In some cases it is possible to prevent mouth ulcers

Switching toothpastes can prevent mouth ulcers from occurring, with some being able to reduce the occurrence of ulcers by approximately 80%.

Sodium dodecyl sulfate (sometimes called sodium lauryl sulfate or simply SLS), a detergent found in most toothpastes, is thought to increase the incidence of mouth ulcers. Using toothpaste free of this compound has been found to help many people by reducing the amount and size of ulcers and in some cases, completely stopping them. A few individuals have noticed that switching to a toothpaste with baking soda prevented re-occurrence of mouth ulcers.

Many people have found that taking Lysine-L supplements can help to reduce the frequency of mouth ulcer appearances.

Additionally, zinc deficiency has been observed in people with recurrent mouth ulcers, and zinc supplementation decreased recurrence of the mouth ulcers.

Some people have reported that the frequency of mouth ulcer occurrences decreased greatly after a particularly large amalgam tooth filling was replaced by some other kind of dental restoration. However, the connection between amalgam fillings and mouth ulcers is not universally accepted, and such replacement can be costly.

Dental braces are a common physical trauma that can lead to mouth ulcers and can be treated with wax to reduce abrasion of the mucosa. Avoidance of other types of physical and chemical trauma will prevent some ulcers, but since such trauma is usually accidental, this type of prevention is not usually practical.