B12 Injections: Who Should Get Them, How Quickly Do They Work, and How Long Do They Last?

  • Medical Reviewer: Mahammad Juber, MD
Medically Reviewed on 8/15/2022

What is vitamin B12?

Vitamin B12 is also important for DNA synthesis and your brain's and nerve cells' normal functions. Vitamin B12 injections should be given to vegetarians, vegans, and those with deficiencies and you may have to take the injections regularly or life long depending on your health situation.
Vitamin B12 is also important for DNA synthesis and your brain's and nerve cells' normal functions. Vitamin B12 injections should be given to vegetarians, vegans, and those with deficiencies and you may have to take the injections regularly or life long depending on your health situation.

You’ll need to take vitamin B12 injections depending on your situation if your deficiency is not diet-related. You may also be given B12 injections if your body cannot absorb B12 from regular tablet supplements. Your body uses B12 to build healthy blood cells and proteins and to metabolize fats and carbohydrates. 

Vitamin B12 is also important for DNA synthesis and your brain's and nerve cells' normal functions.

Vitamin B12 deficiency is pretty common, and many people take B12 injections. We look at who should take B12 injections, how quickly your body starts using them, and how long the effects last.

Vitamin B12 is a water-soluble vitamin. Although naturally present in many foods, it’s also eaten as a dietary supplement. Molecules that have vitamin B12 are called cobalamins because they contain cobalt.

In most food sources, B12 is bound to a protein molecule released by the metabolic activity of hydrochloric acid and other enzymes in the stomach. After it’s released into your system, B12 combines with another molecule called the intrinsic factor that aids its absorption into the small intestine.

Supplements and foods reinforced with this vitamin contain B12 in its free form, which makes it more  readily available for absorption in your system.

There are several B12 supplements available in the market. But you should check with your doctor before using any of them. Just taking large amounts of B12 supplements does not mean that your body will be able to use all of it.

This is determined by the presence of a molecule called the intrinsic factor that is essential for the absorption of B12 into your system. Severe B12 deficiency is widely reported and leads to pernicious anemia since your body needs B12 to make red blood cells, and a deficiency limits its capabilities to build them.

This condition is caused by a lack of intrinsic factor in your system. If you’re severely B12 deficient, your doctor may recommend B12 injections into a muscle, making the vitamin readily available to your system.

Vitamin B12 deficiency

As per the U.S. Department of Health and Human, the daily recommended intake for vitamin B12 is 2.4 micrograms for adults over the age of 18 years. Adult females who are pregnant are recommended to take 2.6 micrograms every day, while the recommended value for lactating women is 2.8 micrograms.

Diet is one of the most important sources of vitamin B12, and animal-based foods are the primary sources of this vitamin. Fish, meat, poultry, and eggs are all rich in B12, while dairy products also contain large amounts of B12. Some breakfast cereals reinforced with B12 make it readily available for absorption into your body.

The amount of vitamins that become available in your body depends on the source and the capacity of the intrinsic factor central to its absorption.

Studies have shown that the bioavailability of B12 is three times greater in dairy products than in fish, poultry, and meat. On the other hand, the bioavailability of B12 from dietary supplements is 50% more than that from food sources.

Most people who eat non-vegetarian food meet their B12 needs. There may be instances where certain conditions such as poor nutrition, intestinal issues, infections, and cancer may lead to B12 deficiency. Serious B12 deficiency may cause anemia and nerve damage.

There are several B12 supplements available, and some (like B12 tablets) have high dosages far above the daily recommended value. But this does not mean that all the B12 contained in the tablet will be absorbed, as it also depends on how much intrinsic factor is available for their absorption.

B12 deficiency is common in people who follow a vegetarian or vegan diet. The results of a study carried out in 2013 indicated that trend.

The study analyzed deficiency rates among different age groups; the vitamin deficiency in the age group that included the elderly varied between 11% to 90%.

It found that B12 deficiency was higher in vegans than vegetarians and that vegetarians report B12 deficiency irrespective of their demography, where they live, their age, and the type of vegetarian diet they eat.

The study also recommended vegans and vegetarians take B12 supplements to make sure that their body gets enough of this vitamin to support essential bodily functions.

Other factors that could lead to B12 deficiency are:

  • Lack of adequate production of intrinsic factor needed for B12 absorption
  • B12 deficiency is common in elderly people as absorption of the vitamin reduces with age
  • People who have had any intestinal surgery such as that for weight loss
  • Those with gut conditions such as Crohn’s Disease, inflammatory bowel disease, or celiac disease


Next to red peppers, you can get the most vitamin C from ________________. See Answer

Vitamin B12 injections

Vitamin B12 supplements are given based on the condition that causes deficiency. While supplements are usually given as tablets, your doctor may recommend B12 injections, depending on your situation.

If your B12 deficiency is due to inadequate vitamins in your diet, you may be prescribed B12 tablets to be taken in between your meals. If your diet does not provide B12 vitamins at all – typical of those on a vegetarian or vegan diet – you may need to take B12 tablets for the rest of your life. In some cases, you may be asked to take injections twice a year.

Once your B12 levels are back to normal, or if a change in your diet gives your body all the B12 it needs, you may be asked to stop taking these tablets. But this does not happen often.

If your B12 deficiency is not diet-related, you’ll need to take B12 injections. Two types of vitamin B12 injections are usually given – hydroxocobalamin and cyanocobalamin.

If you’re taking hydroxocobalamin, keep the following things in mind:

  • Although this injection takes effect right away, it may take some days or even a few weeks for you to get back to health.
  • Typically, to begin with, you’ll be asked to take the injection multiple times a week to increase the B12 levels.
  • In some cases, doctors may recommend hydroxocobalamin injections for the rest of the person’s life.

On the other hand, if you’re taking cyanocobalamin, know that:

  • Cyanocobalamin injections are generally given every day during the first week of your treatment.
  • Once your red blood cells return to normal, you’ll typically be given further injections on alternate days for the next two weeks.
  • This is usually followed by injections every three or four days for the next two to three weeks.
  • Cyanocobalamin injections are mostly given once a month once your B12 deficiency anemia has been treated to prevent a recurrence of symptoms that could lead to nerve damage.

In most cases, you’ll need to take cyanocobalamin injections once a month for the rest of your life, although this may vary. Your doctor will make the call based on your situation and how you respond to the initial treatment.

Side effects of B12 injections

Hydroxocobalamin may not suit everyone. That’s why it’s important to let your doctor know if you have any of the following conditions before you’re prescribed hydroxocobalamin:

  • If you’ve had an allergic reaction to hydroxocobalamin or any other medicines in the past
  • If you’ve been diagnosed with low levels of potassium
  • If you have an irregular or fast heartbeat

Once you’ve started taking hydroxocobalamin, you may have some side effects such as pain, swelling, or itchy skin at the point where the syringe was injected; nausea or vomiting; diarrhea; headaches or dizziness; and hot flashes.

Although extremely rare, hydroxocobalamin injections can also cause serious side effects such as a major allergic reaction or irregular heartbeats (palpitations). Check with your doctor immediately if you have any of these side effects.

  • If you’re prescribed cyanocobalamin injections, bring the following points to your doctor’s attention.
  • If you’re allergic to cobalt
  • If you’re taking antibiotics such as chloramphenicol, colchicine, folic acid, methotrexate, and para-aminosalicylic acid, as your doctor may have to monitor your condition for any possible side effects
  • If you drink or have ever drunk large quantities of alcohol
  • If you’ve ever been diagnosed with Leber’s hereditary optic neuropathy or any kidney condition
  • If you’re pregnant, are planning a pregnancy, or are breast-feeding

Although not common, cyanocobalamin injections can also cause side effects such as diarrhea or a sensation that your entire body is swollen. You should also inform your doctor right away in case you encounter any of the following serious side effects.

Who should take B12 injections?

Vitamin B12 deficiency can cause several symptoms. Pernicious anemia is a serious condition where your red blood cell count becomes dangerously low. You will need to boost your B12 supply to address this situation.

Other common symptoms of B12 deficiency include:

Your doctor may prescribe B12 injections in extreme cases that take effect right away. But you may need to continue taking supplements for the rest of your life, depending on your situation.

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Medically Reviewed on 8/15/2022

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