--for any health related reason that I know of--B12 has no known toxicity level, as the body uses what it needs and excretes excess.

You may be creating some very expensive urine if your body is taking in far more than it needs, but that's the only caveat I would have.

I suspect that if you're taking 5000mcg/daily, your serum B12 level would be much higher than that if the lab that did the measurement uses a methodology that allows for higher values. (My last measurement was 1672. But I do heavily supplement, as you do. I often have to explain this to doctors.)

The real key, I suppose, is how do you FEEL at this dose? Are you having improvements?