How Long Does It Take to Raise Vitamin D Levels?
Vitamin D pharmacokinetics are slow. A new dose changes your serum 25-hydroxyvitamin D level along a first-order approach curve with a time constant of roughly two months  meaning you reach ~63% of the new steady state after one half-life, ~87% after two, and ~95% after three. Practical answers to the most common questions below.
How long does it take vitamin D supplements to work?
Serum 25(OH)D approaches a new steady state along a first-order approach with a time constant of roughly the effective terminal half-life  about 60 days in lean adults and up to 90 days in obese adults. In practical terms, you reach ~87% of the final level by three half-lives, so 3–6 months of consistent supplementation is a fair rule of thumb before your 25(OH)D reflects the full effect of the new dose.
How much does 1,000 IU/day of vitamin D3 raise blood levels?
In lean adults, ~1 ng/mL of serum 25(OH)D per 100 IU/day cholecalciferol at steady state (Heaney 2003). So 1,000 IU/day raises steady-state 25(OH)D by ~10 ng/mL, and 2,000 IU/day by ~20 ng/mL. Response is smaller in obese adults (adipose sequestration) and slightly smaller with ergocalciferol (D2).
How fast can I raise my 25(OH)D with a loading dose?
Loading protocols such as 50,000 IU cholecalciferol weekly for 8 weeks (Endocrine Society) can push serum 25(OH)D from < 20 ng/mL to > 30 ng/mL within 4–8 weeks. High single doses (300,000+ IU) raise 25(OH)D within days, but effects on downstream health markers (PTH, calcium absorption) lag by 2–4 weeks.
Why is my 25(OH)D still low after a month of supplementing?
Two main reasons: dose too low for your body weight/adiposity, or absorption issues. Cholecalciferol needs dietary fat to absorb  take it with a meal containing some fat. Obese individuals often need 2–3× the standard dose. Fat-malabsorption conditions (celiac, IBD, post-bariatric surgery) reduce uptake further.
Does D3 raise levels faster than D2?
Yes. Tripkovic et al.'s 2012 meta-analysis found cholecalciferol (D3) 60–90% more effective at raising serum 25(OH)D than ergocalciferol (D2) at equivalent oral doses. If your prescription is ergocalciferol, ask about substituting cholecalciferol at the same IU dose.
When should I re-test my 25(OH)D after starting supplementation?
Three months after starting or changing a dose is the standard interval  it gives serum 25(OH)D enough time to approach the new steady state. Re-testing sooner captures the trajectory but underestimates the eventual level.
Model your own timeline
Enter your baseline 25(OH)D and target dose into the reserves calculator. It uses a two-compartment pharmacokinetic model to project your serum 25(OH)D over the next 180 days at any daily supplemental dose.