Follow-up: Will hair lost from an active B12 deficiency grow back when deficiency is treated?

B12 deficiency as an actual cause of hair loss is not too common. For most people, correction of B12 levels does not alter hair that much. There are exceptions of course.

If you know for sure that the cause of hair loss is exclusively a B12 deficiency, it may grow back when this is reversed unless there is an underlying genetic problem that has not shown itself. 

if your hair loss is due to nutritional, or vitamin deficiency, it may be temporary. If proper diet and vitamins may help the hair to grow back.