Here's an abstract taken from a blog I have on the different vitamins and minerals and what they do:
Vitamin D3: Cholecalciferol, also nicknamed the “sunshine vitamin” is one that the body can synthesize in the skin when exposed to sunlight. It can also be consumed through diet, but in the case of reptiles, they count on basking to produce a significant amount of it. The purpose of vitamin D is to maintain normal, balanced blood levels of calcium and phosphorus in the body, and plays an important role in the absorption of calcium into the bones, helping them become stronger. It also plays an important part in maintaining immune system health.
There are risks associated with too much of this vitamin, which is why it is not recommended to supplement too heavily with this vitamin via diet. Too much of it will start affecting the nerves and muscles, and more severe toxicity will start to affect calcium levels; calcium will be too highly elevated in soft tissues, such as muscles and kidneys, and begin to actually calcify these tissues, while debilitating the bones.
It also contains pre-formed vitamin A. Levels are fairly low. It is safe to use as you have been using it, every other day (though you may not need it that often, if you are using a wide variety of feeders properly gutloaded). Use a phosphorous and vitamin free supplement on the other days, IF a supplement is required.