Vitamin C megadoses cure colds and cancer

What we know

Vitamin C, or ascorbic acid, is an essential nutrient the human body cannot synthesize and must obtain through diet, playing a genuine, well-established role in collagen synthesis, immune cell function, and antioxidant activity. Its reputation as a cold and disease cure traces largely to chemist Linus Pauling, a two-time Nobel laureate whose 1970 book 'Vitamin C and the Common Cold' argued for very high daily doses as both preventive and curative, and who later extended similar claims to cancer treatment in collaboration with physician Ewan Cameron during the 1970s. Pauling's scientific credentials in unrelated fields lent the claims substantial public credibility despite the underlying clinical evidence not supporting them.

The Cochrane Collaboration, widely regarded as producing the most rigorous available systematic reviews in medicine, has updated its analysis of vitamin C and the common cold multiple times, most substantially in a review covering over 10,000 participants across dozens of trials. It found that regular vitamin C supplementation does not reduce the incidence of colds in the general population, though it modestly shortens the duration of cold symptoms by around 8 percent in adults and 14 percent in children once a cold has already started, and found meaningfully larger preventive benefits only in a narrow subgroup, marathon runners, skiers, and soldiers in subarctic conditions engaged in extreme short-term physical stress, a very different population from ordinary claims about everyday cold prevention.

On cancer, Pauling and Cameron's original claims were tested directly by the Mayo Clinic in a series of randomized controlled trials during the late 1970s and early 1980s, widely regarded as more rigorous than the earlier uncontrolled observational work Pauling had relied on. These trials found no survival benefit for high-dose oral vitamin C in cancer patients compared to placebo, a result that directly contradicted Pauling's claims and that Pauling publicly disputed, arguing the Mayo trials used an inferior oral dosing method compared to intravenous administration he favored, a methodological dispute that has never been resolved in vitamin C's favor through subsequent larger trials. The National Cancer Institute and American Cancer Society both currently state that vitamin C, whether oral or intravenous, has not been shown in rigorous trials to cure or reliably treat cancer, though a limited body of ongoing research continues to examine intravenous high-dose vitamin C as a possible complementary treatment for symptom management alongside, not instead of, standard cancer therapy.

Megadosing vitamin C also carries documented risks rather than being a harmless excess, since doses substantially above the recommended daily allowance of roughly 65 to 90 milligrams for adults can cause gastrointestinal upset, diarrhea, and, in susceptible individuals, an increased risk of kidney stones due to oxalate metabolism, effects documented in clinical nutrition literature and reflected in the tolerable upper intake level of 2,000 milligrams per day set by nutrition authorities including the US National Academies of Sciences.

The claim persists partly due to Pauling's enduring scientific reputation lending it authority disconnected from the actual clinical evidence in this specific area, and partly because vitamin C's genuine, well-documented role in immune function creates an intuitive, though ultimately unsupported, leap to the conclusion that more vitamin C in large doses must mean substantially better immune or anti-cancer outcomes.

Common claims

• High-dose vitamin C prevents the common cold in the general population.Not supported, Cochrane reviews found no preventive effect except in narrow groups under extreme physical exertion.
• Vitamin C shortens cold duration once symptoms start.Partly true, Cochrane found a modest reduction of about 8 percent in adults, not a cure.
• High-dose vitamin C cures cancer.False, Mayo Clinic randomized trials found no survival benefit compared to placebo.
• Taking very large amounts of vitamin C is harmless.False, megadoses can cause gastrointestinal upset and increased kidney stone risk in susceptible individuals.

Evidence hierarchy

All sources

• Vitamin C for preventing and treating the common coldCochrane Library · 2013
• Vitamin C and cancer treatment (PDQ)National Cancer Institute · 2023
• Vitamin C fact sheet for health professionalsNational Institutes of Health, Office of Dietary Supplements · 2023
• Does vitamin C help cancer?American Cancer Society · 2022

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