Vitamin C (also known as ascorbic acid) is a micronutrient found in many foods. Interestingly, it is essential for primates and guinea pigs, but not for any other mammals since most species can manufacture it from other substances in their diets. The discovery that the disease scurvy, common throughout history among sailors and others without access to fresh fruits and vegetables for long periods, was due to Vitamin C deficiency is one of the classic examples of early use of science and scientific methods to solve a serious health problem. Ensuring adequate Vitamin C intake has been one of the most effective public health measures in history.
However, in the 1970s the notion began to gain popularity that extremely high doses of Vitamin C, well beyond any nutritional requirements, could be used as a drug to prevent or treat disease. This was largely due to the efforts of Nobel Laureate Linus Pauling, a brilliant chemist who developed a bit of an obsession with the medicinal use of Vitamin C in his later years. Because of this, he is considered the paragon of the Nobel Disease, in which an accomplished scientist becomes enamored of implausible or pseudoscientific ideas and refuses to abandon them when the evidence dictates they should. Though the verdict is not etched in stone on all aspects of the medicinal use of megadoses of Vitamin C, but as we will see it is clear that Pauling suffered from the Nobel Disease with respect to this practice.
The two primary uses to which Pauling, and many others in his time and since, have suggest Vitamin C could be put are in the prevention and treatment of the common cold and cancer. In terms of the common cold, the evidence indicates there is no benefit for prevention and there might or might not be a small, largely clinically irrelevant benefit for treatment. Pauling was unquestionably wrong on this one. The situation is more complex with regard to cancer therapy.
Does It Work?
The initial studies of Vitamin C as a cancer therapy, reported by Pauling and Dr. Ewan Cameron, appeared to show a benefit in terms of survival when cancer patients received large doses of Vitamin C along with standard therapy. However, these were methodologically terrible studies that were likely only accepted for publication on the basis of Pauling’s prestige. Subsequent research by others replicating this work (e.g. 1, 2) did not find any benefit. A nice review of this history is available on the Science-Based Medicine Blog.
One objection to the negative studies offered by proponents of Vitamin C as a cancer therapy was that they primarily gave the vitamin orally. There is some in vitro and lab animal evidence (e.g. 3, 4) suggesting Vitamin C is more toxic to cancer cells than healthy cells at very high doses (though, of course, there are some limitations to these studies). The concentrations associated with this effect can only be achieved in living animals with intravenous injection of high doses of Vitamin C, so some have argued that the studies showing no benefit from oral use should be re-evaluated with intravenous dosing.
There is little clinical research in humans, and none in companion animals, to show that intravenous high-dose Vitamin C is beneficial for cancer patients. Some small scale uncontrolled studies and case reports have shown some potential effects, but it is unclear if there is any meaningful benefit in terms of survival, quality of life, and other clinically important variables. A narrative review from 2010 concludes:
In view of this lack of data after trials which have included at least 1,591 patients over 33 years, we have to conclude that we still do not know whether Vitamin C has any clinically significant antitumor activity. Nor do we know which histological types of cancers, if any, are susceptible to this agent. Finally, we don’t know what the recommended dose of Vitamin C is, if there is indeed such a dose, that can produce an anti-tumor response.
Similarly, the American Cancer Society position on Vitamin C in cancer patients states,
Although high doses of vitamin C have been suggested as a cancer treatment, the available evidence from clinical trials has not shown any benefit.
And while it is easy, as always, to find supportive anecdotes, it is also easy to find anecdotes that show no benefit. I’ve discussed previously why such anecdotes are only useful in suggesting, not proving, hypotheses. Unfortunately, anecdotal evidence is a bit like a two-headed coin in that proponents of any practice win no matter which side is showing. If a patient seems to improve, that is claimed to demonstrate the therapy works. If a patient doesn’t improve, however, that doesn’t indicate that the therapy doesn’t work in general, only that it doesn’t work in all patients. Since nothing is perfect, this sounds reasonable until you realize that with this kind of spin anecdotes can only ever be used to support a therapy, never to challenge it.
The most positive possible spin one can put on the evidence in humans and lab animals is that there might be a small benefit in some cases, though it is more likely this is simply random noise in the data produced by a small number of studies with significant limitations. No spin at all can be put on the evidence for intravenous Vitamin C in companion animals because there is none, apart from the inevitable anecdotes, of course. One study has shown intravenous Vitamin C generates high levels of the chemical in dogs for only a very short time, so any beenfits would either have to happen from only a brief period of exposure or many injections would have to be given frequently to have any effect.
Is It Safe?
I have always argued that any therapy which has a benefit will undoubtedly have side effects. Living organisms are simply too complex to tinker with their workings and not have unintended, as well as desirable, effects. Though its benefits are still unproven, there is no doubt megadoses of Vitamin C have real physiological effects, and so there is the possibility for harm as well as benefits.
Some research conducted about the same time as Pauling’s studies has suggested that dietary Vitamin C can accelerate the growth of some cancers in laboratory mice. Minor side effects are commonly reported, including nausea, diarrhea, and changes in blood pressure and blood sugar.
And high doses of Vitamin C given intravenously have been documented to cause kidney failure, so it should not be used in patients with any compromise in kidney function. Formation of kidney stones has also been linked to Vitamin C supplementation. Individuals with certain enzyme deficiencies or abnormalities of iron absorption can also be harmed by excessive Vitamin C supplementation. A particularly significant issue in cancer patients is that Vitamin C has been shown to reduce the effectiveness of some anti-cancer drugs. Using a chemical with unproven value that can interfere with the proven benefits of medication in patients with a serious disease is not smart or compassionate care.
Once again, there doesn’t appear to be any formal scientific research on the safety of high doses of intravenous Vitamin C in companion animals. Using such a therapy is a bit like throwing darts blind-folded and hoping to hit the bull’s eye rather than the person standing next to the target.
High doses of Vitamin C given by intravenous injection have not been proven to have any benefit in human cancer patients. There are some studies suggesting such a benefit might exist, but the evidence is weak and contradictory. There is also evidence of both minor and serious side effects associated with this treatment. Vitamin C can interfere with some chemotherapy drugs, thus reducing the benefits of conventional therapy. And, as always, there is a serious risk of harm for patients who elect this unproven therapy over better studied treatments with known risks and benefits.
There is no published clinical research in companion animals evaluating the effects of intravenous Vitamin C as a cancer therapy. The safety and efficacy of this practice is completely unknown despite claims made based on uncontrolled anecdotes and extrapolation from studies in humans.