Vit B12 supplements

Does anyone take these for boosting metabolism i.e., low thyroid problems or fatigue?  I was wondering if they re-act with any of the following - Blood pressure tablets, asprin or Amitriptyline. I am struggling to lose weight despite eating healthily and going to the gym twice a week, I have read B12 can help with weight loss due to low thyroid which I have.   My GP won't put my on thyroxine as it's borderline but I'm sick of putting on weight. Last year he said he would leave it for a year and this year, it's the same and wants to leave it for another year.  I did a food diary for my personal trainer at the gym and, in theory, he says I should be losing eating like I do with the exercise I do but I'm not.