Just got a note from a reader that her dog developed signs of B12 deficiency:

“I was feeding her a [home-cooked] vegan diet, except for a little fish oil, and giving her 500 micrograms B12 every 4 days. She started yelping when I’d open her mouth to give her a pill. The vet couldn’t find a reason. That went away, but she got chronic diarrhea and decreased appetite, also no reason found. Then on a walk, she started limping, then staggering around. Then she collapsed. The vets found no reason for her problems. But B12 deficiency can cause diarrhea, fatigue, loss of appetite and ataxia, and nothing else I know of causes all those symptoms. I’m now giving her 500 micrograms every day, and she’s almost completely back to normal. I wrote the vet a letter suggesting it was B12 deficiency, but I haven’t heard back.”

“She needs 10 mcg B12 if given every day, according to Nutrient Requirements of Dogs – but probably at least 2,000 micrograms if only every 4 days.”