I have been having this recurrent mouthsore since I got back from the UK a year ago. And I have been to hospitals several times because when it comes it hutrs badly. It could be on my tongue or upper palate or gum. I did HIV test and it came out negative. ii was given antibiotics and it didn't help. I was told I am low on Vitamins and I was prescribed vitamin b12. This I have been using religiously but I still get this sore like once in 2 weeks and could last more than a week before it clears. What do you advice I do? Any more test I could go for?

You should be tested for the Herpes Simplex virus.