It isn't as hard to OD on vitamins/minerals by eating foods that naturally have these vitamins and minerals as only so much can be consumed during the day (due to volume a stomach can hold and speed of digestion) However, fat solvable vitamins can still be OD's on even if their source is all natural. These do not pass through the system as waste, when the body uses all it needs every day.
Calcium without phosphrous should be given daily or several times a week depending on age of the dragon. During the first 4 months every feeding is idea as dragons grow fast and need calcium to ensure they don't suffer from not enough calcium. As they grow older this can be cut back to dusting once a day then a couple times a week till they are full grown (roughly at 14 months) then the occasional dusting is all that is needed as most should get all the calcium they need from the greens they eat. Egg laying females (whether the eggs are fertile or not) should get more calcium dustings than males as they need that extra calcium to properly shell eggs.
Multivitamins, once or twice a week for young dragons, a couple times a month for adults is fine. The main reason to give multivitamins to reptiles is because in captivity we are unable to provide the same variety of insects/plant matter dragons would eat in the wild. Not to mention the same kinds. I highly doubt collard greens and dandelions grow in Australia in the wild. To be honest, I don't have a clue what sort of plants and shrubs grow in the habitat the common bearded dragon is found (inland bearded dragons) nor what sort of insect life. So we use multivitamins to compensate for missing nutrients in their diets and what we can give them, due to limitations of our area. Not all stores carry the greens recommended, such as collards, endive, dandelions etc. Or a variety of insects, superworms, butterworms, silkworms etc.
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PHLdyPayne