lump under skin from b12 injection site Reactions: Symptoms, Treatment, and Prevention Where to Safely Inject B12:

It is important to note that these side effects are relatively uncommon and, when they do occur, are usually mild and temporary

The present case reports suggest that optic neuropathy in patients carrying a primary LHON mtDNA mutation may be precipitated by vitamin B-12 deficiency

In the elderly, the symptoms of vitamin B12 deficiency caused by deficient diets and/or digestive and/or absorption problems can be nonspecific, making a diagnosis of deficiency more difficult

However, there is no robust evidence that B12 injections enhance energy, alertness, or performance in individuals whose B12 levels are already adequate