The lack of intrinsic factor is most commonly due to autoimmune gastritis, which causes an autoimmune attack on the parietal cells that create it in the stomach
These vitamin injections are crucial for preventing vitamin deficiencies in human body which can cause diseases like anemia, beriberi, and others
Alternatively, enroll in a residential "special waste" program that will come pick up your needles
Without either, mobilized fatty acids simply get restored to adipose tissue
But for most purposes (like weekly maintenance injections), IM is the preferred route at home because its simpler (the vitamins absorb quickly from muscle into the bloodstream over a few hours)
Full list of articles: How often should you get a B12 shot, and how much B12 should you inject per shot