What happens if you take too much vitamin D?

Not the dramatic poisoning you're picturing — but "more is better" is wrong, and high doses can quietly work against you.

The counterintuitive part: high doses can weaken your bones

A three-year double-blind trial in healthy adults compared 400, 4,000 and 10,000 IU/day. Bone density at the wrist fell in a dose-dependent way: −1.2%, −2.4% and −3.5% respectively. More vitamin D, less bone — with no gain in bone strength to show for it. That is the opposite of what almost everyone takes it for.

Large single "bolus" doses look worse still. An annual 500,000 IU dose increased falls by 15% and fractures by 26% in older women. A monthly 60,000 IU dose increased falls too. These trials were in older, fall-prone people rather than healthy adults, which is why they're weighted accordingly — but the direction is consistent.

Pooled data shows that even 3,200-4,000 IU/day roughly doubles the rate of hypercalcemia (excess blood calcium) and raises falls.

The reassuring part

Actual vitamin D intoxication — the kidney-damaging kind — requires blood levels far beyond anything normal supplementation produces. The documented cases involve manufacturing errors and gross overdosing, at levels orders of magnitude above where you'd land.

And the evidence genuinely cuts both ways: the safety analysis of the same cohort as that bone-density trial concluded that 10,000 IU/day was "safe and well tolerated," with hypercalcemia rare, mild and transient. A trial of 100,000 IU monthly in over 5,000 people for three years found no increase in kidney stones or hypercalcemia at all. Both of those are on the evidence page below, arguing against the claim.

The practical read

You are unlikely to poison yourself. But there is no evidence that megadosing helps, and reasonable evidence that it costs you bone. If you're supplementing, a modest dose to correct a genuine deficiency is the evidence-backed use. Taking 10,000 IU because "more must be better" is not.