<p>Canker sores are small, round sores in your <a href="https://medlineplus.gov/mouthdisorders.html">mouth</a>. They can be on the inside of your cheek, under your tongue, or in the back of your throat. They usually have a red edge and a gray center. They can be quite painful. They are not the same as <a href="https://medlineplus.gov/coldsores.html">cold sores</a>, which are caused by herpes simplex.</p> <p>Canker sores aren't contagious. They may happen if you have a viral infection. They may also be triggered by stress, food allergies, lack of vitamins and minerals, hormonal changes or menstrual periods. In some cases the cause is unknown.</p> <p>In most cases, the sores go away by themselves. Some ointments, creams or rinses may help with the pain. Avoiding hot, spicy food while you have a canker sore also helps.</p>