where is the b12 injection given Injections B12 Injection Dosage and Frequency:

Many patients report feeling increased alertness and reduced fatigue within hours of injection

doi:10.1111/obr.12724

The authors reported that intra-articular AOD-9604 injection in the rabbit osteoarthritis model produced macroscopic, histologic, and biomechanical evidence of cartilage preservation relative to untreated controls, with the combination of AOD-9604 plus hyaluronic acid yielding the most consistent chondroprotective effect [4]

This can be a temporary side effect but should not be ignored, especially if it affects your ability to perform daily activities

megaloblastic anemia

Who Should Avoid Vitamin B12 Injections