While milk provides some calcium, its fortified vitamin D comes as ergocalciferol (vitamin D2), which studies show is poorly absorbed compared to the cholecalciferol (vitamin D3) naturally found in foods like sardines. (mindbodygreen.com)
Pets and humans require vitamin D. However, humans can convert ergocalciferol (D - 2) into active vitamin D - 3, while pets can not. (2ndchance.info)
So be sure the vitamin tablet or liquid that you purchase contains cholecalciferol (D - 3) and not ergocalciferol (D - 2). (2ndchance.info)