Signs and symptoms of vitamin B12 deficiency can include fatigue, pale skin, headaches, shortness of breath, fast heartbeat, gastrointestinal issues, mouth inflammation, paresthesias (abnormal sensations), decreased cognitive ability, depression, muscle weakness, imbalance and incoordination, vision problems, and erectile dysfunction

Here are some possible side effects to keep in mind: You might feel some mild pain or see redness where you got the shot, but it usually goes away quickly Some people feel a bit dizzy right after the injection You might get some itching or a rash if you're sensitive Allergic reactions are rare, but if you have one, get medical help right away Some medications might not play nice with B12 shots, so tell your doctor about all your meds You might get a headache or feel a bit nauseous, but it's usually mild and short-lived In very rare cases, high doses might cause acne-like skin issues Your body usually gets rid of extra B12, but super-high doses could stress your liver Who Should Consider B12 Shots for Weight Loss
Why Do Some Cats Have a Vitamin B12 Deficiency
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The dosing context below summarizes published study arms and common research-planning structures