How do vitamin B12 injections work, and who can actually benefit from them?

• B12 shots will benefit only those with a B12 deficiency and people with some underlying conditions.
• No research indicates that B12 shots will help with weight loss or benefit healthy individuals.
• B12 shots can have mild side effects like diarrhea and swelling at the injection site.

Vitamin B12 injections are an effective way to treat a deficiency, but they're not for everyone. In fact, if you don't have a B12 deficiency or an underlying health condition that places you at risk of a deficiency, you will see no benefits from high dose B12 shots, says Dr. Jorge Moreno, a Yale Medicine internist and assistant professor of medicine.

Here's what you need to know about vitamin B12 injections, from where and how to get them to potential benefits and side effects.

What is a B12 shot?

B12 is an essential vitamin that plays an important role in making DNA, maintaining nerve health, and producing red blood cells. If you don't get enough vitamin B12, you might experience adverse side effects like numbness, tingling, and canker sores.

B12 shots contain about 100 micrograms of B12, which is roughly 40 times the recommended daily value for an average healthy adult (2.4 micrograms).

But these high doses are necessary to help treat people with a deficiency. Especially for those who cannot absorb all of the B12 they ingest due to an underlying condition like inflammatory bowel disease, says Mary Wirtz, a nutrition consultant for Mom Loves Best.

B12 shots are also highly effective. Depending on the individual, they start working within 48 to 72 hours of the first dose, but they might take three months to completely correct a B12 deficiency.

Important: Vitamin B12 is water soluble, meaning any excess is flushed out through urine, and as a result, there's no real risk of overdose.

A typical dosage regimen starts with higher amounts and gradually tapers off. For example, a doctor might prescribe the following:

• 100 micrograms daily for about a week
• Followed by every other day for seven doses
• After that, every three to four days for two to three weeks
• Once monthly until the deficiency is treated

B12 injections require a prescription and can be administered at a doctor's office or at home with the approval of a healthcare provider. Common injection sites include the thigh, shoulder, and buttocks.

Who could benefit from a B12 shot?

Since most people are able to consume enough vitamin B12 through their diet, only those who have trouble getting or absorbing enough of the nutrient can benefit from the shot.

For example, these shots are often used to prevent or treat pernicious anemia - a rare blood disorder that results from the intestines not absorbing enough B12.

Others who might be prescribed B12 shots are those at the highest risk for a B12 deficiency, including:

• Vegans, since vitamin B12 is found only in animal products
• People ages 60 and up since their bodies don't absorb B12 as efficiently
• People with Crohn's disease or pancreatic disease, since they prevent nutrient absorption
• People who had weight-loss surgery, which decreases B12 absorption
• People with an eating disorder
• People with HIV
• People with diabetes​​

Since a mild B12 deficiency might not cause symptoms, it can go undetected for years since levels are not checked during routine blood work.

Therefore, if you are at high risk of a B12 deficiency, consult with your healthcare provider, who can run a blood test to check your B12 levels.

Treating a B12 deficiency is crucial since a severe deficiency can lead to nerve damage or neurological problems.

Warning: In recent years, vitamin B12 shots have increased in popularity due to claims they help with weight loss and boost energy. But there is no evidence supporting these claims.

Side effects of B12 shots

Vitamin B12 injections are considered safe, but common side effects include diarrhea and swelling. While these typically subside within a few days, you should always notify your doctor just in case, Wirtz says.

In rare cases, some people might have severe side effects like:

• Skin rash, itching, or hives
• Swelling of the face, lips, or tongue
• Chest pain or tightness
• Blue tint to the skin
• Difficulty breathing or wheezing
• A swollen, painful area on the leg
• Dizziness

If you experience any of the above side effects, reach out to a healthcare provider immediately.

Insider's takeaway

Vitamin B12 injections are for those with a severe B12 deficiency or conditions that prevent their body from effectively absorbing the vitamin, Moreno says.

If you don't have a deficiency, you probably won't notice any benefits from a B12 shot, especially since any excess is simply excreted through urine.

If you think you might have a vitamin B deficiency, your doctor can test your blood levels and discuss whether shots might be beneficial.