Your doctor will give you the right dosage of vitamin B12 supplement, especially if you will inject it. Taking too much can cause side effects like nausea, headache, dizziness, and, in some cases allergic reactions.

Everyone needs vitamin B12, and most people get enough through their diet. However, it’s important to know what side effects occur when you take too much.

Vitamin B12 is water-soluble and absorbed in the intestines. After it’s absorbed, it’s used to make DNA and red blood cells. Vitamin B12 that isn’t being used is stored in the liver. But if you take supplements, you might ingest more than your body needs.

Oral vitamin B12 is generally safe to take at recommended doses. If you require injectable vitamin B12, your doctor will advise on the dosage.

Injectable vitamin B12, which can treat significant deficiencies, may cause the following common side effects:

More serious side effects of vitamin B12 injections can include:

Vitamin B12 can cause very rare but serious allergic reactions, or anaphylaxis. If this occurs after you take vitamin B12, call 911 or your local emergency services immediately.

Vitamin B12 injections can contain cobalt, which can also cause an allergic reaction.

Don’t take vitamin B12 supplements if you have sensitivities or allergies to vitamin B12, cobalt, and any other ingredients in the medication.

Anaphylaxis is a severe reaction to a trigger, such as an allergy, and is potentially life threatening.

Symptoms can develop quickly and include:

  • feeling faint or lightheaded
  • difficulty breathing or wheezing
  • rapid heartbeat
  • confusion
  • anxiety
  • clammy skin
  • collapsing or losing consciousness
  • hives
  • swelling
  • stomach pain, nausea, or vomiting

If you or someone nearby develops these symptoms, you should:

  1. Check to see whether they have an epinephrine pen. If they do, read and follow the instructions to dispense the medication.
  2. Dial 911 (or a local emergency number).
  3. Lay them down. If they have vomited, lay them on their side.
  4. Stay with them until emergency services arrive.

It is possible for someone to need more than one injection with an epinephrine pen. If symptoms do not begin to clear after 5 minutes, give a second injection if one is available.

If you notice any adverse effects after you start taking vitamin B12 supplements, you should discontinue use immediately. Seek medical attention if your symptoms get worse or are severe.

Your doctor may recommend changing the dosage. They can also recommend treatments for specific side effects where necessary.

You can also consult your doctor to establish an appropriate dose if it’s determined that you don’t get enough B12 from food sources.

According to the National Institutes of HealthTrusted Source, the recommended dietary amounts (RDAs) are as follows:

  • 2.4 micrograms (mcg) daily for ages 14 years or older
  • 2.6 mcg daily for pregnant people
  • 2.8 mcg daily for people who are breastfeeding or nursing

It may be possible to get enough vitamin B12 from foods. However, doctors may recommend taking B12 supplements after the age of 50 years as your ability to absorb B12 decreases with age.

Examples of foods with vitamin B12 include:

  • eggs
  • cheese
  • fish
  • shellfish
  • liver
  • kidney
  • red meat

Learn more about daily vitamin B12 intake.

Common side effects of vitamin B12 injections include mild diarrhea, itching, nausea and vomiting, dizziness, headaches, and hot flashes.

In rarer cases, vitamin B12 can cause more serious side effects such as an irregular heartbeat or heart palpitations, hives, and anaphylaxis.

If you develop any side effects from taking vitamin B12, it’s important to contact your doctor.

Seek immediate medical help if you develop symptoms of a severe allergic reaction, such as breathing difficulties, confusion, anxiety, clammy skin, and fainting.