The evidence
Backed by the research, not the hype.
Every On Call formula starts in the clinical literature. Here is the evidence behind the two doses we chose, and the reasoning our physicians used to choose them.
One study is a headline. The evidence is a pattern.
We don't formulate around a single sponsored trial. Every dose we ship has to hold up across the weight of the published research, reviewed by board-certified physicians in internal medicine and neurology.
01 / Vitamin D3
2,000 IU, in the middle of the window.
The NIH lists an adult RDA of 600 IU and a tolerable upper intake level of 4,000 IU per day. Those numbers answer different questions: the RDA is an intake reference, while the upper limit is a safety boundary.
The NIH puts the adult RDA at 600 IU and the tolerable upper limit at 4,000 IU per day. We chose 2,000 IU, the same daily dose used in VITAL, the largest randomized vitamin D trial ever run, where more than 25,000 adults took it for over five years.2 It is a meaningful amount that stays comfortably inside the safe range.
The vitamin D3 window
We also use D3, not D2. Head to head, cholecalciferol (D3) raises and holds blood levels of vitamin D more reliably than ergocalciferol (D2).1 On Call D3 uses vegan vitamin D3 from algae.
From source to your bloodstream
02 / Vitamin B12
Why can a B-12 label be much higher than the RDA?
The adult RDA for vitamin B-12 is 2.4 mcg. An RDA and the amount printed on a supplement label are not the same kind of number. The body absorbs oral B-12 in more than one way. Smaller amounts use a pathway that depends on intrinsic factor, a protein made in the stomach. Passive diffusion contributes at larger oral amounts.
A 2025 model of B-12 absorption reproduced that pattern across oral doses. The researchers used pregnancy as an example application. The absorption model helps explain why the amount on a B-12 label can sit well above the RDA. It does not establish that 500 mcg is universally optimal, and it does not predict what any one person will absorb.
Source: PMID 40750040.
Does methylcobalamin automatically mean better?
No form name settles that question by itself. After absorption, the body processes cobalamin before it can help two enzymes, methionine synthase and methylmalonyl-CoA mutase, do their normal work. That pathway does not prove that one supplement form produces a better health outcome.
A separate review concluded that methylcobalamin and adenosylcobalamin are unlikely to be inherently superior to cyanocobalamin. That review is framed around deficiency treatment, so On Call Gummies does not use it to claim a product benefit. The narrower point is the useful one: the word methyl on a label is not a clinical outcome.
Sources: PMID 30693532 and PMID 25820384.
Why is the old 500 mcg chart gone?
The earlier chart adapted a dose-finding trial in 120 older adults with mild vitamin B-12 deficiency. The observed marker response was the same at 500 and 1,000 mcg, but the trial's primary model estimated 647 to 1,032 mcg for 80 to 90 percent of the maximal marker change. That evidence does not support calling 500 mcg a universal plateau. The chart was removed rather than asking one study to say more than it measured.
Source: PMID 15911731.
These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.
Evidence notes
What do D3 and D2 studies actually measure?
D3 and D2 are both forms of vitamin D, but equal amounts do not always produce the same change in the blood marker used to assess vitamin D status. In a 12-week randomized trial of 335 healthy women, 15 mcg per day of D3 increased total serum 25(OH)D more than the same amount of D2. A smaller crossover study in 12 healthy men also found that equal microgram doses of D2 and D3 did not have equal effects on serum vitamin D status.
That is a finding about a laboratory marker. It does not show that D3 prevents or treats a disease, and it does not turn a higher number into a guaranteed health outcome.
Sources: PMID 28679555 and PMID 29295513.
Does vitamin D3 need to be taken with food?
The careful answer is that food and formulation can both matter. In one randomized absorption study of 50 healthy older adults, a single 50,000 IU D3 dose produced a 32 percent higher peak plasma D3 level when taken with a fat-containing meal rather than a fat-free meal. In a separate crossover study of 88 subjects, an oily D3 formulation produced no significant fed-versus-fasting difference.
Those studies used different formulations and very high single doses. Together, they do not support a blanket rule that every vitamin D product must be taken with fat. They support the narrower conclusion that absorption depends partly on how a study dose is formulated and administered.
Sources: PMID 25441954 and PMID 27213447.
These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.
How we work
How we choose a dose.
Start with the clinical literature, never a trend.
Find the amount the body can actually use.
Confirm it stays below the safe upper limit.
Formulate, then third-party test every batch.
References
- Tripkovic L, et al. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status. Am J Clin Nutr. 2012;95(6):1357-1364. PMID 22552031
- Manson JE, et al. Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease (VITAL). N Engl J Med. 2019;380(1):33-44. PMID 30415629
- Eussen SJ, et al. Oral cyanocobalamin supplementation in older people with vitamin B12 deficiency: a dose-finding trial. Arch Intern Med. 2005;165(10):1167-1172. PMID 15911731
- Vidal-Alaball J, et al. Oral vitamin B12 versus intramuscular vitamin B12 for vitamin B12 deficiency. Cochrane Database Syst Rev. 2005;(3):CD004655. Cochrane CD004655
- Tripkovic L, Wilson LR, Hart K, et al. Daily supplementation with vitamin D2 compared with vitamin D3 in healthy women. Am J Clin Nutr. 2017. PMID 28679555
- Jakobsen J, Maribo H, Bysted A, Sommer HM, Hels O. Vitamin D vitamers affect vitamin D status differently in young healthy males. Nutrients. 2017. PMID 29295513
- Dawson-Hughes B, Harris SS, Lichtenstein AH, et al. Dietary fat increases vitamin D3 absorption. J Acad Nutr Diet. 2015. PMID 25441954
- Cavalier E, Jandrain B, Coffiner M, et al. Influence of food on serum vitamin D3 after supplementation. Nutrients. 2016. PMID 27213447
- National Institutes of Health, Office of Dietary Supplements. Vitamin D and Vitamin B12 Health Professional Fact Sheets. ods.od.nih.gov
- Green R, et al. Physiologically Based Pharmacokinetic Modeling of Vitamin B-12 Incorporating Mechanistic Absorption: An Example Application for Intake Estimation During Pregnancy. J Nutr. 2025. PMID 40750040
- Froese DS, Fowler B, Baumgartner MR. Vitamin B12, folate, and the methionine remethylation cycle: biochemistry, pathways, and regulation. J Inherit Metab Dis. 2019. PMID 30693532
- Obeid R, Fedosov SN, Nexo E. Cobalamin coenzyme forms are not likely to be superior to cyano- and hydroxyl-cobalamin in prevention or treatment of cobalamin deficiency. Mol Nutr Food Res. 2015. PMID 25820384
Read it, then try it.
Shop the essentialsThese statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. The studies referenced inform our formulation choices and do not constitute a claim about any specific health outcome.