When deficiency is treated early enough, damage may be repaired quickly and completely. The longer treatment is delayed, the more likely repairs will take many months to years and be incomplete.
Do not despair if a symptom becomes worse at some point after treatment begins. This sometimes happens when the body is addressing that particular problem, and often the symptom becomes dramatically worse temporarily (briefly to much longer). Symptoms can even multiply previous to great improvement. No guarantees, but this often happens when damage has been serious.
Many people who have serious damage experience what I call a honeymoon period within days or weeks of starting treatment. At that point it may seem that improvement is so rapid they begin planning to dance on tables within a few months. Then, in those who experience great fatigue and increased symptoms as their bodies begin very hard work on serious repairs, fear is common and reasonable.
Recovering from damage is often not linear. It isn't like getting over the flu or a cold, or healing from a wound. Nerve repair is slow and can be very weird. Many of us have settled on "weird" as the best way to describe it.
And very few medical people have the slightest understanding of that process. Many in fact assume, irrationally, that as soon as the B12 deficiency is corrected, the body will miraculously be back as good as new. As if even a simple knife wound would be healed as soon as the knife is removed. Ridiculous.
More to come...