When I initially tried the human B12 injection, I gave 1/10th of a cc. It is basically a little diabetic needle, to the first mark, it is only enough to probably fill a pea or small bean.
In humans, some people are unable to properly absorb B12 due to a deficiency of a chemical called intrinsic factor (sp?) which is found at some small area of the intestine. These people who are deficient have to either take b12 orally daily, for life or 1 injection, about 1cc about once a month. Most people are able to absorb B12 fine and do not need any supplements. In fact, my doctor told me it is nearly impossible to not get enough from a normal, Western diet. (so my doc told me, I am B12 deficient)
I think this is somehow the same with chickens.... I don't think there is a lack of anything in their diet, I think maybe something just seems to go "awry" during maturity. From what I read, it seems to affect younger birds. My little one seems entirely normal now. I have a zip tie on her leg, otherwise I likely could not tell her from the other BR's. I only gave her that little dose twice.
I doubt you can overdose, it is water soluble.
