Almost everyone has heard that vitamin C is “good for colds”. Few people know that the vitamin C you take by mouth and the vitamin C given into a vein behave almost like two different substances. In this article I explain what science has measured, what remains unproven and why a high dose of intravenous vitamin C always needs a medical assessment first.
The human body cannot make vitamin C and needs it every day. It is the “spark” that allows eight of the body’s enzymes to work (Institute of Medicine, 2000). These are among its best-established functions:
It is the essential cofactor of the enzymes that give collagen fibres their strength: three of those eight enzymes are devoted to this. Without vitamin C, collagen forms weakly, which is why severe deficiency (scurvy) damages the skin, gums, blood vessels and wound healing. Healthy skin holds high concentrations of vitamin C, and the cells that make collagen are the ones that accumulate the most (Pullar, Carr and Vissers, Nutrients, 2017).
The white blood cells that “eat” microbes (neutrophils) accumulate vitamin C. In them, it helps them reach the site of infection, trap the microbe and destroy it (Carr and Maggini, Nutrients, 2017).
Two other enzymes need it to make carnitine, the molecule that carries fats into the mitochondria so they can be turned into energy.
It is needed to make noradrenaline from dopamine. That is why the adrenal glands have one of the highest concentrations of vitamin C in the body.
It neutralises free radicals and helps “recharge” vitamin E.
Taken with food, it improves the absorption of iron from vegetables and pulses.
An important nuance: all of these functions depend on having enough vitamin C. They have been shown to correct a shortfall, not to be “boosted” above normal in someone whose levels are already good. In trials with healthy people, giving more vitamin C to those who already had enough barely changed how their white blood cells worked. The question that high-dose studies ask is a different one: what happens when levels are reached that the body never achieves through diet.
The gut absorbs vitamin C up to a point and, beyond that, the body regulates it and gets rid of it. So even if you take more tablets, the amount that reaches the blood barely rises.
The reference study was carried out by the team at the US National Institutes of Health (Padayatty and colleagues, Annals of Internal Medicine, 2004). They measured blood vitamin C in healthy volunteers given the same dose by both routes:
Using their model, they calculated that the maximum tolerated oral dose (3 g every 4 hours) would not exceed about 220 µmol/L. A 50 g infusion, by contrast, would reach about 13,400 µmol/L: around 60 times more. The authors themselves warn that these high figures are a calculation and had not been confirmed in patients.
What this means: the “megadoses” the studies talk about can only be reached intravenously. Taking a lot of vitamin C in tablets does not reproduce that effect.
At normal doses, vitamin C is an antioxidant. At very high concentrations, such as those only reached through a vein, it does something different in the laboratory: it generates hydrogen peroxide in the fluid surrounding the cells.
Laboratory study
In 2005, Chen, Levine and colleagues (PNAS) tested this in cultured cells. Normal cells withstood it without any problem, and five of the ten tumour cell lines they studied died at much lower concentrations. It is an important finding, but it is a laboratory one: it shows that the mechanism is plausible, not that it works the same way in people.
It is one of the uses backed by a better-designed clinical trial. Suh and colleagues (Nutrition Journal, 2012) gave 141 healthy office workers, aged 20 to 49, a single infusion of either 10 g of vitamin C or a drip without the vitamin, without them or the researchers knowing who was receiving which. Those who received vitamin C reported less tiredness at 2 hours and the following day. The effect was clearer in those whose vitamin C was low at the start. It is a small, single-dose study, so it is a promising signal, not a settled conclusion.
There is a lot of interest here, and a lot of exaggeration too. A systematic review (Fritz and colleagues, Integrative Cancer Therapies, 2014) concluded that intravenous vitamin C might improve quality of life and some side effects of chemotherapy, such as tiredness, nausea or insomnia. The authors themselves stress that good-quality evidence is scarce and that well-controlled trials are lacking.
What no serious study says: that vitamin C cures cancer or replaces cancer treatment. If you are having treatment, any drip has to be discussed with your oncologist.
This is the example of why it is not enough for something to “sound good”. For a few years, intravenous vitamin C was used in ICU patients with sepsis. The LOVIT trial (Lamontagne and colleagues, New England Journal of Medicine, 2022), with 872 patients, compared it with placebo. The vitamin C group did not improve: it had slightly more cases of death or persistent organ damage at 28 days (44.5% versus 38.5%). The authors recommend not using it in sepsis outside clinical trials.
The lesson: intravenous vitamin C is not a solution for everything. Each use has to be judged on the evidence available for that specific use.
It is generally well tolerated, but it is not harmless and it is not suitable for everyone. A review of 74 studies (Yanase and colleagues, Critical Care Medicine, 2020) did not find, in double-blind trials, that it was more harmful than placebo. It did record specific problems that call for monitoring:
A genetic disorder of the red blood cells that often causes no symptoms. With high doses of vitamin C it can cause haemolysis (the breakdown of red blood cells). That is why it is ruled out with a blood test before high doses.
Part of the vitamin C is converted into oxalate, which can be deposited in the kidneys. Isolated cases of kidney damage and stones have been described. The US National Cancer Institute warns that the risk is greater if there is already kidney disease.
High doses can distort glucose readings on finger-prick meters. If you are diabetic and monitor yourself this way, the doctor needs to know.
This has been described in some cases.
The same institute adds haemochromatosis (iron overload) to the situations in which it can cause problems.
Before any high dose of vitamin C there is a medical assessment: your history and your medication. From 15 g upwards we also ask for a G6PD test and a kidney function test. The doctor decides the dose for you. There is no “standard” drip that suits everyone.
If you want to know whether it makes sense in your case, the first step is an assessment.
This article is signed by Dr. Mariana Goldstücker, medical registration (colegiada) no. 032892127. This page is for information purposes and does not replace a medical consultation. The assessment is an individual professional evaluation; treatments are determined case by case and do not guarantee results. Your data is processed under GDPR (EU) 2016/679 and the Spanish LOPDGDD.