There have been a couple of posts about sore mouth in the last month or so, so I would like to offer this:
I've had so-called Burning Mouth Syndrome for some years, which has fluctuated, particularly with pred. The dentist says it nearly always turns out to be an allergy, but I couldn't track it down. I always suspected toothpaste, and have tried lots of different brands with little success, though the ones without SLS did seem a a bit better.
However, the last month I have been using 'Dr Organic' and the burning has gone! Slight downsides are that it has a stupid name, and only 1000 ppm fluoride instead of the usual 1400ppm. In some of the 5 flavours I think it has none.
I hope this might help someone!