While most people get their B12 supply from the foods they consume, there are others who resort to a B12 supplement to make up for a deficiency. In some cases though, there might be an overdose, and this can lead to various vitamin b12 side effects.
In this article, we take a look at the various vitamin B12 side effects that you should know about before adding one of these supplements to your daily health regimen.
Vitamin B12 Side Effects
Vitamin B12 helps your body in keeping the nerve and blood cells healthy and contributes to making the DNA, which is the genetic material in all cells, as per the NIH. Your daily requirement of B12 varies as per your age, from 0.4 mcg to 2.8 mcg. However, there might be times when there is an overdose of this vitamin in your body and this can lead to certain side effects, ranging from mild to severe. Let us take a look at these side effects below. 
Vitamin B12 Overdose
Vitamin B12 is a water-soluble vitamin and is therefore considered safe even at higher doses. In order to curb a B12 deficiency, your doctor or health care professional may prescribe you an oral or injectable supplement. There is no upper limit (UL) to a B12 dosage, because of its low toxicity nature. However, in some cases, B12 is known to have potential side effects. These include the following.
- Skin rashes
As per the NIH Factsheet, it can also have some other severe side effects, such as the following.
- Allergic Reaction or Anaphylaxis: In some people, B12 can cause a very rare form of allergic reaction or anaphylaxis, in which they may experience swelling on the face, tongue, and throat and difficulty in breathing, as per a study published in 2018.  If you experience any of these symptoms, treat it as a medical emergency.
- Drug Interactions: B12 can interact with the other medicines you take. In other cases, medicines taken regularly can lead to a decrease in the body’s absorption of B12. Some of these medicines include antibiotics like Chloramphenicol, medicines used for acid reflux like Omeprazole and Lansoprazole, and histamines like Tagamet, Zantac and more. It can also interfere with diabetes medication, such as with Metformin. 
If you detect any of these side effects, discontinue use and speak with your doctor about the symptoms you experienced. 
Note: Breastfeeding women should speak with a doctor before taking B12 supplements, etc.
A Brief on What is Vitamin B12
Vitamin B12 is a water-soluble vitamin in the B vitamin complex and is actually used to describe a number of different forms of a compound containing cobalt, known as cobalamins. One of the most common cobalamins that are found in food is known as methylcobalamin. Vitamin B12 is bound to proteins in food and is released into the body upon absorption. However, when you take supplements of vitamin B12 additives, the vitamin is already in an unbound form. 
The body requires vitamin B12 for a number of important functions, including the production of blood cells and proper nervous system activity. It is also crucial for DNA synthesis and various other functions in the body, as per a study published in the journal Nutrients. Various health conditions can cause a deficiency in this vitamin, which may lead to supplement use. However, there are some people who should not consume vitamin B12 or must be careful when using these supplements due to potential side effects.