It generally takes a few weeks of daily vitamin D supplementation for blood levels of vitamin D to rise noticeably. Specifically, each 1,000 IU of vitamin D3 taken daily can increase blood 25(OH)D levels by about 10 ng/ml after a few weeks

. For resolving vitamin D insufficiency, research shows improvement within about 12 weeks of weekly high-dose vitamin D supplementation

. However, the time to feel symptom relief or fully correct a deficiency can vary widely depending on initial vitamin D levels, dosage, individual factors like obesity or malabsorption conditions, and the form of vitamin D taken (D3 tends to work faster than D2)

. Some sources indicate that vitamin D levels can start to increase within 10 days to 2 weeks of supplementation, especially with higher doses (e.g., 5,000 IU daily), but it may take 6 to 12 weeks or longer to fully correct deficiency and notice health improvements

. Symptom improvement such as reduced fatigue, better mood, or less joint pain may be noticed within weeks to months, but blood testing is the definitive way to confirm effectiveness

. In summary:

• Blood vitamin D levels begin to rise within a few weeks of daily supplementation

• High-dose supplementation (e.g., 5,000 IU daily) can increase levels significantly within 2 weeks

• It may take 4 weeks to 4 months to notice health improvements depending on severity and individual factors

• Consistency and proper dosing are key; vitamin D is fat-soluble and absorbed better with meals containing fat

Therefore, vitamin D "works" by increasing blood levels within weeks, but full symptom relief or correction of deficiency may take 1 to 3 months or more depending on the situation

References

• Vitamin D levels rise after a few weeks of supplementation; insufficiency resolved by 12 weeks of high-dose vitamin D

• Health improvements typically seen between 4 weeks and 4 months of daily supplementation

• High-dose (5,000 IU) supplementation can significantly raise levels within 2 weeks

• Vitamin D absorption improves when taken with fat-containing meals