Please read the following for info on B12:
http://www.felinecrf.org/vitamin_b.htm#methylcobalamin
Why Methylcobalamin is Helpful
Vitamin B12 in the form of methylcobalamin (rather than the more commonly available cyanocobalamin) is the neurologically active form of Vitamin B12, and is used by the body to correct or prevent neurological problems. A lack of Vitamin B12 may also cause cognitive dysfunction. It is also essential for red blood cell production, so too low a level of Vitamin B12 in your cat's body may cause or contribute to anaemia.
Cats with IBD or pancreatitis tend to have low levels of cobalamin so are often given methylcobalamin. It is also often recommended for cats with diabetic neuropathy. The prevalence of hypocobalaminaemia in cats with spontaneous hyperthyroidism (2011) Cook AK, Suchodolski JS, Steiner JM & Robertson JE Journal of Small Animal Practice 52(2) pp101–106 found that a sizeable percentage of cats with hyperthyroidism have low levels of cobalamin.
Plasma homocysteine, B vitamins, and amino acid concentrations in cats with cardiomyopathy and arterial thromboembolism (2000) McMichael MA, Freeman LM, Selhub J, Rozanski EA, Brown DJ, Nadeau MR, Rush JE Journal of Veterinary Internal Medicine 14(5) pp507-12 found that cats with heart disease who have thromboembolism (thrown a clot or saddle thrombus) have significantly lower levels of vitamin B6, vitamin B12 and arginine, an amino acid. The study concludes "We interpret the results of this study to suggest that vitamin B12 and arginine may play a role in CM and ATE of cats." If your cat has heart disease, therefore, you may wish to discuss using methylcobalamin with your vet.
Vitamin B12 may be a particular concern for older cats. Vitamin B12 deficiency (2003) OH RC & Brown DL American Family Physician 67(5) pp979-86 mentions that one study found that 15% of people over the age of 65 had a Vitamin B12 deficiency. Addressing age-related changes in feline digestion (2010) Patil AR & Cupp CJ Nestlé Purina Companion Animal Nutrition Summit 2010 Focus on Gerontology states "if Vitamins E & B12 are at low levels, then processing of fat and protein are likely compromised in older cats."
Methylcobalamin has been found by members of Tanya's CRF Support Group to be very helpful for various CKD-related problems, including anaemia, incontinence, appetite loss and constipation. If you are using famotidine (Pepcid AC), it may reduce the absorption of vitamin B12 from food, so it might be worth supplementing it.
Oregon State University has some information about Vitamin B12.
Chronic renal failure promotes severe variant of Vitamin B12 deficiency (2006) Duning T, Nabavi DG, Dziewas R, Kugel H & Schäbitz W-R European Neurology 56 pp62–65 reports on the case of a human CKD patient with Vitamin B12 deficiency and concludes that CKD patients "may require earlier and much larger therapeutic cobalamin doses than previously considered."
PDR Health has some information about methylcobalamin in humans.
Net Doctor has some information about Vitamin B12 deficiency in humans.
I had a cat with horrible pancreatitis issues, and when I gave her B12 shots weekly, the pancreatitis issues disappeared. As for her levels, I don't think it much matters because she had a problem that is mentioned in your signature, pancreatitis, I gave her weekly shots of 25units of B12, and the problem went away.
B12 worked for my cat.