Vitamin D helps the body absorb calcium. Skin makes vitamin D in sunlight, but people who rarely go outside, exclusively breastfed babies and older adults may need a supplement on medical advice. D3 K2 products add vitamin K2, usually as MK-7. Drops suit small children, while tablets or sprays suit adults. Read the amount in IU or mcg per drop or per tablet carefully, because products differ widely. Vitamin D is fat-soluble and too much can be harmful, so use only the dose on the label or as directed by a professional.