Vitamin C for Common Cold
For the general population, vitamin C does not prevent the common cold, and taking it once symptoms start has no reliable benefit. Routine daily supplementation may modestly shorten how long a cold lasts and take some edge off severe symptoms, but the effect is small and is not a reason to load up at the first sniffle.
Why this grade7-layer evidence engine
This earns a Weak (C) grade because the evidence splits sharply by question. The anchor Cochrane review (PMID 23440782, ~11,300 participants) found that regular supplementation did not lower cold incidence in ordinary people (RR 0.97), and a GRADE umbrella review reached the same negative conclusion on prevention (PMID 30113569). Crucially, vitamin C started after symptoms appear showed no consistent therapeutic effect. The one trial set reporting dramatic post-onset cures (PMID 33102597) was China-only with biologically implausible results and is flagged low-quality, so it is not treated as real support.
The grade stays above 'does not work' because two signals are genuine but modest. The same Cochrane review showed regular intake trimmed cold duration (about 8% in adults, 14% in children), a 2023 meta-analysis found roughly 15% lower severity at 1 g/day or more (PMID 38082300, concentrated in severe symptoms), and a 2022 meta-analysis found shorter duration but no significant effect on incidence (PMID 34967304). A notable caveat: the only strong prevention benefit (halved risk, RR 0.48) was limited to extreme-exertion groups such as marathoners and skiers and does not generalize.
Regulators and clinics line up with this cautious read. The FDA and EFSA back vitamin C for basic roles like antioxidant protection and normal immune function, not as a cold cure, while the NHS stresses a balanced diet and warns against high doses. Mayo, Cleveland Clinic, and Harvard all say it does not prevent colds (Harvard: doses above 500 mg show no significant effect), and the CDC's cold-care guidance omits vitamin C entirely. Confidence is also tempered because the key positive studies share one lead author and the main Cochrane review has not been fully updated in over a decade.
Scoring transparency
All scores computed by a 7-layer evidence engine — fully auditable▸View the full decision path (audit trail)
- compute_raw_score — 加權公式: L2×0.30 + L3×0.25 + L5×0.25 + L11×0.10 + L1×0.10 = 0.613
- tier_from_score — 依分數區間映射至 tier letter
- apply_hec_rules — 高品質 SR/MA 顯示 positive (2 篇 > 0 negative)
- tier_strict_requirement_check — | B→C 因 scope.conflation_risk=true 且 L11 獨評較低 (B7-2 tier cap)
- detect_disputes — 偵測到 0 個 hard + 0 個 soft dispute
- decide_status — 依 tier + dispute 結果決定 status