One old wives' tale blames canker sores on tomatoes. Experts admit some sort of allergic reaction to foods may be to blame, but others point out that food allergies can cause lesions that resemble canker sores. If you're plagued with frequent canker sores, pay attention to whether an outbreak seems to be linked with any particular food. Likely offenders include nuts, shellfish, chocolate and tomatoes. If you discover a connection, avoid the offending food.
You'll also want to stay away from foods that are hot, in terms of temperature or spiciness. They'll burn and sting a tender canker sore. And you may want to avoid rough, scratchy foods such as chips for the time being as well.