people in treatment to check with their oncologist, especially before high doses. The second involves cholesterol care. In people taking niacin with simvastatin, vitamin C combined with other antioxidants may dampen the hoped-for rise in HDL (“good”) cholesterol, and ODS advises clinicians to monitor lipid levels1.
Plain ascorbic acid does the job
The vitamin C in most pills is ascorbic acid. ODS says the body absorbs it as well as the vitamin C in orange juice or broccoli1.
Labels also list sodium ascorbate, calcium ascorbate and other mineral forms, blends with plant compounds called bioflavonoids, and branded forms such as Ester-C1. Our articles on natural versus synthetic vitamins and on supplement forms review the head-to-head studies, which measured vitamin C in blood or white blood cells, not colds.
Seals from USP, NSF or Informed Choice check what’s in the bottle, not whether it prevents colds.
The vitamin C decision card
Ratings use four levels: Strong evidence, Promising but early, Not enough evidence, and Evidence says no, or unsafe.
| Your situation | What trials show | Dose tested | Risk notes | Rating |
|---|---|---|---|---|
| Healthy adult hoping for fewer colds | Almost no change in cold risk in trials with 10,708 people; the “no” is about benefit, not safety2 | Daily pills, 200 mg or more2 | Food can supply 200 mg or more1 | |
| Short bursts of heavy strain, such as an ultramarathon or subarctic military exercises | Cold risk about halved in five small trials23 | 250 mg to 1 g a day3 | Older, small trials; one company sponsored5 | |
| Daily use to shorten colds | Slightly shorter colds2 | 1 to 2 g a day in children2 | Stay under your age’s upper limit1 | (for a small effect) |
| A big dose once a cold starts | No consistent effect in seven comparisons2 | Up to 8 g in one trial2 | Above the upper limit; diarrhea and cramps1 | |
| IV vitamin C for sepsis | More death or organ failure in LOVIT; panel advises against68 | 50 mg/kg every 6 hours for 4 days7 | A hospital decision | |
| IV vitamin C with cancer treatment | Small trials, mixed results; not FDA approved10 | 75 g three times a week, or 1 g/kg10 | Screening for G6PD deficiency and kidney problems10 |
Questions readers ask
Should I take vitamin C when I feel a cold coming on?
Trials that started vitamin C after symptoms began found no consistent benefit2. One large trial reported a benefit from a single 8-gram dose, and the Cochrane authors called a personal trial reasonable2. That dose is well above the 2,000 mg upper limit, so check with a clinician first if any item in the box above applies to you1.
I’m training for a marathon. Should I take it?
The best results came from runners, skiers and soldiers taking 250 mg to 1 gram a day during short bursts of hard effort23. The trials were small and older, so treat this as promising but early. A day of varied fruits and vegetables can supply more than 200 mg1.
Are vitamin C drips at wellness clinics worth it?
For sepsis, the biggest trial found more harm than benefit6. For cancer, the FDA hasn’t approved it and trials are small10. If you’re considering a drip, ask about screening for G6PD deficiency and kidney problems first10.
The bottom line
Vitamin C matters for health, but extra pills do far less than the hype suggests. For most people, daily vitamin C barely changes the odds of catching a cold and makes colds only slightly shorter. The clearest benefit showed up in small trials of people under brief, heavy physical strain.
Food covers most people’s needs and fills tissues on its own. Megadoses by mouth hit an absorption ceiling and bring kidney stone and iron questions. IV vitamin C is a different treatment: it failed in sepsis, and in cancer it remains an experiment to discuss with your oncologist.
Related reading
About this article
This article was researched using NIH and National Cancer Institute summaries, Cochrane and other systematic reviews, randomized trial reports, a clinical practice guideline and university news releases, all listed below. It was reviewed in October 2026. It is general education, not medical, legal or financial advice.
Sources
- NIH Office of Dietary Supplements. “Vitamin C: Fact Sheet for Health Professionals.” National Institutes of Health, updated July 31, 2025. ods.od.nih.gov
- Hemilä H, Chalker E. “Vitamin C for Preventing and Treating the Common Cold.” Cochrane Database of Systematic Reviews, 2013, Issue 1, CD000980 (Cochrane plain-language and abstract page). cochrane.org
- Hemilä H, Chalker E. “Vitamin C for Preventing and Treating the Common Cold.” Cochrane Database of Systematic Reviews, 2013, full text hosted by the University of Helsinki. mv.helsinki.fi
- Himmelstein SA, et al. “Vitamin C Supplementation and Upper Respiratory Tract Infections in Marathon Runners.” Journal of Exercise Physiology Online, 1998;1(2) (background section describing Peters et al., 1993). asep.org
- Letters to the editor on Peters EM, Goetzsche JM, Grobbelaar B, Noakes TD, “Vitamin C Supplementation Reduces the Incidence of Postrace Symptoms of Upper-Respiratory-Tract Infection in Ultramarathon Runners,” American Journal of Clinical Nutrition, 1993;57:170-174 (compiled by H. Hemilä, University of Helsinki). mv.helsinki.fi
- Centre de recherche du CHUS, Sunnybrook Health Sciences Centre and Université de Sherbrooke. “LOVIT Trial” news release on Lamontagne F, et al., New England Journal of Medicine, 2022. crchus.ca
- The Bottom Line (critical care trial summaries). “LOVIT Trial: Intravenous Vitamin C in Adults with Sepsis in the Intensive Care Unit.” thebottomline.org.uk
- Reintam Blaser A, Alhazzani W, Belley-Cote E, Møller MH, et al. “Intravenous Vitamin C Therapy in Adult Patients With Sepsis: A Rapid Practice Guideline.” Acta Anaesthesiologica Scandinavica, 2023;67(10):1423-1431 (Capital Region of Denmark research record). research.regionh.dk
- Hemilä H, Chalker E. “Vitamin C for Patients With Sepsis?” Frontiers in Medicine, 2024;11:1450091. pmc.ncbi.nlm.nih.gov
- National Cancer Institute. “Intravenous Vitamin C (PDQ)–Health Professional Version.” NCI, updated May 13, 2025. cancer.gov
- Karolinska Institutet. “Vitamin C Supplements Linked to Kidney Stones” (on Thomas LDK, Elinder CG, Tiselius HG, Wolk A, Åkesson A, JAMA Internal Medicine, 2013;173(5):386-388). News release, February 5, 2013. news.ki.se
- Potts M. “Vitamin C Increases Men’s Risk of Kidney Stones.” Consultant360, February 6, 2013. consultant360.com