Bearded Dragons need two lights, one heat and one UVB. SOME bulbs (Mercury Vapor Bulbs) produce both but your light is only a heat lamp, so it produces no UVB. UVB mimics radiation from the sun and allows dragons to make D3, which they need to absorb their calcium. Without UVB, you can feed them as much calcium as you want and their bodies won't absorb it, which is what happened here. UVB lights should be long strip lights like: http://www.lightyourreptiles.com/24-ho-t5-double-bulb-fixture-with-arcadia-12-and-6-5k-day-light-bulbs-included/
These bulbs need to be replaced approximately every year as UVB output goes down. Even if the bulb is still working, it may not be producing UVB.
Just curious, what substrate do you use? Meaning, what is on the bottom of your tank? Do they have problems pooping? It would be best to fix the reason they have problems instead of giving medication if not necessary.