im sites for b12 injection Best Vitamin Site: Administering B12 Shots Best Vitamin B12 Injection Sites:

Boost Energy Levels

Vitamin B12 supports red blood cell formation, brain function, and energy production - helping to combat fatigue, low mood, and brain fog while enhancing overall vitality

Cycling Approach: Some practitioners implement protocols such as 12 weeks on, 4-8 weeks off to theoretically prevent receptor desensitization, reduce long-term costs, or provide physiological breaks

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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