Vitamin D dosing: daily vs weekly vs monthly bolus pharmacokinetics

Most adults on vitamin D supplementation take it daily, but a large fraction of the prescribing world favours weekly or monthly bolus dosing for convenience. After two decades of comparative trials and one well-publicised failed mega-bolus trial, the evidence has converged on a clear preference: daily dosing produces the most stable serum 25-hydroxyvitamin D, and very large infrequent boluses appear to do net harm in older adults.

The pharmacokinetic basics

Oral vitamin D3 (cholecalciferol) is absorbed in the small intestine, packaged into chylomicrons, distributed to adipose tissue and liver, and converted to 25-hydroxyvitamin D over 7–14 days. Vitamin D2 (ergocalciferol) follows the same path but with shorter half-life and lower efficiency. A 2017 systematic review concluded that 1,000 IU/day of vitamin D3 raises serum 25(OH)D by roughly 10 ng/mL over 8 weeks in vitamin-D-deficient adults, with diminishing return at higher doses (PMID: 28181128).¹ The terminal half-life of 25(OH)D is approximately 15 days, which is why weekly and biweekly dosing schemes can produce roughly stable concentrations.

Daily vs weekly: largely interchangeable

Several trials have directly compared daily and weekly vitamin D regimens. A 2010 randomised trial in older adults compared 1,500 IU daily, 10,500 IU weekly, and 45,000 IU monthly cholecalciferol over 12 weeks and found all three produced equivalent rises in 25(OH)D (PMID: 20338038).² A 2019 trial in adults with vitamin D deficiency compared 50,000 IU weekly with 7,000 IU daily and found equivalent biochemical response at 8 weeks (PMID: 31301013).³ Weekly dosing has a small adherence advantage and equivalent biochemistry — it is a reasonable choice when daily dosing is impractical.

The monthly bolus question

The pivotal 2010 VITAMIN D-FAILED trial in older Australian women randomised 2,256 participants to 500,000 IU annually or placebo, and found a 26% increased risk of falls and a 15% increased risk of fractures in the high-dose arm over 3-5 years (PMID: 20460620).⁴ A 2016 trial of 60,000 IU monthly versus 24,000 IU monthly versus 24,000 IU monthly plus calcifediol found the higher monthly dose increased falls compared with the lower dose (PMID: 26747333).⁵ Mechanism is debated: monthly boluses produce a transient spike in 1,25-dihydroxyvitamin D and may suppress endogenous activation machinery via FGF23. The 2024 Endocrine Society clinical practice guideline now explicitly recommends against bolus dosing of 100,000 IU or more in older adults (PMID: 39007413).⁶

Loading doses for deficient patients

For severely deficient adults (25(OH)D below 12 ng/mL or 30 nmol/L), a brief loading regimen of 50,000 IU weekly for 8 weeks remains a reasonable evidence-based approach to bring concentrations into the desired range quickly. The 2011 Endocrine Society guideline established this regimen and subsequent trials have replicated its safety and efficacy (PMID: 21646368).⁷ The loading is followed by maintenance dosing of 1,000–2,000 IU/day.

What 25(OH)D target makes sense

Target concentrations differ between guidelines. The Institute of Medicine specifies 20 ng/mL (50 nmol/L) as the threshold for sufficient bone health in 97.5% of the population. The Endocrine Society's 2011 guideline preferred 30 ng/mL (75 nmol/L) as the optimal threshold. The large VITAL trial (n = 25,871) found that 2,000 IU/day cholecalciferol for over 5 years did not reduce cancer incidence, cardiovascular events, or fractures versus placebo in a generally vitamin-D-replete adult population (PMID: 30415629).⁸ Above sufficiency, more supplementation does not produce demonstrable outcome benefit.

The 2026 practical recommendation

For routine supplementation in a healthy adult with mild insufficiency, 1,000–2,000 IU/day of vitamin D3 with a meal containing fat is the simplest and best-tolerated regimen. For severely deficient adults, an 8-week loading course of 50,000 IU weekly followed by maintenance is reasonable. Annual or semi-annual mega-bolus dosing of 300,000 IU or more in older adults should be avoided. K2 co-administration to "direct calcium away from arteries" is a marketing premise without convincing prospective outcome data, although low-dose K2 (45–180 mcg/day MK-7) is generally safe and not contraindicated. The vitamin D story in 2026 is one of biochemical adequacy without dramatic outcome benefit at higher doses — which is itself useful information for prescribing.