Vitamin D3, or cholecalciferol, is the form of vitamin D that human skin makes when exposed to ultraviolet B (UVB) light. It is also the form found in fatty fish, one of the few foods that naturally contain much vitamin D. On its own it is inactive: the liver and kidney convert it into the hormone that controls calcium absorption and bone mineralization.

Vitamin D3 is often discussed as a testosterone aid, because observational studies link low vitamin D levels with low androgen levels in men. The single most important fact is that large randomized trials have not borne out most of these hopes. In healthy adults, extra vitamin D3 did not prevent cancer, heart disease or fractures, and controlled trials have not shown that it reliably raises testosterone.

This page summarizes what the Institute of Medicine (IOM) report, the 2024 Endocrine Society guideline and the major clinical trials report. It is for education only and is not medical advice.

Property	Detail
Class	Fat-soluble vitamin; precursor of the steroid hormone calcitriol
Chemistry	Cholecalciferol, C27H44O, 384.6 g/mol
Sources	Skin synthesis under UVB light; fatty fish; supplements and fortified foods
Route	Oral (supplements); endogenous synthesis in skin
IOM daily intake (RDA)	600 IU for ages 1–70; 800 IU for ages 71 and older
IOM upper limit (UL)	4,000 IU/day from age 9
Doses in major trials	2,000 IU/day (VITAL); 60,000 IU once a month (D-Health)
Half-life	25(OH)D3 ≈ 15 days; calcitriol ≈ 15 h; body stores ≈ 2 months
US status	Sold as a dietary supplement and in fortified foods
WADA status	Not on the WADA 2026 Prohibited List

What it is

Vitamin D3 is a secosteroid: a steroid whose B ring has been broken open. PubChem lists it as C27H44O with a molecular weight of 384.6 g/mol (PubChem).

In skin, UVB light in the 280–320 nm range splits the B ring of 7-dehydrocholesterol, a cholesterol precursor. The product, pre-vitamin D3, then rearranges into vitamin D3 by heat alone (Bikle, Chem Biol 2014). Several things limit this process:

melanin in darker skin, which blocks UVB;
clothing and sunscreen;
season and latitude, since UVB intensity falls further from the equator.

Because skin synthesis varies so much, the IOM set its intake recommendations on the assumption of minimal or no sun exposure (Ross et al., JCEM 2011).

Vitamin D3 vs vitamin D2

Feature	Vitamin D3 (cholecalciferol)	Vitamin D2 (ergocalciferol)
Origin	Skin (from 7-dehydrocholesterol); fatty fish	UVB-irradiated ergosterol in plants and fungi
Binding to vitamin D binding protein	Higher	Lower, so cleared faster from blood
Raising blood 25(OH)D	More effective, especially as a large single dose	Less effective as a bolus; difference lost with daily dosing

The structural and binding differences come from Bikle’s review. The comparison of potency comes from a meta-analysis of randomized trials that compared the two forms head to head (Tripkovic et al., AJCN 2012).

How it works in the body

Vitamin D3 is activated in two steps (Bikle, Chem Biol 2014):

Liver. The enzyme CYP2R1 (the main 25-hydroxylase) converts it to 25-hydroxyvitamin D, written 25(OH)D. This is the main circulating form and the one measured in a “vitamin D blood test.”
Kidney. The enzyme CYP27B1 converts 25(OH)D into 1,25-dihydroxyvitamin D, or calcitriol, the active hormone. Parathyroid hormone (PTH) stimulates this step, while FGF23 and calcitriol itself inhibit it.

Calcitriol acts through the vitamin D receptor (VDR), a transcription factor in the steroid hormone nuclear receptor family that partners with the retinoid X receptor to switch genes on or off. The VDR is found in nearly every tissue, which is why vitamin D has been studied for so many conditions beyond bone. The enzyme CYP24A1 breaks down calcitriol and 25(OH)D, preventing them from building up to toxic levels.

The three forms turn over at very different speeds (Jones, AJCN 2008):

Form	Approximate half-life	Notes
Vitamin D3 (whole-body stores)	≈ 2 months	Fat-soluble; stored in adipose tissue
25(OH)D3	≈ 15 days	Main circulating form; usual range 25–200 nmol/L
Calcitriol, 1,25(OH)2D3	≈ 15 h	Active hormone; tightly regulated

The long half-life of 25(OH)D is why blood levels change slowly, over weeks, after a change in intake. It is also why the effects of an overdose can take a long time to clear (see below).

Reference intakes and blood levels

IOM 2011 Dietary Reference Intakes

The IOM committee reviewed both skeletal and non-skeletal outcomes. It concluded that bone health was the only outcome with enough evidence of cause and effect to set intake requirements. For cancer, heart disease, diabetes and autoimmune disease, the evidence was “inconsistent, inconclusive as to causality, and insufficient” (Ross et al., JCEM 2011; IOM report).

Age group	RDA	Upper limit (UL)
0–6 months	400 IU (adequate intake)	1,000 IU
6–12 months	400 IU (adequate intake)	1,500 IU
1–3 years	600 IU	2,500 IU
4–8 years	600 IU	3,000 IU
9–70 years	600 IU	4,000 IU
71 years and older	800 IU	4,000 IU
Pregnancy and lactation	600 IU	4,000 IU

One microgram of vitamin D equals 40 IU, so 600 IU is 15 µg and 4,000 IU is 100 µg.

The IOM’s interpretation of blood 25(OH)D levels:

Serum 25(OH)D	IOM interpretation
16 ng/mL (40 nmol/L)	Covers the needs of about half the population
20 ng/mL (50 nmol/L)	Covers the needs of at least 97.5% of the population
Above 30 ng/mL (75 nmol/L)	Not consistently associated with more benefit
Above 50 ng/mL (125 nmol/L)	Should raise concern about possible adverse effects

The committee also concluded that the prevalence of vitamin D inadequacy in North America had been overestimated.

Endocrine Society 2024 guideline

The Endocrine Society’s 2011 guideline focused on patients at risk of deficiency and recommended testing 25(OH)D in that group (Holick et al., JCEM 2011). Its 2024 guideline addresses something different: vitamin D for disease prevention in people with no established reason for treatment or testing (Demay et al., JCEM 2024). Using GRADE methods and a supporting systematic review of 151 studies (Shah et al., JCEM 2024), the panel:

suggests empiric vitamin D (above the DRI, without testing) for ages 1–18, adults 75 and older, pregnancy, and high-risk prediabetes;
suggests against empiric vitamin D above the DRI for healthy adults under 75;
suggests against routine 25(OH)D testing in all the populations it considered, including people with obesity or dark skin;
suggests daily dosing rather than intermittent high doses for people over 50 who take vitamin D.

The panel found no clear evidence defining a target 25(OH)D level for disease prevention. Its recommendations do not replace the IOM intake values.

What the research shows

Cancer, heart disease and death: VITAL

VITAL randomized 25,871 US men aged 50 and older and women aged 55 and older to vitamin D3 2,000 IU/day or placebo, in a factorial design with fish oil. Participants were not selected for low vitamin D. Median follow-up was 5.3 years (Manson et al., NEJM 2019):

Outcome	Hazard ratio, vitamin D vs placebo (95% CI)
Invasive cancer (primary)	0.96 (0.88–1.06)
Major cardiovascular events (primary)	0.97 (0.85–1.12)
Death from cancer	0.83 (0.67–1.02)
Death from any cause	0.99 (0.87–1.12)
Death from cardiovascular causes	1.11 (0.88–1.40)

Neither primary outcome was reduced. No excess hypercalcemia or other adverse events were found.

Two VITAL ancillary studies added more results:

Fractures: no effect on total fractures, HR 0.98 (0.89–1.08), or hip fractures, HR 1.01 (0.70–1.47). The result did not change with baseline 25(OH)D (LeBoff et al., NEJM 2022).
Autoimmune disease: 123 confirmed cases on vitamin D vs 155 on placebo, HR 0.78 (0.61–0.99), a 22% reduction of borderline significance (p = 0.05) (Hahn et al., BMJ 2022).

Monthly high doses: D-Health

The D-Health trial randomized 21,315 Australians aged 60–84 to 60,000 IU of vitamin D3 or placebo once a month for up to 5 years. During the trial, mean 25(OH)D was 115 nmol/L on vitamin D and 77 nmol/L on placebo (Neale et al., Lancet Diabetes Endocrinol 2022).

Outcome	Result, vitamin D vs placebo (95% CI)
All-cause mortality (primary)	HR 1.04 (0.93–1.18)
Cancer mortality	HR 1.15 (0.96–1.39)
Cardiovascular mortality	HR 0.96 (0.72–1.28)
Major cardiovascular events	HR 0.91 (0.81–1.01)
Myocardial infarction	HR 0.81 (0.67–0.98)
Stroke	HR 0.99 (0.80–1.23)
Any fall in the past month	OR 1.02 (0.95–1.10)

Sources: mortality from Neale 2022; cardiovascular events from Thompson et al., BMJ 2023; falls from Waterhouse et al., J Cachexia Sarcopenia Muscle 2021.

Key points:

Monthly vitamin D3 did not reduce deaths. In an exploratory analysis that excluded the first 2 years, cancer deaths were higher on vitamin D (HR 1.24, 95% CI 1.01–1.54). The authors wrote that this regimen “might not be appropriate” for people who are already vitamin D replete.
Major cardiovascular events were 6.0% vs 6.6%. This is a small difference (number needed to treat 172) whose confidence interval included no effect.
Falls did not change overall. In people with a body mass index under 25, vitamin D was linked to more falls (OR 1.25, 95% CI 1.09–1.43).
Hypercalcemia and kidney stones were no more common than with placebo.

Testosterone

The idea that vitamin D3 raises testosterone rests mostly on one small early trial. Later, larger trials designed to test the question did not confirm it.

Trial	Men	Dose and duration	Testosterone result
Pilz et al., Horm Metab Res 2011	54 overweight men in a weight-loss program, 25(OH)D under 50 nmol/L	3,332 IU/day (83 µg) vs placebo, 1 year	Total T rose from 10.7 to 13.4 nmol/L (≈ 309 to 386 ng/dL) vs baseline; no change on placebo
Heijboer et al., Clin Endocrinol 2015	Three small trials (92, 49 and 43 men), normal baseline T	600–2,000 IU/day, 6–16 weeks	Median change 0–0.5 nmol/L; no effect
Lerchbaum et al., JCEM 2017	98 healthy men, normal T, 25(OH)D under 75 nmol/L	20,000 IU/week vs placebo, 12 weeks	No effect (p = 0.497); insulin sensitivity index (QUICKI) fell slightly
Lerchbaum et al., Eur J Nutr 2019	94 men with low T (under 10.4 nmol/L, ≈ 300 ng/dL)	20,000 IU/week vs placebo, 12 weeks	No effect on total T or other hormones

The Pilz trial was a secondary analysis of a weight-loss study, and only 23 men were in the placebo group. Its testosterone result was a change from baseline within the vitamin D group. The two Graz trials (Lerchbaum 2017 and 2019) were designed with testosterone as the primary outcome, measured it by mass spectrometry, and included men with either normal or low testosterone. Neither found an effect.

Meta-analyses of randomized trials point the same way:

Meta-analysis	Trials	Effect on total testosterone (95% CI)
Hosseini Marnani et al., Andrologia 2019	8 studies	+0.20 nmol/L (−0.20 to 0.60); not significant
Abu-Zaid et al., Diseases 2024	15 studies	+0.38 nmol/L (0.06 to 0.70); high heterogeneity (I² 67%)
Paez-Allendes et al., Nutrients 2026	11 comparisons	+0.47 nmol/L (−0.50 to 1.44); low-certainty evidence

Even the one positive pooled estimate, 0.38 nmol/L, is about 11 ng/dL. The 2024 analysis found no effect on free testosterone, LH, FSH, SHBG or estradiol. The most recent meta-analysis concluded that randomized evidence does not show a clear or reproducible effect on testosterone or its bioavailable fractions.

Muscle strength and mass

A meta-analysis of 30 randomized trials with 5,615 participants (mean age 61) found that vitamin D supplementation had a small effect on muscle strength, a standardized mean difference of 0.17 (Beaudart et al., JCEM 2014). It found no significant effect on muscle mass or muscle power.

The strength benefit was larger in people aged 65 and older and in people starting with 25(OH)D under 30 nmol/L. In younger adults, the pooled effect was close to zero (SMD 0.03, 95% CI −0.08 to 0.14).

Side effects and risks

At intakes up to the UL, vitamin D3 has a good safety record. VITAL (2,000 IU/day) and D-Health (60,000 IU/month) found no excess hypercalcemia. The risks below involve high intermittent doses, long-term intake above the UL, product errors, or specific medical conditions.

Why the upper limit is 4,000 IU/day

The IOM started from 10,000 IU/day, the level below which intakes had not been linked to hypercalcemia or acute toxicity. It then lowered the figure to 4,000 IU/day to allow for uncertainty about long-term outcomes, including all-cause mortality, and concerns about 25(OH)D above 50 ng/mL (125 nmol/L). The IOM notes that the UL is not a target intake (Ross et al., JCEM 2011).

Vitamin D toxicity

Toxicity (hypervitaminosis D) causes high blood calcium. A clinical review lists these as the most common symptoms (Marcinowska-Suchowierska et al., Front Endocrinol 2018):

confusion and apathy;
repeated vomiting and abdominal pain;
heavy urination, thirst and dehydration.

Effects on the kidney can progress from high urinary calcium to calcium deposits in the kidney (nephrocalcinosis) and kidney failure.

Thresholds reported in the literature:

Source	Threshold
IOM 2011	Concern above 50 ng/mL (125 nmol/L)
Jones, AJCN 2008	Toxicity likely needs 25(OH)D above 750 nmol/L (300 ng/mL); a “prudent” upper limit of 250 nmol/L (100 ng/mL)
Marcinowska-Suchowierska 2018	25(OH)D above 150 ng/mL (375 nmol/L) is the hallmark of overdose; acute toxicity probably needs intakes above 10,000 IU/day

High-dose and intermittent dosing

More is not better, and in some trials high doses caused harm:

Annual megadose. In 2,256 women aged 70 and older, a single 500,000 IU dose each autumn or winter increased falls (rate ratio 1.15, 95% CI 1.02–1.30) and fractures (rate ratio 1.26, 95% CI 1.00–1.59) compared with placebo. In a post hoc analysis, the excess in falls was greatest in the first 3 months after each dose (Sanders et al., JAMA 2010).
Daily doses at or above the UL. In the 3-year Calgary trial, 311 healthy adults aged 55–70 took 400, 4,000 or 10,000 IU/day (Burt et al., JAMA 2019). Radial bone density fell more with higher doses: −1.2%, −2.4% and −3.5%. Bone strength did not differ significantly between groups.

The Endocrine Society’s preference for daily over intermittent high-dose vitamin D reflects this. Its supporting review found that high-dose intermittent vitamin D may increase falls compared with lower daily doses (Shah et al., JCEM 2024).

Who is at higher risk

Some conditions make people sensitive to vitamin D, because the body makes calcitriol outside the kidney or cannot break it down normally (Marcinowska-Suchowierska et al., Front Endocrinol 2018):

granulomatous diseases such as sarcoidosis and tuberculosis, where immune cells produce calcitriol;
some lymphomas;
rare inherited disorders such as idiopathic infantile hypercalcemia and Williams-Beuren syndrome.

In these conditions, hypercalcemia can occur even at doses considered safe for the general population. D-Health excluded people reporting hypercalcemia, hyperparathyroidism, kidney stones, osteomalacia or sarcoidosis.

Quality and contamination

Vitamin D3 is mostly sold as a dietary supplement, and errors in content have been documented. In two documented US cases, manufacturing and labeling errors meant patients took more than 1,000 times the recommended daily dose of vitamin D3 (Araki et al., JCEM 2011). Both developed hypercalcemia. It took about a year for their 25(OH)D levels to return to normal, though both recovered without lasting harm. The authors also noted that patients often do not report supplement use to their doctors unless asked repeatedly.

Vitamin K2, another fat-soluble vitamin sold as a supplement, has its own page.

Key takeaways

A prohormone, not a hormone. Vitamin D3 is converted to 25(OH)D in the liver and then to calcitriol in the kidney. 25(OH)D, with a half-life of about 15 days, is the form measured in blood tests.
Intake needs are modest. The IOM RDA is 600–800 IU/day, set for minimal sun exposure, and a 25(OH)D of 20 ng/mL (50 nmol/L) meets the needs of at least 97.5% of people.
Large trials were mostly null. In VITAL (2,000 IU/day) and D-Health (60,000 IU/month), vitamin D3 did not reduce cancer, cardiovascular events, fractures or deaths in older adults who were not selected for deficiency.
No reliable testosterone boost. The 2011 Pilz result was not reproduced by larger trials designed to test it, including in men with low testosterone. Meta-analyses show no clear effect.
More is not better. The UL is 4,000 IU/day. Annual megadoses increased falls and fractures, and 10,000 IU/day for 3 years lowered bone density compared with 400 IU/day.
Guidelines have narrowed. The Endocrine Society’s 2024 guideline suggests against extra vitamin D above the DRI and against routine 25(OH)D testing in healthy adults under 75.

Sources

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Institute of Medicine. Dietary Reference Intakes for Calcium and Vitamin D. Washington, DC: The National Academies Press; 2011. National Academies Press.
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Shah VP, Nayfeh T, Alsawaf Y, et al. A systematic review supporting the Endocrine Society clinical practice guidelines on vitamin D. J Clin Endocrinol Metab. 2024;109(8):1961–1974. PubMed.
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World Anti-Doping Agency. The 2026 Prohibited List. WADA 2026 Prohibited List.

Vitamin D3