A daily 2,000 unit tablet showed no harm over five years
In 25,871 adults taking 2,000 international units of vitamin D3 daily for five years, there was no excess of high blood calcium or any other adverse event.
What the study found
Five years of daily vitamin D3 produced no safety signal
In 25,871 adults taking 2,000 international units of vitamin D3 daily for five years, there was no excess of high blood calcium or any other adverse event. The set upper limit is 4,000 international units a day, and intoxication needs blood levels above 150 nanograms per millilitre.
The caveat · Daily doses only, not huge rare ones
That upper limit was set on the risk of high blood calcium with an allowance for uncertainty, not on any observed pattern of harm at 4,000 international units. Reaching a toxic blood level takes very large doses sustained over a long period. Safety at 2,000 international units daily says nothing about the very large intermittent doses that were followed by more falls in other trials.