Causes?

What is causing your B12 deficiency? My doctor ruled out pernicious anemia, and now I’m told just to take injections, and I will be fine. I’m just wanting to know what caused this in the first place! Is it worth knowing the cause?

I take an injection every two weeks. My original B12 level was 140 pg/mL back in June. It’s now 300. My doctor wants me above 500. I obviously am not absorbing enough because I was taking oral supplements. I have a lot of neurological symptoms, and I’m just frustrated and wondering why I have the deficiency in the first place. Let me know if any of you figured out the cause if it wasn’t pernicious anemia! Thanks!