Vitamin D3

Best for

Ease seasonal allergy symptoms

Likely strong benefit · 400–60000 IU/day for 4–12 weeks · 2 meta-analyses , n=1.3k

77 papers · 16 claims · 129 outcomes scored · 9 positive

Outcomes

What vitamin d3 actually does, by outcome

Each row is one outcome with effect size, evidence base, the dose that worked in trials, and time to first effect. Magnitude tiers come from native-unit MCID where available, Cohen's d otherwise.

Outcome Effect Evidence Dose Weeks Rank
Ease seasonal allergy symptoms Likely strong benefit
large
2 meta-analyses n=1.3k Meta-analyzed 400–60000 IU 4–12 wk #1/7
Reduce everyday fatigue Likely strong benefit
large
1 RCT n=120 100000 IU #2/44
Lower fracture risk in older adults Likely benefit
moderate
3 meta-analyses n=50k Meta-analyzed 800–2000 IU 276 wk #3/3
Raise vitamin D levels Likely benefit
moderate
29 meta-analyses n=22k Meta-analyzed 5–600000 IU 0–156 wk #1/2
Replenish immune fuel during TB treatment Likely benefit
moderate
29 meta-analyses n=22k Meta-analyzed 5–600000 IU 0–156 wk #1/2
Improve asthma control Faint early signal
small
5 meta-analyses n=4.2k Meta-analyzed 125–4000 mcg 12–48 wk #1/6
Raise key nutrient levels in pregnancy Likely modest benefit
small
5 RCTs n=2.8k 20–150000 IU 4–26 wk
Build stronger nutrient stores in babies Faint early signal
small
3 RCTs n=305 20–150000 IU 4–26 wk
Catch fewer colds and infections Likely real but unnoticeable
trivial
2 RCTs n=7.5k 3200–100000 IU 26–52 wk #5/17
Reduce falls in older adults Probably doesn't help
trivial
3 meta-analyses n=19k Meta-analyzed 400–4800 IU 52–104 wk #2/5
Lower COVID infection risk Probably doesn't help
trivial
2 RCTs n=7.5k 3200–5000 IU 26–39 wk
Move more easily with aging Probably doesn't help
trivial
3 RCTs n=1.9k 1000–100000 IU 12–104 wk
Build stronger bones in growing kids Probably doesn't help
trivial
1 RCT n=665 1000 IU
Build and maintain bone density Doesn't appear to help
trivial
2 meta-analyses n=610 Meta-analyzed 800–2000 IU 104 wk
Strengthen bone structure Probably doesn't help
trivial
1 RCT n=574 2000 IU 104 wk
Lower the chance of any cancer diagnosis Not enough research
1 RCT n=5.1k 100000–200000 IU

Risk profile

Adverse events and known drug interactions

Safety events

gastrointestinal adverse events (bloating, constipation, diarrhea, nausea, heartburn) moderate
muscle cramps moderate
other musculoskeletal adverse events (muscle soreness, joint pain, tendonitis, falls injuries) moderate
dizziness mild
nausea mild
hypercalciuria mild
hypercalcemia mild
headache mild

Sources

  1. 1. Effect of Adjuvant Vitamin D Therapy on Total Nasal Symptoms Score, IgE, and Eosinophil Levels in Allergic Rhinitis: A systematic review and meta-analysis. (2025)
  2. 2. Effect of vitamin D3 on self-perceived fatigue: A double-blind randomized placebo-controlled trial. (2016)
  3. 3. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. (2012)
  4. 4. Effects of high doses of cholecalciferol in normal subjects: a randomized double-blinded, placebo-controlled trial. (2014)
  5. 5. Moderate amounts of vitamin D3 in supplements are effective in raising serum 25-hydroxyvitamin D from low baseline levels in adults: a systematic review. (2015)
  6. 6. A Trial of Calcium and Vitamin D for the Prevention of Colorectal Adenomas. (2015)
  7. 7. A randomized two way cross over study for comparison of absorption of vitamin D3 buccal spray and soft gelatin capsule formulation in healthy subjects and in patients with intestinal malabsorption. (2015)
  8. 8. The effect of Vitamin D supplementation in disease activity of systemic lupus erythematosus patients with Vitamin D deficiency: A randomized clinical trial. (2017)
  9. 9. Effects of vitamin D3 in clinically isolated syndrome and healthy control participants: A double-blind randomised controlled trial. (2017)
  10. 10. Bovine Lactoferrin Enhances TLR7-Mediated Responses in Plasmacytoid Dendritic Cells in Elderly Women: Results From a Nutritional Intervention Study With Bovine Lactoferrin, GOS and Vitamin D. (2018)

Generated August 14, 2026

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