Hemilä & Chalker 2013, vitamin C for preventing and treating the common cold

This Cochrane systematic review of 29 trials (11,306 participants) found that regular vitamin C supplementation does not reduce cold incidence in the general population (RR ~0.97), but in five trials of people under extreme physical stress — marathoners, soldiers, and skiers (598 participants) — it halved the risk of catching a cold. Regular supplementation did modestly reduce cold duration in the general population. Strong evidence.