After evaluation by vet, you can administer vitamin b12 injection for cats
A dose of 1000 mcg daily for 6 or 7 days should be administered by intramuscular or deep subcutaneous injection
The rats were euthanized, and tissue samples (brain, heart, kidneys, liver, spleen, lung, stomach, intestine, muscle, grease, ovaries, womb, testicles, and thymus) were collected at 3 min, 10 min, 1 h, and 24 h after administration (three males and three females at each time point)
In some counties, it may also be available as an injection
PMID 18459613
"Is there a "gold standard" for human serum vitamin B12 assay?"