Vit D in concentrated form, is a very small amount really and GI reactions to it are not common. The units convert into micrograms and while the units seem "high" in reality.
1000IU = 25micrograms. This is really tiny, and not likely to cause upsets.

Some D is in oil gel caps, mostly soybean oil.
Some is dry in tablets.
Some in drops.

The D2 in RX caps, contains sorbitol, which is a laxative to some people. But it is still a small amount of sorbitol, which is given by the teaspoonful or more for the laxative action to work.
The D2 on RX has been shown to not work...this is the form many doctors still use unfortunately, and if you want results don't use it!

The 5000IU would be 5 X 25mcg or 125micrograms/dose. This is also written as .125 milligrams. A milligram is 1000th of a gram. About the size of a grain of sugar or salt -- it is really minute. Thyroid hormone is expressed this way too.
Thyroid hormone is typically given in micrograms.. 25 mcg to 300mcg

The lower end of Vit D ranges remain in use in US as "normal".
But 30 is not really normal anymore. The new goal is 50ng/ml.
For MSers, the target is much higher than for other people, and is closer to 100ng/ml.

The average dose guide is 1000IU for ever 10ng needed to raise. People who carry more weight on their bodies typically need more, some people may need less. So if a person tests at 30 then 2000IU may raise them to 50.

There are two test ranges also...which may be confusing.
in US typically ng/ml are used. In other countries nmol/L are used. This is a link to convert from one to the other:
http://www.unc.edu/~rowlett/units/sc...ical_data.html

Vit D is near the bottom. People in Canada and UK typically test in nmol so their reading may appear actually higher when US ranges are discussed.

Please visit the links on my thread that was given above, to get more information. Some physicians are recommending that when you take high dose D3 to raise your levels to the new guidelines, that you consume no more that 600mg of calcium as a supplement. As you raise your D you absorb more calcium from the GI tract, and to avoid any risk of hypercalcemia...you should not take high doses of calcium as a supplement. I think this is good advice.