Short answer: Neither prevents colds in most people. Taking vitamin C every day shortened colds by about 8% in adults, but starting it once a cold began showed no consistent effect.1 Zinc, mostly as lozenges, started during a cold may shorten it by about 2 days, on low-certainty evidence and with more side effects.2 Both reviews tested each against placebo, not against each other.
| Vitamin C | Zinc | |
|---|---|---|
| Preventing colds | No effect in the general public (10,708 people); about half the risk in 598 marathon runners, skiers and soldiers1 | Little or no effect (9 trials, 1,449 people, low certainty)2 |
| Cold length when taken daily | 8% shorter in adults, 14% shorter in children1 | Probably little or no difference (3 trials)2 |
| Started after a cold begins | No consistent effect in 7 trial comparisons1 | May shorten colds by about 2.4 days (8 trials, 972 people, low certainty)2 |
| Amounts studied | At least 0.2 g a day in the trials reviewed1 | Zinc gluconate lozenges at 45 to 276 mg a day for 4.5 to 21 days2 |
| Side effects | High doses can cause diarrhea, nausea and cramps3 | More non-serious side effects in treatment trials (risk ratio 1.34)2 |
| NIH upper limit | 400 to 2,000 mg a day, depending on age3 | 40 mg a day for adults; 50 mg or more for weeks can block copper absorption4 |
A Cochrane review pooled placebo-controlled trials of at least 0.2 g a day. In the general public, daily vitamin C did not lower the chance of catching a cold, but in people under heavy physical stress, such as marathon runners and soldiers in subarctic training, the risk was about halved. Taken every day, it shortened colds by 8% in adults and 14% in children. Taken only once symptoms started, it had no consistent effect.1 NIH reaches the same conclusions.3
A 2024 Cochrane review of 34 trials with 8,526 people found zinc may do little or nothing to prevent colds. Used as a treatment, mostly as lozenges, it may shorten colds by about 2.4 days, but the evidence is low certainty, trials differed a lot, and non-serious side effects were more common.2 A separate meta-analysis of 7 lozenge trials using more than 75 mg a day found colds 33% shorter, with no sign that more than 100 mg a day worked better.5 NIH says zinc lozenges or syrup taken early in a cold can shorten it but do not seem to make symptoms milder.4
For a cold that has already started, zinc has more supporting data than vitamin C, though it is low certainty. For prevention, neither helps most people. Zinc lozenge trials used daily amounts above the NIH adult upper limit of 40 mg, and zinc can interact with some antibiotics.4 This is general information, not personal advice. More detail: the Vitamin C (Ascorbic Acid) and Zinc guides.