should I take b12 without a blood test

I have been chronically fatigued for over six years now with a constant "brain fog" for the past two.

My HGB is within normal limits but my vit D was a little low (I have been taking 50k IU per week for a couple weeks now). I have not had a b12 blood test done yet to see if I am actually deficiant but have started taking the suppliment anyways. Is it safe to take 10000 mcg a day or would you advise against this without a blood test. I was thinking I would try these theraphy for a week or two and stop if I don't feel better

-Joseph