I have read this important tip from a magazine...
THE CAUSE: A reaction to one or more allergy-causing compounds. The usual culprits are dyes used for coloring, and suds-producing surfactants in your toothpaste.
HOW TO FIX IT: Stay away from colored toothpastes and gels, as well as toothpastes with other additives like "sparkles" or "crystals," and stick to plain white paste formulas instead.