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B12 Side Effects


Vitamin B12 or known as cyanocobalamin or cobalamin is a type of B vitamins, which is water soluble. Vitamin B12 is an important nutrient used in the normal functioning of the nervous system especially the brain as well as the normal formation of the red blood cells. Vitamin B12 is specifically used in the DNA synthesis and regulation used in the metabolism of every cells in the body. Vitamin B12 also plays a role in the synthesis of fatty acid as well as energy production. Vitamin B12 is prepared into tablets, injections and gel.

Uses and Advantages of Vitamin B12

Vitamin B12 is used for the treatment of conditions that affect the above functions of vitamin B12. Indications of vitamin B12 supplementation include:

  • Pernicious anemia

Pernicious anemia is caused by a disruption in the intrinsic factor in the stomach; which is responsible for vitamin B12 absorption. Without the intrinsic factor, it leads to vitaminB12 deficiency causing pernicious anemia. Intramuscular injections of vitamin B12 are thereby needed.

  • Prevention of Vitamin B12 deficiency
  • Megaloblastic anemia because it may perform the role of folic acid
  • Hereditary deficiency of transcobalamin II
  • Memory loss
  • Alzheimer’s disease
  • Mood boosting
  • Cyanide poisoning
  • Improve immunes system in cases of allergic reactions
  • Hyperhomocysteinemia
  • Age-related macular degeneration
  • Sleep disorders

Aside from this accepted use of vitamin B 12, off label use also include treatment various types of diseases such as Lyme disease and many others. The vast uses of vitamin B12 make it a good nutrient since vitamin B12 is responsible for the most basic function in the body, which are DNA syntheses. It is safe for use in pregnant women. One of its advantages is also having a low toxicity level so even intake of large doses may not result in harmful effects in the body.

B12 Side effects

The use of vitamin B12 is rendered safe as long as the right amount of the vitamin is taken in. B12 side effects may be related with its function in the body and B12 side effects tend to be uncommon. B12 side effects may be related to its interaction with other medications and diseases, but the sole use of B12 does not result in serious B12 side effects. B12 side effects are also associated with the non-oral administration of B12. Oral B12 has not been associated with any adverse effects. B12 side effects include:

1. Dermatologic B12 side effects

Itching and Exanthema

This B12 side effect is related to the use of parenteral and intranasal gel of vitamin B12. This is related to the possible allergic-like reactions to vitamin B12.

Folliculitis and Inflammatory acne

These B12 side effects are rare and may only include exacerbation of the above conditions and never a cause for its occurrence. Caution is warranted for people having these conditions. The exact mechanism for its occurrence is unknown, but these B12 side effects have been seen in patients taking high doses of B12 of up to 10 mg per week. This occurs on the face during the first and second B12 injections. These B12 side effects usually disappear after 8 to 10 days of stopping the therapy.

Acne Rosacea

This is the eruption of tiny, reddish lesions on the face associated with the ingestion of 100 mcg of B12 every day.  The dermatologic B12 side-effects have been associated with the presence of iodine and sorbitol in ampoules of parenteral B12.

2. Gastrointestinal B12 side effects


Dyspepsia has also been reported with the use of parenteral cyanocobalamin. Dyspepsia is the slowing down of the gastric emptying time resulting in bloating and abdominal pain.

Nausea and Vomiting

These B12 side-effects are also related to the use of parenteral cyanocobalamin.

Transient diarrhea

Transient diarrhea may be associated with parenteral injections of cyanocobalamin.


Glossitis or the inflammation of the tongue was also seen in patients using intranasal gel form of vitamin B12

3. Hematologic and Cardiovascular B12 side effects

Vascular thrombosis

Presence of blood clots in the blood vessels was reported in some patients with risk factors of blood clotting such as use of oral contraceptives and sedentary lifestyle.

Congestive Heart Failure

The use of parenteral cyanocobalamin has also resulted to rare cases of congestive heart failure.


Hypokalemia was also reported in patients with megaloblastic anemia so care is warranted when using B12 in this type of anemia.

4. Nervous system B12 side effects


Although vitamin B is responsible for the improvement in the nerve impulse transmission, some cases of paresthesia were reported with the use of parenteral and intranasal cyanocobalamin.


There was also reported anxiety, but the exact mechanism is unknown as to why some patients taking B12 have experienced such.

Abnormal gait and problems in coordination

Some patients also experienced these cerebellar B12 side effects although the exact cause is unknown.


Hypoesthesia is the presence of reduced sensation to pain. This may be related to the affectation of the pain impulse transmission in the brain.


Headache is also a common B12 side effect as a result of affectation of the nerve impulse transmission in the brain.

5. Musculoskeletal B12 side effects

Joint and muscle pains

Intake of parenteral vitamin B12 has also resulted in joint and muscle pains. These B12 side effects may be due to the function of B12 in the metabolism and production of energy although the exact mechanism is unknown.

Back ache

Patients also may suffer back ache as a musculoskeletal B12 side effect.

6. Respiratory B12 side effects


The intranasal administration of cyanocobalamin causes rhinitis in some patients. Rhinitis may be due to a local reaction of the nasal passage to the cyanocobalamin gel.


Dyspnea may be secondary to rhinitis as a result of inflammation of the nasal airways contributing to difficulty of breathing.

7. Immunologic B12 side effects


This B12 side effect was seen in painters taking parenteral and intranasal cyanocobalamin.

8. Other B12 side effects

Hypersensitivity reactions

Hypersensitivity reactions were seen in patients receiving parenteral cyanocobalamin that may have impurities. However, reactions tend to be mild and there were no reported deaths as a result of hypersensitivity reactions to cyanocobalamin.

Pain at injection site

Patients receiving intramuscular cyanocobalamin may experience pain and swelling at the injection site. This B12 side effect is usually relived after hours to days.

 The B12 side effects tend to be rare and occur in individuals with idiosyncratic reactions to vitaminB12. Vitamin B12 side effects have never been fatal so vitamin B12 supplements are still essential. People who want to take vitamin B12 may also have this through natural sources in foods such as green leafy vegetables and meat. Vitamin B12 is very essential for the body so people should never worry about B12 side effects when they are taking in what is prescribed by the physician. Other essential nutrients such as protein may also have protein side effects, so taking any supplements normally produce side effects that should not cause worry to people.

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