Vitamin D Supplementation — The Complete Guide
Vitamin D supplementation is one of the simplest, cheapest, most well-studied interventions in medicine — and one where people commonly make small mistakes that reduce effectiveness. This guide covers dose selection, formulation choice, timing, cofactors, and monitoring so you can supplement effectively.
Step 1 — Test 25(OH)D
Before starting any regimen, know your starting number. A single 25(OH)D test costs $30–$100 out of pocket and is often covered by insurance with an ordering diagnosis. Without a baseline, you're guessing. Categories used in this guide: deficient < 20 ng/mL, insufficient 20–29 ng/mL, sufficient 30–60 ng/mL, high normal 60–100 ng/mL, excess > 100 ng/mL.
Step 2 — Pick a dose
- Sufficient starting 25(OH)D (≥ 30 ng/mL) — 1,000–2,000 IU/day maintenance keeps you there in most adults.
- Insufficient (20–29 ng/mL) — 2,000–4,000 IU/day for 8–12 weeks, then re-test.
- Deficient (< 20 ng/mL) — loading regimen: 50,000 IU weekly × 8 weeks (Endocrine Society) or 6,000 IU/day × 8 weeks, then re-test and titrate to maintenance.
- Severe deficiency (< 12 ng/mL) or symptomatic — same loading, expect longer time to full replenishment; see specialist if any osteomalacia symptoms.
- Obesity (BMI ≥ 30) — 2–3× standard doses (see our obesity page).
- Malabsorption — often need 4,000–10,000 IU/day; consider water-miscible or calcifediol.
Step 3 — Pick a form
- D3 (cholecalciferol) is preferred — raises 25(OH)D more effectively than D2 at equivalent dose.
- Softgel with olive/MCT oil — cheapest, best absorption.
- Liquid drops — good for children, adults with dysphagia, precise titration.
- D2 (ergocalciferol) — used in some US prescription 50,000 IU capsules and by vegans not accessing lichen D3. Less potent per IU than D3.
- Lichen D3 — the vegan-friendly D3; equivalent to sheep-lanolin D3.
Step 4 — Take it correctly
- Take with a meal containing at least 10–20 g fat — vitamin D absorption improves 30–50% vs an empty stomach.
- Consistency beats timing — daily is better than weekly for compliance and 25(OH)D stability.
- Don't stack calcium in the same dose over 500 mg — split intake for absorption.
- See our best time of day page.
Step 5 — Consider cofactors
- Magnesium 300–400 mg/day — required cofactor for every vitamin D activation enzyme.
- Calcium 1,000 mg/day total (food + supplement) — needed for the calcium-related benefits of vitamin D; not needed alone with vitamin D unless dietary intake is low.
- Vitamin K2 100 µg/day — theoretical benefit for directing calcium to bone; evidence modest but low harm.
- See D + Mg + K2 page.
Step 6 — Re-test and titrate
Re-test 25(OH)D 8–12 weeks after starting or changing dose (that's roughly the steady-state time for daily oral supplementation). Adjust dose:
- Under target: increase by 1,000–2,000 IU/day.
- Over target: reduce by 1,000 IU/day.
- Over 100 ng/mL: stop for 4 weeks, then restart at half the dose.
Once you find your maintenance dose, re-test yearly. Adjust for season if you pause supplementation in summer.
Step 7 — Know when to see a doctor
- You take 4,000+ IU/day and 25(OH)D still won't rise — possible malabsorption, genetic variation, or drug interaction.
- You have symptoms of vitamin D toxicity — nausea, vomiting, constipation, kidney stones, confusion — while supplementing.
- You have chronic kidney disease, sarcoidosis, hyperparathyroidism, or lymphoma — dosing requires specialist input.
- Bone pain, proximal myopathy, or hypocalcaemic symptoms — need workup, not just supplementation.