Vitamin D3

Essential hormone-like vitamin crucial for bone health and immune function

Scientific nameCholecalciferol FormulaC27H44O Molecular weight384.64 g/mol CAS67-97-0

85 absorption score

9 safety / 10

8 USD per month

5 documented interactions

Check it against something

Evidence level A

Vitamin D3 with:

99 out of 100

Works better together Compatible. Timing differs, so follow each supplement's own window.

Check it against your whole stack →

What it does

mechanism and evidence for each claim

Enhanced bone strength

Increases calcium absorption by up to 40%

Activates calcium-binding proteins in intestines Evidence A-grade 8-12 weeks

Immune system support

Reduces risk of respiratory infections by 25%

Modulates T-cell and macrophage responses Evidence A-grade 4-8 weeks

Mood enhancement

May reduce seasonal depression symptoms by 30%

Influences serotonin production pathways Evidence B-grade 6-10 weeks

Cardiovascular health

Supports healthy blood pressure regulation

Modulates renin-angiotensin system Evidence B-grade 12-16 weeks

Muscle strength

Improves muscle function and reduces fall risk

Enhances muscle protein synthesis Evidence A-grade 8-12 weeks

Others take these for the same thing

Same goal, different mechanism — check any of them against this one

Probiotics for Autoimmune Disease Support Gut health & immune modulation 10-100 billion CFU daily B 60% Calcium Bone health & muscle function 1,000-1,200 mg A 30% Turmeric Anti-inflammatory & antioxidant 300-1000mg A 3% Collagen Peptides Skin health & joint support 5-15g B 85% Calcium Carbonate Affordable bone health & antacid 1000-1200 mg A 25% Phosphorus Bone health & energy metabolism 700-1250mg A 80%

Or a different form of the same thing

the comparison people search most

Vitamin D3 vs Vitamin D2 (Ergocalciferol) absorption 85 against 60 · $8 against $15 a month 60%

All Vitamin D forms ranked by absorption →

Every pair we checked

12 combinations scored by the engine

Vitamin D3 + Prebiotic Fiber Complex May reduce absorption by 10-15% Needs attention 58

Vitamin D3 + Caffeine No documented interaction either way No known interaction 70

Vitamin D3 + Creatine Monohydrate No documented interaction either way No known interaction 70

Vitamin D3 + Whey Protein No documented interaction either way No known interaction 70

Vitamin D3 + Turmeric No documented interaction either way No known interaction 70

Vitamin D3 + Calcium Carbonate Reduces vitamin D conversion efficiency Needs attention 76

Vitamin D3 + Lactase Improved calcium absorption when consuming dairy Works better together 82

Vitamin D3 + Bovine Collagen Essential for collagen synthesis and cross-linking Works better together 82

Vitamin D3 + DHA Prevents oxidation of DHA and enhances stability Works better together 82

Vitamin D3 + Boron - Essential Trace Mineral for Bone and Hormone Health Extends vitamin D half-life Works better together 83

Vitamin D3 + Vitamin C (Ascorbic Acid) Prevents arterial calcification from vitamin D Works better together 90

Vitamin D3 + Magnesium Oxide Essential cofactor for vitamin D activation Works better together 94

Dosage

Morning with fat

Daily1000-2000 IU

Maintenance1000-2000 IU

Therapeutic5000-10000 IU

Maximum10000 IU

Toxic50000+ IU daily for months

Upper limit 100 mcg (4000 IU) NIH ODS

Before you take it

Pregnancygenerally safe at standard doses

VeganYes

When to expect it

First Effects2-4 weeks for blood level changes

Peak Benefits8-12 weeks for optimal tissue saturation

Deficiency Development2-4 months without intake

Toxicity OnsetSeveral months of megadoses

How to measure it

$50-80

Marker25(OH)D

Optimal40-60 ng/mL (100-150 nmol/L)

Deficient below20 ng/mL (50 nmol/L)

FrequencyEvery 6 months initially, then annually

What people actually reported to the FDA

CAERS, 276 reports naming it as the suspect

Most reported effects

Dyspnoea 25

Coeliac disease 14

Excluded from the list above: choking (76), dysphagia (38), foreign body trauma (22), foreign body (16). Those are people struggling with the tablet itself, not with the compound.

Named as the suspect 276 reports of 1,000+ that mention it at all

Serious outcome 88% hospitalisation, emergency room or worse

Other Serious or Important Medical Event211

Hospitalization58

Other Serious Outcome30

Reports are voluntary and unverified — they record what someone experienced while taking something, not what the compound caused. Counts skew towards products people take a lot of, and a serious outcome is more likely to be reported than a mild one, so these are not rates. How we filter them →

Works better with

3 documented

Magnesium

Essential cofactor for vitamin D activation

Required for 25-hydroxylase and 1α-hydroxylase enzymes · level A

Vitamin K2

Prevents arterial calcification from vitamin D

Activates matrix Gla protein and osteocalcin · level B

Boron

Extends vitamin D half-life

Reduces 24-hydroxylase activity · level C

What not to take with Vitamin D3

2 documented

Excessive Calcium

Reduces vitamin D conversion efficiency

Competitive inhibition of hydroxylase enzymes · moderate significance

Fiber

May reduce absorption by 10-15%

Binds to vitamin D in intestines · mild significance

Medication interactions

Discuss these with a pharmacist before combining

thiazide-diuretics Major

Increases calcium retention risk

What to do: Monitor calcium levels monthly

steroids Moderate

Reduces vitamin D effectiveness

What to do: Increase vitamin D dose by 2-3x

Available forms

bioavailability compared

Ranks 2 of 3 among vitamin d forms. Vitamin D3 (Calcidiol) absorbs better — 95%. Compare all forms →

Cholecalciferol 85

Calcidiol (25(OH)D3) 95

Cholecalciferol: Most studied form, Identical to skin production, High absorption

Calcidiol (25(OH)D3): Bypasses liver conversion, Faster acting, More predictable

Food sources

before you supplement

Cod liver oil 1360 IU/100g · 90

Salmon (wild) 988 IU/100g · 85

Mackerel 388 IU/100g · 85

Egg yolks 87 IU/100g · 80

Too little

deficiency signs

Fatigue and weakness
Bone pain and muscle aches
Frequent infections
Depression and mood changes
Hair loss
Delayed wound healing
Muscle weakness

Too much

toxicity signs

Nausea and vomiting
Kidney stones
Hypercalcemia
Heart rhythm abnormalities
Confusion
Excessive thirst and urination

Who benefits most

office-workers

Limited sun exposure increases deficiency risk

2000-3000 IU daily

elderly

Reduced skin synthesis and absorption capacity

3000-4000 IU daily

dark-skinned-individuals

Melanin reduces vitamin D synthesis by up to 90%

3000-5000 IU daily

athletes

Higher needs for muscle function and recovery

2000-4000 IU daily

pregnant-women

Critical for fetal bone development

2000-4000 IU daily

Where to buy it

compare forms first

Look for Vitamin D3 at 1000-2000 IU per serving. The form that matters here is Cholecalciferol.

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What else affects it

Beyond other supplements

Excessive calcium

Reduces vitamin D conversion efficiency

Competitive inhibition of hydroxylase enzymes

How it works

Calcium absorption in intestines (increases 3-5x)
Immune system modulation via VDR receptors
Gene expression regulation (900+ genes)
Hormone production support (testosterone, insulin)
Phosphate homeostasis regulation
Parathyroid hormone suppression

Key findings

what the evidence supports

Reduces mortality by 11% (BMJ 2014, n=95,766)
Improves immune function against respiratory infections (Cochrane 2017)
Reduces fracture risk by 20% in elderly (NEJM 2019)
May reduce cancer risk by 13% (JAMA 2019)

Sources: NIH Examine.com · data reviewed 2026-08-02

Compiled by Denis, who maintains this database and holds no medical qualification — every figure here is sourced or derived, never asserted. How we calculate it →

Common questions

What are the benefits of Vitamin D3?

Documented effects: Enhanced bone strength; Immune system support; Mood enhancement; Cardiovascular health. Increases calcium absorption by up to 40%

How much Vitamin D3 should you take per day?

The common daily range is 1000-2000 IU. By goal — maintenance: 1000-2000 IU; therapeutic: 5000-10000 IU; maximum: 10000 IU; toxic: 50000+ IU daily for months.

When is the best time to take Vitamin D3?

Morning with fat. First effects are typically reported within 2-4 weeks for blood level changes.

What are the side effects of Vitamin D3?

At excessive doses: Nausea and vomiting, Kidney stones, Hypercalcemia, Heart rhythm abnormalities, Confusion. Safety rating at normal doses is 9 out of 10.

What should you not take with Vitamin D3?

Separate it from Excessive Calcium, Fiber by two to four hours. Reduces vitamin D conversion efficiency

Does Vitamin D3 interact with medications?

Documented interactions with thiazide-diuretics, steroids. Discuss these with a pharmacist before combining.

How much Vitamin D3 is too much?

The tolerable upper intake level is 100 mcg (4000 IU) per day for adults, per NIH ODS. Above that, the risk of adverse effects rises: Nausea and vomiting, Kidney stones, Hypercalcemia.

How do you know if you need Vitamin D3?

The marker is 25(OH)D, optimal range 40-60 ng/mL (100-150 nmol/L). Common deficiency signs: Fatigue and weakness, Bone pain and muscle aches, Frequent infections, Depression and mood changes.

Taken for

59 Immunity

Vitamin D3 · Vitamin C (Ascorbic Acid) · Comprehensive Immune Support Complex

38 Joints & Bones

Vitamin D3 · Calcium Carbonate · Bovine Collagen