When I first read about people topping off/up in my ignorance believed that clearly they did not understand how B12 injections worked in the body. I came to marvel at the ability to 'just know'.
With 2 mg every 4 hours during the day and injecting if I wake up in the night I am now one of the cool kids and can tell when I need more B12.
The way I can tell is my head gets stuffy. That is right now the first sign where prior it was teeth pain and was not alleviated quickly by injecting.
I get a stuffy head in about 6 hours and some times at the end of the day prior to my next injection.
Yesterday was a good but arduous day. I started to get a stuffy head late in the day and considered moving up my injections. I chose to wait the 1/2 hour. In 20 minutes after the injection the stuffy head was all but gone and in 40 was alleviated. So now I am one of the cool kids. π
In hindsight the reason I could not tell when I needed more B12 is I did not have enough at the frequency where my body could tell me that I needed more. It was always starving for B12 and starving a bit more was not that dramatic.