Vitamin C: How Much Is Actually Useful

The RDA for Vitamin C — 90 mg for adult men, 75 for women — was set decades ago to prevent scurvy. Scurvy is the floor, not the ceiling. The relevant question for most people isn't "am I getting enough to avoid frank deficiency?" but "am I getting enough to do what I'm actually asking my immune system to do?"

Here's the part the supplement industry doesn't advertise: oral Vitamin C absorption plateaus. Around 200 mg single-dose, the gut down-regulates and you stop absorbing the rest. Take 1 gram orally and you absorb roughly the same as you would from 200 mg. The extra goes through.

Oral Vitamin C maxes out at 200 mg/dL plasma. IV reaches 20,000 mg/dL. That's not a marketing claim — that's pharmacokinetics.

IV Vitamin C bypasses that bottleneck entirely. Studies routinely use 10, 15, even 25 grams intravenously and reach plasma concentrations that produce pharmacological effects — not just nutritional ones. Concentrations high enough to act as a pro-oxidant inside cancer cells, for example, which has driven a small body of oncology research.

For everyday use, the relevant point is more practical. If you take oral Vitamin C at the first sign of a cold, you're absorbing maybe 200 mg of it. An IV at the same moment delivers 50× that into your bloodstream within an hour. The cellular environment is genuinely different.

None of this means oral Vitamin C is useless. As a daily nutritional supplement at 250–500 mg, it does what it does. As an acute therapeutic dose, IV is in a different category.

The honest verdict: oral for maintenance, IV for the moments that actually matter — illness, post-race depletion, pre-flight immune armor, recovery from extended stress.