High-Dose Vitamin D Bolus Therapy: Why Large Single Doses Increased Fractures and Falls

Vitamin D supplementation is normally given as daily or weekly oral dosing, but for decades clinicians experimented with annual or quarterly "bolus" injections or single large oral doses, on the theory that occasional megadoses would solve compliance problems in frail older adults. Two well-conducted randomized trials forced reconsideration, and current guidelines from the Endocrine Society and Society for Bone and Mineral Research advise against bolus regimens at the doses studied.

Sanders 2010: 500,000 IU annually raised falls and fractures

The pivotal trial was Sanders and colleagues 2010, published in JAMA. The investigators randomized 2,256 community-dwelling Australian women aged 70 and older to either 500,000 IU of cholecalciferol once a year or placebo, for 3 to 5 years. The primary endpoints were falls and fractures. The vitamin D arm had a 26 percent higher rate of falls and a 31 percent higher rate of fractures, with the excess concentrated in the first three months after each annual dose [1]. The result was directionally opposite to what most expected based on the daily-dosing literature.

Smith 2007 and intramuscular megadoses

Smith and colleagues 2007 reported that 300,000 IU of intramuscular ergocalciferol given annually for 3 years to elderly UK residents produced a non-significant increase in hip fracture in the active arm, with a hazard ratio of 1.49 [2]. The Khaw and colleagues 2017 ViDA trial in New Zealand used monthly oral doses of 100,000 IU and reported no increase in falls or fractures over a median 3.4 years, but also no fracture reduction [3]. Together, these trials suggest that the harm signal is concentrated in very large annual doses, not in monthly 100,000 IU regimens.

Why might bolus doses increase falls?

Several mechanisms have been proposed. Large boluses produce supraphysiological 25-hydroxyvitamin D peaks (often above 100 ng/mL) that may disrupt local activation of 1,25-dihydroxyvitamin D in muscle and bone. They also induce 24-hydroxylase, the catabolic enzyme that degrades active vitamin D, producing a counter-regulatory phase of relative deficiency weeks after the dose. The peak-trough kinetics may transiently impair muscle function and increase fall risk during the post-bolus weeks.

The VITAL signal in healthy adults

The VITAL trial of 2,000 IU per day cholecalciferol in 25,871 generally healthy US adults found no reduction in cancer or major cardiovascular events over 5.3 years [4]. Importantly, VITAL also found no excess of falls or fractures in the daily-dosing arm. Daily 800 to 2,000 IU regimens have not produced the harm signal seen with annual boluses.

Repletion of severe deficiency

For severe deficiency (25-OH vitamin D less than 12 ng/mL), the Endocrine Society 2024 guideline supports 50,000 IU once weekly for 6 to 8 weeks followed by daily maintenance, rather than a single annual megadose [5]. This regimen achieves repletion without the supraphysiological peaks of annual dosing. Avoiding very-high single doses is the safety lesson.

Practical takeaways

Daily 800 to 4,000 IU cholecalciferol is the dosing the evidence supports for most adults needing supplementation. Avoid single oral or intramuscular doses above 100,000 IU outside specific clinical contexts, and avoid the historical 300,000 to 500,000 IU annual regimens entirely. Patients with sarcoidosis, granulomatous disease, primary hyperparathyroidism, or chronic kidney disease stages 4 to 5 require specialist supervision regardless of dose, because they handle vitamin D abnormally.