A couple of years ago I had a problem I thought was my balance, but having a routine blood test for my med levels, I was asked to have another blood test, a few weeks later I had a letter and a very good print-out about PA, it said that taking anti-convulsants can have an effect on your Intrinsic Factor, that attaches to B12 and then absorbed.
It must have been the combination of age, I was 62 at the time, and the epilepsy meds that caused the PA, I had been taking meds for 40 years, I now have to have a B12 shot every 8 weeks, I think it was age that tipped the balance.