What causes oral ulcers?

Multiple causes: Ulcers have many causes, some are known, while others are poorly understood. Viruses, bacteria, ill-fitting dentures, stress, nutritional deficiencies and compromised immune system and drug side effects are all known factors.

No certain reason: Unfortunately there hasnt been a sure reason found for why ulcers occur. Some of the reasons that contribute are increased stress, insufficient sleep, and vitamin deficiencies.

Ulcer: Mostly auto immune attack. But sometimes cancers present as ulcers. Also there are other diseases such as erythema multiform, hand-foot-mouth, behćet, reiter syndrome that cause ulcers in the mouth.