People with signs and symptoms of B-12 deficiency
Anyone who is displaying any of the signs and symptoms of a B-12 deficiency or pernicious anemia should consult their doctor immediately.
Some common signs and symptoms include:

• decreased cognitive function, such as issues with memory or understanding
• fatigue
• lethargy
• constipation
• feeling faint
• depression or irritability
• headache
• difficulty maintaining balance
• sore, swollen tongue, which may be pale yellow or very red
• heart palpitations
• mouth ulcers
• paraesthesia (pins and needles)
• vision changes

B-12 deficiency risk factors

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People who are more a t risk of deficiency 

Both diabetes and some medications for type 2 diabetes can increase the risk of vitamin B-12 deficiency.
Also, the following risk factors can increase the chance of developing vitamin B-12 deficiency:

• alcohol abuse
• smoking
• certain prescription medications, including antacids and some type 2 diabetes drugs
• having an endocrine-related autoimmune disorder, such as diabetes or a thyroid disorder
• eating a vegetarian or vegan diet
• certain gastrointestinal disorders, such as Crohn's disease
• gastric bypass or the removal of parts of the stomach
• aging

People with gastrointestinal issues
A gastrointestinal tract that does not function normally may inhibit vitamin B-12 release or absorption. For this reason, people with disorders such as celiac disease and Crohn's disease may be at higher risk of vitamin B-12 deficiency.
Individuals who have undergone gastrointestinal surgery, including weight loss surgery, may have less of the cells necessary to secrete stomach acid and intrinsic factor. As a result, they may struggle to absorb B-12.
Vitamin B-12 shots, rather than oral supplements, may be particularly helpful for people who have gastrointestinal issues because injections tend to bypass these areas.
Older adults

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Vegetarians and vegans
As vitamin B-12 is found mainly in fish, meat, eggs, and dairy, people who do not consume these foods are at risk of B-12 deficiency and may benefit from regular B-12 shots or other supplements.

Common side effects of Cyanocobalamin include the following symptoms
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injection site reactions (pain, redness, swelling, irritation),
diarrhoea,
nausea,
upset stomach,
itching,
rash,
headache,
dizziness,


if you suffer from any of those reaction after B12 injection you must see your GP