b12 levels after injection Level Back Below "Normal" Initial Set of Shots : r/B12_Deficiency Why You Shouldn't Worry About
Description
However, cats with certain health conditions, especially cats with gastrointestinal disease, may not be able to absorb enough of the nutrient from their food and require supplementation

In many cases, individuals with B12 deficiency may receive injections once a week for several weeks, followed by less frequent maintenance injections, such as once a month

It is typically administered by a healthcare professional in a clinic or hospital, but in some situations, patients may need to administer this medication in the home environment

But for some people it may take longer or multiple shots to feel the benefits
Cordero, P., Gomez-Uriz, A