Vitamin C

At normal dietary doses it does the boring, essential thing exceptionally well: it’s the cofactor your body needs to build collagen, so adequacy keeps skin, blood vessels, gums and connective tissue intact — and frank deficiency is scurvy, which is dramatic, real, and completely reversible with vitamin C. It also recycles other antioxidants and helps you absorb iron. None of that is in dispute. What people actually seek it out for — IV drips for ‘immune boosting,’ faster recovery, or as an add-on to cancer care — is a separate and far shakier claim than the textbook biochemistry.

The genuinely interesting fact is pharmacokinetic, not magical: your gut tightly caps how much vitamin C you can absorb from pills, so oral dosing tops out around 150–250 µmol/L in plasma no matter how much you swallow. Bypass the gut with an IV and that ceiling disappears — gram-scale infusions reach plasma levels 25–70× higher than oral can ever produce, into millimolar territory. At those concentrations ascorbate stops acting like a vitamin and starts acting like a pro-oxidant drug, generating hydrogen peroxide that’s selectively toxic to some tumor cells in a dish. That mechanism is real and is exactly why the IV-cancer idea exists — but a plausible mechanism in cell culture is not the same as helping patients, and that’s where the story falls apart.

Ascorbate is the electron-donor cofactor for prolyl and lysyl hydroxylase, the enzymes that hydroxylate proline residues so collagen’s triple helix can form and stay stable — lose it and connective tissue literally falls apart (scurvy). It also regenerates vitamin E and other antioxidants and reduces dietary iron to its absorbable form. The high-dose IV mechanism is a different beast: at pharmacologic millimolar plasma levels ascorbate auto-oxidizes and generates extracellular hydrogen peroxide, which some cancer cells clear poorly — the proposed selective-cytotoxicity pathway. Oral dosing physically cannot reach those levels because intestinal transporters saturate and the kidney clears the excess.

At dietary and even generous oral doses it’s about as safe as nutrients get — excess is water-soluble and largely peed out, with loose stools and GI upset the main complaint at high oral doses, plus a modest kidney-stone signal in predisposed people. High-dose IV is a different risk profile entirely: oxalate nephropathy and stone formation (oxalate is an ascorbate breakdown product), with fatal acute renal failure reported in patients with pre-existing kidney dysfunction. The most dangerous is hemolysis in G6PD-deficient patients — severe, sometimes fatal red-cell destruction — which is why G6PD screening before infusion is mandatory. High-dose IV also fouls finger-stick glucometers, giving false readings. Notably, in blinded trials the overall adverse-event rate for high-dose IV was similar to placebo in non-G6PD-deficient people — the danger is concentrated in the specific failure modes above, not diffuse toxicity.

There’s no peptide-style stacking story here. In dietary use it’s paired with iron-rich meals because it boosts non-heme iron absorption, and it regenerates vitamin E, so the two are biochemically complementary. In the clinic, high-dose IV vitamin C shows up bundled into ‘wellness’ drips alongside B-vitamins, glutathione and magnesium, and into experimental sepsis cocktails with thiamine and hydrocortisone — but those combinations are marketing or unproven protocol, not validated synergy. The honest framing: there is no evidence-backed reason to ‘stack’ it for the boosted effects people are sold.