b12 injection swelling Why do I feel worse after a injection? Vitamin B12 Side Effects

I don't know if this helps but I'v always found the upper deltoid far less painful than the thigh

Gently, blow your nose to clear both nostrils

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Journal of Clinical Investigation

As to the aches and pains, breathlessness etc, this could be due to B12 needing potassium to build RBC, so I would check out low potassium symptoms and eat a banana or maybe two until you have fathomed it out

Learn more about the difference between GHK-Cu and AHK-Cu below