Although some people claim vitamin B12 shots can help with weight loss, experts do not recommend them. They can cause side effects and, in some cases, allergic reactions.

According to a 2019 study, vitamin B12 levels are lower in people with obesity than in those at a moderate weight. However, it did not prove that the vitamin can help a person lose weight.

While B12 shots are necessary for some people who cannot absorb the vitamin in other ways, they have some risks and side effects. Some of the risks can be serious, such as fluid buildup in the lungs or a blood clot.

Learn about what B12 shots are, how they work, risks and side effects, and more.

A person drawing liquid into a syringe.Share on Pinterest
SimpleImages/Getty Images

B12 is a water-soluble vitamin present in some foods. It is available as an oral dietary supplement in tablet form, or doctors can also prescribe it as a shot. B12 supplements may be necessary for some people as the body cannot make it.

B12 is important for the following:

  • development and functioning of the central nervous system
  • formation of red blood cells
  • synthesis of DNA, the genetic material in the cells
  • functioning of two enzymes responsible for an array of chemical reactions

Compounds containing B12 are also called cobalamins. Two common forms include cyanocobalamin and hydroxocobalamin.

Doctors typically use B12 shots to treat vitamin B12 deficiency. One cause of B12 deficiency includes pernicious anemia, a condition that results in a reduction of red blood cells when the intestines cannot absorb enough vitamin B12.

A healthcare professional delivers the shot into a muscle, bypassing the intestinal tract. This way, the body can receive what it needs.

Research from 2019 notes an inverse relationship between obesity and low vitamin B12 levels. This means that people with obesity tend to have lower levels than those with a moderate weight.

However, the research authors emphasize that this does not indicate the shots can help a person lose weight, as there is no proof that a causal relationship exists. They could not determine whether obesity lowers B12 levels or if low B12 levels predispose someone to obesity.

In explaining such research findings, Pernicious Anemia Relief (PAR) notes that obesity may stem from the habits of individuals who have a B12 deficiency or their co-occurring conditions. Conversely, B12 deficiency may have an effect on metabolism that plays a role in obesity.

PAR recommends that the only people who should take B12 shots are those with B12 deficiency who cannot absorb the vitamin when they take it orally.

In rare situations, B12 shots can cause allergic reactions, including:

  • itching
  • rash
  • anaphylaxis, a life threatening condition that can cause a fast heartbeat and difficulty breathing

Other side effects that can occur include:

  • shortness of breath, fast weight gain, and swelling
  • congestive heart failure, the inability of the heart to adequately pump blood
  • pulmonary edema, the buildup of fluid in the lungs
  • peripheral vascular thrombosis, the obstruction of a vein with a blood clot
  • hypokalemia, low potassium levels that can cause irregular heartbeats and muscle weakness
  • fever
  • tingling, numbness, and joint pain
  • swollen tongue
  • polycythemia, an increase in the red blood cell mass

Additionally, B12 may interact with certain medications.

B12 shots are not necessary for weight loss. For most people, a balanced diet provides the necessary nutrients for wellness, including vitamin B12.

Some food sources of the vitamin include:

However, a person with B12 deficiency may not be able to absorb enough of the vitamin from their diet. When this occurs, they may need supplements or B12 shots.

People with obesity or those who are concerned about their weight can contact a doctor. They can provide recommendations on how to achieve a moderate weight in a healthy, sustainable manner.

Additionally, individuals interested in B12 should consult their doctor before taking an oral supplement. If they think they may have a B12 deficiency, a blood test can determine this.

Learn more about ways to lose weight.

Experts do not recommend B12 shots for weight loss. Some studies indicate people with obesity have low vitamin B12 levels. However, researchers do not know if the effects of obesity can cause a reduction in B12 levels or if reduced B12 levels may be a contributing factor to obesity.

More studies are necessary to determine what factors may underlie the link.

B12 shots can cause side effects, some of which are serious. Most individuals who follow a balanced diet get enough B12, but doctors may prescribe the shots for those who cannot absorb it.