I get mouth ulcers all the time. Every time I work too hard, get sick, bite my mouth - always an ulcer.
I have lived off this stuff for the last few years now - boots used to have their own (cheaper) brand that looks like it got ditched. Bonjela once is now my saviour and I cannot rate it highly enough.
I was mortified this week to find that every branch of boots seems to have sold out, but replaced it with an awful replacement product that doesn't work at all. Thank god for Amazon.
As the other reviews say time and time again - you literally cannot underestimate just how painful this stuff is to apply. We are talking picking up a hot pan, salt in a wound, chilli in the eye levels of pain. However, the pain lasts only seconds and then it is gone forever (until the next one).
Other big tips - don't hold it there for too long and definitely don't reapply (unless straight away). These only prolong the ulcer and ruin the surrounding tissue.
If the ulcer is between the tongue and teeth (ie, those uber-painful ones that make eating impossible) make sure you really really dry the ulcer first with the other end of the stick - the saliva in your mouth will otherwise make it useless.