You dont say whether you have B12 deficiency or Pernicious Anaemia.
There is a list showing:-
Who’s at greatest risk for B12 Deficiency?
Anyone at any age, can become B12 deficient. Thus you need to be tested immediately if you develop the symptoms described in this chapter. However, certain people are at an elevated risk. They include the following:
Vegetarians, vegans and people eating macrobiotic diets.
People aged sixty and over
People who’ve undergone any gastric and/or intestinal surgery, including bariatric surgery for weight loss purposes (Gastric bypass).
People who regularly use proton-pump- inhibitors. H2 blockers, antacids, Metformin, and related diabetes drugs, or other medications that can interfere with B12 absorption.
People who undergo surgeries or dental procedures involving nitrous oxide, or who use the drug recreationally.
People with a history of eating disorders (anorexia or bulimia).
People with a history of alcoholism.
People with a family history of pernicious anaemia.
People diagnosed with anaemia (including iron deficiency anaemia, sickle cell anaemia and thalassaemia).
People with Crohn’s disease, irritable bowel syndrome, gluten enteropathy (celiac disease), or any other disease that cause malabsorption of nutrients.
People with autoimmune disorders (especially thyroid disorders such as Hashimoto’s thyroiditis and Grave’s disease) Type 1 diabetes, vitiligo, lupus, Addison’s disease, ulcerative colitis, infertility, acquired agammaglobulinemia, or a family history of these disorders.
Women with a history of infertility or multiple miscarriages.
Infants born to and/or breast fed by women who are symptomatic or are at risk for B12 deficiency.Can you identify yourself in any of the above?
If you have been diagnosed the B12 injections are for life and it is not unusual for things to start to feel worse before they get better and depending on how long you have had low B12 for whatever reason will determine the length of time for repairing the damage done to your nervous system.
As to your faintness, sickness and heart pounding, hopefully these have subsided and won't be repeated after your next injection but I would advise your doctor of these so that he is aware. I'm assuming you are on hydroxocobalamin.
I wish you well nicnic - take care.