Dr. Martin Healy:

On how long it takes for vitamin D levels in one's system to increase once one starts taking a supplement, in the general population it takes a few weeks if one is coming from a very low base. It can take a while. This is the case when using the commonly available vitamin D supplement, vitamin D3. There are other supplements which act faster but they are only used in acute situations such as in the ICU when a person's level must be increased very quickly. When one takes a supplement, one may be anxious to see if one's levels have gone up, but a repeat test cannot be carried out within three months. One must wait three months before a repeat test can be carried out for the level of vitamin D in the blood. This is because of the way vitamin D is handled in the body. It takes a while to get to what is called a steady state. It takes a minimum of three months and sometimes longer. While it is inappropriate, we often see requests for vitamin D testing two weeks after supplementation has begun. This does not give an accurate picture.