Vitamin D Deficiency: Symptoms, Causes, and How to Fix It (2026)

What are the signs of vitamin D deficiency?

Common symptoms include persistent fatigue, bone pain (especially back and hips), muscle weakness and cramps, frequent respiratory infections, depression or low mood, slow wound healing, and hair loss. These symptoms are non-specific — the only way to confirm vitamin D deficiency is a blood test measuring 25-hydroxyvitamin D (25-OH-D). A level below 20 ng/mL indicates deficiency; 20–29 ng/mL is insufficiency. If you have multiple symptoms from this list, testing is warranted before treating empirically.

How much vitamin D should I take daily?

Dosing depends on your current blood level: For maintenance with adequate levels (30–50 ng/mL): 1,500–2,000 IU D3 daily. For insufficiency (20–29 ng/mL): 2,000–4,000 IU D3 daily for 3 months then retest. For deficiency (below 20 ng/mL): 4,000–5,000 IU D3 daily for 3 months then retest. For severe deficiency (below 12 ng/mL): prescription-supervised repletion (50,000 IU weekly for 8–12 weeks). Official RDAs (600–800 IU) are widely considered insufficient to achieve optimal blood levels. Always test before and after supplementing to titrate to optimal blood levels (40–60 ng/mL).

What is the difference between vitamin D2 and D3?

Vitamin D3 (cholecalciferol) is produced in human skin and found in animal foods. It raises 25-OH-D approximately twice as effectively as D2 per IU and has a 2x longer half-life. Vitamin D2 (ergocalciferol) is produced by fungi and plants — once used in prescription-strength supplements. Recommendation: choose vitamin D3 for supplementation unless medically directed otherwise. Vegan note: most D3 is derived from lanolin (sheep wool); lichen-derived vegan D3 is available and equally effective. D2 is naturally vegan if preferred, but lower potency means higher doses needed.

Should I take vitamin D with vitamin K2?

Taking vitamin D3 with vitamin K2 is a well-reasoned pairing. Vitamin D increases calcium absorption. Vitamin K2 (specifically MK-7 form, 100–200mcg daily) activates proteins that direct this absorbed calcium into bones rather than arteries. If taking 2,000+ IU vitamin D3 long-term, adding vitamin K2 MK-7 is prudent. Important: if you take warfarin or any blood thinning medication, do NOT take vitamin K2 without medical clearance — K2 counteracts warfarin's mechanism.

How long does it take for vitamin D supplements to work?

Blood levels typically rise within 2–4 weeks and reach a new steady state at approximately 3 months. At 1,000 IU/day, expect blood levels to rise approximately 5–10 ng/mL. At 2,000 IU/day, roughly 10–20 ng/mL rise. Symptom improvement: fatigue and mood typically improve within 4–8 weeks; bone-related symptoms take 3–6 months. Retest 25-OH-D approximately 3 months after starting supplementation to assess response and adjust dose.

Can you get too much vitamin D?

Vitamin D toxicity requires blood 25-OH-D levels above 100–150 ng/mL and typically prolonged daily doses above 10,000 IU. Symptoms of toxicity (hypercalcemia): nausea, vomiting, weakness, frequent urination, kidney stones, confusion. From sun exposure: cannot occur — skin photodegrades excess previtamin D3. Tolerable upper limit: 4,000 IU/day per the Institute of Medicine. At 1,000–4,000 IU D3/day, toxicity risk is negligible for healthy adults. Have blood levels tested every 3–6 months if taking over 4,000 IU daily long-term.