Canker sores last about one to two weeks, according to Mayo Clinic; however, the majority of the pain associated with canker sores usually subsides after approximately seven to 10 days, says Healthline. Canker sores that have not healed after three weeks may require medical attention.
Canker sores are not contagious, according to WebMD; however, it is also not possible to prevent them, and doctors are not really sure why they occur. Women are more susceptible to canker sores than men, and they are more common during periods of physical or emotional stress. They may also be triggered by acidic foods, such as tomatoes, and by ingredients in toothpaste.