Taking vitamin D and a vitamin B complex supplement together is generally safe, highly common, and clinically acceptable. Because vitamin D is a fat-soluble nutrient that requires dietary fats for optimal absorption, while B-complex vitamins are water-soluble and can be taken with or without food, they function through completely separate metabolic pathways. Combining them poses no harmful chemical conflict, though taking them alongside a balanced meal maximizes the overall absorption efficiency of the fat-soluble component.