Potentially harmful. Oral D3 supplementation can be overdosed with or without UVB. As the animal 'manufactures' D3 in it's own body when exposed to sunlight, it's impossible for it to overdose itself. The entirely unnatural oral supplementation which cannot be correctly dosed has been shown to cause long term kidney damage in the case of overdose and ineffective calcium uptake in the case of underdose. Hence why UVB should be provided instead of oral D3 supplement.