Common side effects of Vitamin B12 injections can include:

1. Pain/Redness at Injection Site: This is the most common side effect, with some individuals experiencing tenderness or discomfort where the injection was given.
2. Mild Diarrhea: Some people may experience mild gastrointestinal disturbances post-injection.
3. Headache: A headache may occur shortly after the administration of a B12 injection.
4. Nausea: This can happen in some individuals, though it’s generally mild and temporary.
5. Feeling of Swelling: A sensation of the entire body or parts of the body swelling, although actual swelling is not typically observed.
6. Dizziness or Light-headedness: Some may feel dizzy following the injection.
7. Low Potassium Levels: In rare cases, rapid increase in red blood cell count can cause potassium levels to drop, a condition known as hypokalemia.

It’s important to note that while these side effects are possible, they are not experienced by everyone, and Vitamin B12 is generally considered safe when used as directed. Severe side effects are rare but require immediate medical attention. These might include allergic reactions, such as hives, difficulty breathing, or swelling of the face, lips, tongue, or throat. Always consult with a healthcare professional before starting any new supplement regimen.