For decades, patients told they were low on vitamin B12 have walked out of the doctor's office with a shot in the arm. Wellness clinics now sell the same jab as an energy booster.
A systematic review and meta-analysis published in Frontiers in Pharmacology in December 2025 pooled data from 16 studies with 6,098 participants. It found no statistically significant difference between oral, sublingual and intramuscular B12 in raising blood levels of the vitamin.
The pooled increase in serum cobalamin was 402.6 pg/mL across all routes. Homocysteine, a marker that climbs when B12 is low, dropped by about 4.83 μmol/L. In head-to-head comparisons, the differences between routes disappeared.
Vitamin B12 is a water-soluble nutrient needed for red blood cell formation, nerve function and DNA synthesis. It is found naturally in fish, meat, poultry, eggs and dairy, and added to many breakfast cereals. Adults need 2.4 micrograms a day, according to the National Institutes of Health.
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Where the injection habit came from
B12 absorption depends on intrinsic factor, a protein secreted in the stomach that carries the vitamin into the bloodstream. In people who make little or none of it, only about 1 percent of an oral dose is absorbed passively.
That 1 percent turns out to matter. Because a small fraction of a very large pill is still a meaningful amount, the NIH fact sheet on B12 for health professionals states that a high-dose oral regimen "might be as effective as intramuscular administration."
The same fact sheet notes that evidence "suggests no difference in efficacy between oral and sublingual forms," despite marketing claims of superior bioavailability for lozenges and sprays.
Who still needs the shot
Injections have not been made obsolete. They remain the standard first-line treatment for pernicious anemia, an autoimmune condition that destroys the stomach cells that make intrinsic factor.
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Shots are also standard after gastrectomy, the surgical removal of all or part of the stomach, or in severe malabsorption from advanced Crohn's disease. The meta-analysis authors flagged that oral B12 is unsuitable for patients with certain gastric disorders, and that sublingual forms may be preferred where intrinsic-factor-mediated absorption is impaired.
The NIH also notes that older adults are more likely to have atrophic gastritis, a condition that reduces the stomach acid needed to release B12 from food. This group can usually absorb the synthetic B12 in fortified foods and supplements, but sometimes needs doses above the RDA.
What this says about the wellness B12 boom
B12 shots and IV drips are frequently sold as fatigue fighters or performance enhancers to people whose blood levels are already normal. The NIH fact sheet is blunt about that claim. Vitamin B12 supplementation appears to have no beneficial effect on performance in the absence of a nutritional deficit.
Dietary B12 deficiency is more common in strict vegetarians and vegans, in adults who avoid animal foods, and in some long-term users of metformin or proton pump inhibitors like omeprazole. For most of these people, an oral or sublingual supplement is often enough. Doses of 500 to 2,000 mcg are widely used, though only about 10 mcg of a 500 mcg tablet is actually absorbed in a healthy person.
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Anyone with symptoms like fatigue, tingling in the hands or feet, memory problems or a sore tongue that could point to B12 deficiency should speak with a doctor before self-treating. Blood tests can confirm whether the vitamin is truly low, and the right form and dose depend on the underlying cause.
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This article is made and published by Jesper Bengtson, who may have used AI in the preparation.