It looks like the 1500mg is the calcium (as pointed out by prunus ) -- it's 800 IU a day of Vit D.3 grams/day?
That might be for the pack or something... I'm on one 800IU tablet; 20mg. Also prescribed... Did get initial heroic dose thing, which was something like 6000IU. But that was once/week (which, come to think of it, isn't much more than 800/day, though I may have forgotten the exact dose/how many times).