Vitamin C does real work in the immune system. White blood cells concentrate it at fifty to a hundred times blood level and burn through it fighting microbes ¹. The open question was only whether taking more shows up as fewer or shorter colds.

Across 29 trials and over 11,000 people the answer splits cleanly. On prevention in the general population, daily vitamin C is a null: the risk of catching a cold is essentially unchanged ². On duration, regular daily use trims episodes by about 8% in adults and 14% in children — half a day off a six- or seven-day cold. On severity, a later re-cut found a 26% drop in severe-symptom days ³. It doesn't stop colds; it compresses the bad fraction of the ones you get.

One group gets a large prevention effect: marathon runners, cross-country skiers, and soldiers on heavy cold-weather blocks. Across five trials, daily vitamin C halved cold incidence during the stress period ². If you're heading into a marathon build, an expedition, or a week of hard outdoor labour, start a daily dose a week before and stay on it. This is the one place the recommendation is confident, not hedged.

Plain ascorbic acid runs about $10–20 a year at this dosing.

Honest forecast at the personal scale: modest. You catch two or three colds a year; daily vitamin C trims roughly half a day off each, landing mostly on the worst days ³. Over a decade those half-days add up to about a week of fevered indoor time you don't spend, quietly, without ever knowing which colds you compressed. For the endurance athlete the number is bigger and faster: half the colds through a hard block, felt in the season rather than the decade.