Is more vitamin D always better?

Short answer

No. People who took 4,000 or 10,000 IU a day for 3 years lost more bone density than those on 400 IU, and daily 3,200-4,000 IU made too-high blood calcium 2.2 times as common.

Is more vitamin D always better?
'More vitamin D → better health' — does this formula hold?
Vitamin D runs low easily, so some people take big doses like 10,000 IU a day. We checked whether more is better, and what happened when people took more.
Does blood vitamin D keep rising with dose?
5 times the dose, only 9% higher after 3 years
311 people took 400 IU, 4,000 IU or 10,000 IU every day for 3 years. After 3 years, blood vitamin D levels were 77 in the 400 IU group, 132 in the 4,000 IU group and 144 in the 10,000 IU group. Going from 4,000 to 10,000 IU, 2.5 times the dose, raised the level by only 9%.
Did bones get stronger?
The higher doses lost more bone density
In the same 311 people, wrist bone density fell 1.2% with 400 IU, 2.4% with 4,000 IU and 3.5% with 10,000 IU over 3 years. Shin bone density also fell most with 10,000 IU, by 1.7%. The calculated strength of the bone did not differ between the three groups.
What about one big monthly dose?
The bigger dose group fell more often
200 people aged 70 or older who had fallen before took vitamin D once a month for 1 year. The 60,000 IU group reached higher blood levels than the 24,000 IU group, but their leg function did not improve more. More of them fell at least once: 67% vs 48%.
Is 3,200-4,000 IU a day fine?
Too-high blood calcium was 2.2 times as common
An analysis pooling 22 trials with 12,952 people who took 3,200-4,000 IU daily for over 6 months found too-high blood calcium (hypercalcemia) 2.2 times as often, about 4 in 1,000 people. Falls were 1.25 times and hospital stays 1.16 times as common. Kidney stones and deaths did not differ.
Labels were wrong, too
2 people took products with 100-1,000 times the stated dose
2 people developed life-threatening high blood calcium, and 1 needed a long stay in intensive care. Their vitamin D products held 100-1,000 times the amount on the label. Their blood vitamin D levels were 1,372 and 644, far above the 144 seen after 3 years of 10,000 IU.
Summary: more did not mean better
From 4,000 IU, losses showed up in bone density and blood calcium
People taking 4,000 or 10,000 IU a day for 3 years lost more bone density than those on 400 IU. Daily 3,200-4,000 IU made hypercalcemia 2.2 times as common, and big monthly doses meant more people fell. 4,000 IU a day is the set upper intake limit for the general public, and harm already showed up around that level.