I just had a call from the vet. Seamus had a series of B12 loading doses last November/December, and then a blood test yesterday (note, results in a day!!!). His B12 a month after the loading doses ended is bang in the middle of the range. If it was a one-off, she said, it would be sky high - therefore he needs regular (monthly) injections. And if he seems to be droopy despite that, then it will need to be more frequent!
I cannot imagine a similar response if those results had been for a human! In any case, he was only at the bottom of the range when we started the loading doses, not hugely deficient.
Add to this a general policy with vets to keep dogs on thyroxine at levels significantly HIGHER than they would be if it was naturally produced - because they seem to do better at higher levels. You can see that dogs get treated by symptoms, whereas their owners have test results followed slavishly.