why give vitamin b12 injections Best Injection Sites: Where to Inject B12 · PA Relief B12 Vitamin shot - GlamourEyes

Serious side effects are very rare

Huang ZH, Luque RM, Kineman RD, Mazzone T

Cobalt sensitivity reactions can result in different symptoms, such as chronic or subacute allergic contact dermatitis, which can occur immediately or up to 72 hours after administration, but the likelihood of a cobalt allergy is low and affects between 1% and 3% of the general population

In others an injection will be needed every 2 - 3 months

Before receiving folic acid or methylcobalamin, patients should be assessed for deficiency and appropriate therapy started concurrently

Since there is no known hazardous threshold for vitamin B12, there is no upper limit