Vitamin D and the Common Cold
Vitamin D is one of the few over-the-counter supplements with reasonable trial evidence for reducing acute respiratory infections — including colds. The effect is small in the vitamin D-sufficient and larger in the deficient. As always, daily/weekly dosing works; bolus doesn't.
Martineau meta-analysis
Martineau 2017 BMJ (25 RCTs, 10,933 participants) and 2021 update (46 trials, 75,541) both showed vitamin D reduced acute respiratory tract infections (which include colds, flu, and lower respiratory infections). Reduction ~10% overall, ~70% in profoundly deficient (25(OH)D < 25 nmol/L) subgroups. See our fuller flu page.
Mechanism against viruses
Vitamin D upregulates cathelicidin (LL-37) — an antimicrobial peptide with broad activity against respiratory viruses including rhinoviruses (the most common cold viruses), influenza, and RSV. It also modulates T-cell responses, dampening the excessive inflammation of severe upper respiratory infection.
Doesn't treat active colds
Vitamin D's evidence is for prevention, not treatment. Taking a mega-dose when you already have a cold has no evidence for shortening it, and one-off high-dose regimens don't reduce future infection rates. If you catch a cold, rest, fluids, and symptomatic care (analgesics, decongestants, saline nasal) are what actually help.
Practical guide
- Maintain 25(OH)D ≥ 30 ng/mL year-round with 1,000–2,000 IU/day cholecalciferol.
- Daily dosing preferred; weekly acceptable for compliance.
- Don't rely on vitamin D as a substitute for other cold-prevention basics — handwashing, adequate sleep, minimising exposure during peak cold season.
- Zinc lozenges started within 24 hours of symptom onset have modest evidence for reducing cold duration (see our zinc page).