Vitamin D Loading Dose Protocols
When 25(OH)D is severely low, a loading dose can normalise it in weeks rather than months. Standard-dose supplementation takes ~3–4 months to reach steady state (see our time-to-steady-state page); loading doses compress that to 4–8 weeks. Not every deficient patient needs one — and one loading regimen (annual bolus) has been shown to cause harm.
Standard Endocrine Society loading regimen
- Adults: 50,000 IU cholecalciferol (D3) once weekly × 8 weeks = 400,000 IU total, then maintenance 1,500–2,000 IU/day.
- Alternative: 6,000 IU daily × 8 weeks (equivalent total).
- Children 1–18: 2,000 IU daily × 6 weeks OR 50,000 IU weekly × 6 weeks.
- Infants < 1 year: 1,000–2,000 IU daily × 6 weeks.
- Re-test: 25(OH)D at 8–12 weeks; move to maintenance.
When loading makes sense
- Baseline 25(OH)D < 12 ng/mL (severe deficiency, rickets-territory)
- Symptomatic deficiency — bone pain, proximal myopathy
- Approaching a time-critical intervention (surgery, pregnancy, bariatric bypass)
- Malabsorption where daily dosing keeps failing
- Compliance is easier with weekly than daily
When loading is unnecessary
Mild deficiency (20–30 ng/mL) without symptoms rises adequately on 2,000 IU/day over 2–3 months. Loading buys speed, not endpoint difference. If you don't need normalisation urgently, standard dosing is fine.
The annual mega-bolus that backfired
The 2010 Sanders trial gave 500,000 IU cholecalciferol once yearly to community-dwelling women ≥ 70 years. Result: increased falls and fractures compared with placebo, particularly in the 3 months after each annual dose. A separate trial (Smith 2007) with 300,000 IU IM annually showed similar risk. Very-high infrequent bolus dosing appears to acutely dysregulate calcium handling and is not recommended. Weekly or monthly loading avoids this problem.
D2 vs D3 for loading
D3 (cholecalciferol) is preferred — it raises 25(OH)D more effectively than D2 (ergocalciferol) per unit dose. If your prescription only has D2 (still common in US "50,000 IU vitamin D" scripts), consider asking for D3 or an OTC equivalent. See D2 vs D3.
Safety monitoring
- Loading up to 50,000 IU/week × 8 weeks is well tolerated in adults with normal calcium metabolism.
- Check serum calcium if there's any concern about hyperparathyroidism, sarcoidosis, granulomatous disease, or lymphoma (which can convert 25(OH)D to calcitriol unregulated).
- Don't stack loading doses with concurrent very-high-dose calcium.