In some users, toothpaste may cause canker sores if it contains the ingredient called sodium lauryl sulfate or SLS, states the Massachusetts Dental Society. Sodium lauryl sulfate is an emulsifier and a cleansing agent used in many cosmetic products, body wash, shampoos and toothpaste, as noted by the Environmental Working Group.
SLS can irritate skin and may produce canker sores in people who have sensitivity to this ingredient. By using a toothpaste that does not contain SLS, the problem with canker sores may be avoided.
Cancer sores, which are also called aphthous ulcers, are lesions that can form in the mouth. Some symptoms of canker sores are pain and fever in severe cases, notes WebMD. Although SLS in toothpaste may be a factor that causes canker sores, there can be many other causes for this condition, including stress and a deficiency in either folic acid or iron.