What causes mouth ulcers?

Mouth ulcers - also known as canker sores - are one of those pesky things that can really ruin your week. And even though they can be small - oh the pain - so with up to one in four of us get them coming back again and again - we thought we should chat to an expert. So, what causes them, what makes them come back, and how can we know if it's that's worth getting help? Joining Emile is Dr Anita Lim, an Oral Medicine Specialist and PhD Candidate at Western Sydney University's School of Medicine.