Vitamin B12 Injections

Liver therapy and the 1934 Nobel Prize

A review in the Journal of the Royal Society of Medicine traces treatment of pernicious anemia to experiments by George Whipple on anemia in dogs. George Minot and William Murphy then described results in 45 patients given a liver-rich diet at a meeting in Boston on 4 May 1926 and published them in the Journal of the American Medical Association that year. The review says the finding was soon confirmed by physicians worldwide.

Whipple, Minot and Murphy shared the 1934 Nobel Prize in Physiology or Medicine, which the Nobel Foundation describes as awarded for discoveries concerning liver therapy in cases of anemia.

Liver extracts, intrinsic factor and vitamin B12

Because a diet of raw liver was hard to take, liver extracts were developed for intramuscular injection, and the review says this became part of standard management of pernicious anemia until the 1950s. The factor was isolated in 1948 and named vitamin B12, and Dorothy Hodgkin and colleagues later worked out its structure by X-ray crystallography.

The same review states that William Castle named a gastric component of absorption intrinsic factor in 1930. Pernicious anemia is described by the NIH Office of Dietary Supplements as an irreversible autoimmune disease of the stomach lining, which is why absorption from food, not only intake, can be the problem.

Cobalamin and its forms

Compounds with vitamin B12 activity are collectively called cobalamins because they contain cobalt. The NIH Office of Dietary Supplements lists hydroxocobalamin and cyanocobalamin as forms that become biologically active after conversion, and says these two are the forms given by injection as a prescription medication, usually into a muscle.

Injection, not drip

This page belongs to the family of pages on IV Therapy, where B12 appears as the intramuscular option. An intramuscular B12 injection is a single shot into a muscle. It is not an intravenous drip and does not involve an infusion into a vein. A BMJ clinical review notes that intravenous hydroxocobalamin is used in a different setting, cyanide poisoning.

Names such as B12 shots, energy shots or lipotropic injections are commercial or informal. The sources reviewed do not define them, and this page uses vitamin B12 injections for any injected cobalamin.

Established medical treatment of deficiency

The NHS describes vitamin B12 supplements for deficiency anemia as usually given by injection at first, followed by tablets or regular injections depending on whether the deficiency is related to diet. For deficiency not caused by diet, it says injections of hydroxocobalamin are usually needed, and that hydroxocobalamin is preferred to cyanocobalamin in the UK because it stays in the body longer.

The NIH says injections bypass barriers to absorption and are typically used for deficiency from pernicious anemia and other malabsorption conditions. A BMJ clinical review describes lifelong intramuscular injections as the default when absorption is impaired. Dosing schedules are set by clinicians, and this page does not give them.

Oral treatment as an alternative

A 2018 Cochrane review found that oral and intramuscular B12 had similar effects on serum B12 levels in the small number of trials available. The British Society for Haematology guideline says oral therapy may be suitable provided appropriate doses are taken and adherence is not an issue. Prescribing data for English general practice show that tablets doubled to about 2 million prescriptions a year between 2019 and 2024 while injections returned to pre-pandemic levels.

Elective shots and who gives them

Injections for energy or wellness in people without deficiency are described in the sources only as a promotion. The NIH says B12 is often promoted as an energy enhancer. The clinical guidance reviewed covers treatment of diagnosed deficiency delivered by physicians and nurses. It does not describe clinic or weight-loss injection offers, and no source reviewed defines who may give them.

Uses within medicine

In clinical guidance the uses are treatment of B12 deficiency, including deficiency from pernicious anemia, gut surgery or malabsorption, and, in a BMJ review, nitrous oxide toxicity. Reported symptoms of deficiency on the NHS page include extreme tiredness, pins and needles and memory problems, and it warns that some nervous system problems can be irreversible if diagnosis is delayed.

Elective energy and wellness claims

Outside deficiency, promoters present B12 shots as a way to raise energy, improve mood or support weight loss. The NIH notes that, because of its role in energy metabolism, B12 is often promoted as an energy enhancer and performance booster. The sources reviewed do not endorse these claims, and a role in energy metabolism is not evidence that an extra injection changes how a person feels.

Deficiency treatment

A BMJ clinical review states that injections restore normal metabolism, alleviate symptoms and reverse hematologic and some neurological complications of deficiency, with anemia usually resolving within weeks and neurological symptoms taking months or years. It adds that little robust evidence supports current recommended dosing schedules.

The Cochrane review of oral versus intramuscular B12 included three randomized trials with 153 participants and 3 to 4 months of follow-up. It rated the evidence for similar serum B12 levels as low quality, found no trial reporting symptoms or quality of life, and could not run meta-analyses. The BMJ review says the review's evidence that oral B12 is as efficacious is of very low quality, and that serum levels are not the same as symptom relief.

People without deficiency

The NIH states that B12 supplementation appears to have no beneficial effect on athletic performance or endurance in the absence of a nutritional deficit. A 2021 systematic review of 16 randomized trials with 6,276 participants without overt deficiency found no evidence of an effect on cognitive function and no overall effect on depression. Only one trial reported on idiopathic fatigue, so no analysis was possible. Most of these trials tested B12 or B-vitamin supplementation, not specifically injections.

Searches of Europe PMC for trials of B12 injections for weight loss or energy in non-deficient adults returned no relevant trials. This is a limit of the searches run here and not a finding that none exist.

Reported harms

The British Society for Haematology guideline says hydroxocobalamin is generally well tolerated, with side effects including itching, rash, chills, fever, hot flushes, nausea and dizziness and, exceptionally, anaphylaxis. It reports acneiform eruptions rarely. A BMJ review lists acne or rosacea with unknown frequency and notes that some people improved after switching to a preparation without the preservative benzyl alcohol.

A 2025 review says hypersensitivity to cobalamin is rare but more likely after intramuscular or subcutaneous administration than oral, and a case report describes an acneiform eruption beginning weeks after starting injections for pernicious anemia. The guideline adds that transient hypokalemia can follow treatment of severe anemia, with clinical significance unknown, and the BMJ review calls it usually mild.

Contraindications and interactions

The BMJ review says preparations containing benzyl alcohol are contraindicated for children and pregnant women, cyanocobalamin is contraindicated in Leber hereditary optic neuropathy, and cyanocobalamin but not hydroxocobalamin is contraindicated in renal insufficiency. Subcutaneous administration is sometimes recommended when intramuscular injection is contraindicated, such as with anticoagulants.

The NIH says some medicines, including metformin and acid-reducing drugs, can lower B12 status. The NHS notes that folic acid treatment can mask an underlying B12 deficiency, which is why B12 is checked first.

Care boundaries and oversight

Tiredness has many causes, and the NHS advises seeing a GP if B12 deficiency is suspected, as diagnosis is by blood test and symptoms. Elective shots should not delay diagnosis or necessary care. The British Society for Haematology guideline is the professional-body statement used here and addresses diagnosed deficiency only. No professional-body statement on elective B12 shots for people without deficiency was found in the searches run.

How this page relates to others

Related pages include IV Therapy and Myers' Cocktail, which cover vitamin infusions given into a vein, and Nutritional Therapy and Dietetics, which cover food-based approaches. The NHS notes that a vegan diet without supplements or fortified foods can lead to deficiency, which is covered on Vegan Diet. Peptide Therapy covers other injected products. This page does not describe those practices beyond noting the differences in route.

Cross-condition caution

The evidence for injections in diagnosed deficiency applies to deficiency and should not be carried to elective use. The reverse also holds: the null findings in non-deficient people do not call into question treatment of confirmed deficiency, where guidelines describe injections as an established option alongside oral treatment.