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 recommended dietary allowance of 600 IU and a tolerable upper intake level of 4,000 IU per day. These are different reference values. The recommended dietary allowance is an intake target. The upper intake level is a safety boundary.
VITAL randomized more than 25,000 adults to 2,000 IU of vitamin D3 daily or placebo for about five years. That trial documents the dose studied. It does not establish that 2,000 IU is the right dose for every person or guarantee an outcome. 2
The 2024 Endocrine Society guideline does not recommend routine vitamin D testing for generally healthy adults, and it does not establish that one supplemental dose is right for everyone. Testing, symptoms, abnormal laboratory results, pregnancy, medications, and clinician-directed plans belong with a clinician who knows the full history. 13, 14
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.
Vitamin B12 FAQ
What does vitamin B12 do in normal metabolism?
Vitamin B12 is required for normal red blood cell formation and DNA synthesis. It also helps two enzymes do their normal work in methionine and propionate metabolism. Those normal roles do not make B12 a stimulant, and they do not support a promise of extra energy from taking more.
Source: NIH Office of Dietary Supplements, Vitamin B12 Fact Sheet for Health Professionals.
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.
Who is more likely to have low vitamin B-12?
Vitamin B-12 status depends on more than food intake. NIH identifies higher risk among older adults, people who eat little or no animal food without reliable fortified foods or supplements, people whose gastrointestinal history affects absorption, and people who use metformin or acid-suppressing medications for long periods. A risk factor does not diagnose a deficiency. Symptoms, abnormal laboratory results, pregnancy, surgery, or medication questions belong with a clinician who knows the full history.
Sources: NIH Office of Dietary Supplements, Vitamin B12 Fact Sheet for Health Professionals; Langan RC, Goodbred AJ. Vitamin B12 Deficiency: Recognition and Management. PMID 28925645.
This is general education, not medical advice. 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.
Vitamin D metabolism
What happens to vitamin D3 after you take it?
The vitamin D3 in a supplement is cholecalciferol. The liver changes it into 25(OH)D, the main form measured in common vitamin D blood tests. Further conversion produces 1,25-dihydroxyvitamin D, the hormonally active form that binds the vitamin D receptor and influences gene transcription.
Most vitamin D in circulation travels bound to vitamin D binding protein. Transport, conversion, receptor binding, and breakdown are separate steps. A higher 25(OH)D result does not by itself prove a better health outcome. These pathways also do not support the idea that more intake produces a proportional increase in biological effect.
Sources: PMID 32911795, PMID 28257826, and PMID 24095930.
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
- Demay MB, et al. Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2024. PMID 38828931
- Shah VP, et al. A Systematic Review Supporting the Endocrine Society Clinical Practice Guidelines on Vitamin D. J Clin Endocrinol Metab. 2024. PMID 38828942
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.
What does ‘free form’ mean for vitamin B-12?
B-12 in food is attached to protein and must be released during digestion. B-12 in supplements and fortified foods is already in free form, so it enters the later transport pathway without that first separation step. It still relies on the body’s regulated absorption process.
Source: NIH Office of Dietary Supplements, Vitamin B12 Fact Sheet for Health Professionals.
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.