hi claire. the first time i megadosed was per my hospital drug info desk's recommendations for how to boost a person's d3 level 50nmol/L in 10 days. you take 50,000IU per day for 10 days. the first time i did this i went from the 70s to the high 140s. that was in 2006.
in the meantime i identified and fixed a zinc deficiency. in spring 2009 i tested 103 for d3 and i wanted to be over 150 so i decided on 50,000IU per day for 8 days. this time instead of going up the anticipated 50nmol/L or so, i shot up to 271 nmol/L d3. (over 250 puts you at risk of developing hypercalcemia).
so it appears that d3 absorption may be related to zinc status (FYI average zinc level in healthy controls is 18.2 umol/l, zinc is considerably lower in MS patients, this may also be reflected in the low uric acid levels seen in many ms patients).
interactions like these are a great reason not to megadose blind, ie without some idea of where you are starting from and what you're aiming to achieve.
for d3 it's best to have levels over 150 but not over 250 nmol/L.
a daily intake from all sources should be 4000IU for basic maintenance.
if you take 8000IU per day that will boost your baseline level slowly over time. obviously it will be faster if you optimize other nutrients, or take much larger doses such as those mentioned above, for a week or thereabouts.
in the short and long term, be careful to get enough calcium and magnesium as well as zinc. never take your whole day's worth of magnesium at the same time as d3. split it up so that your body can take advantage of magnesium for its own sake, not just as part of the d3 process.
if you have any more questions, feel free