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Canker sores or aphthous ulcers themselves are considered non-contagious. They are generally an autoimmune reaction to an immune stress, whether it is a viral infection, an food or chemical irritation/sensitivity, or trauma (like braces). However, there can be recurrent inner mouth sores that can look and feel like canker sores that are in fact Herpes Simplex type 1 infections and it can be problematic to discern the difference. The Herpes virus can be spread to another person's genitals.

The wisest choice is to avoid oral mucous membrane contact with another individual (oral or genital) while you have a canker sore. The sores can last up to a week, so that is a significant period of time, but there are other sexual activities you can participate in that would not potentially put your partner at risk.