Cold sores vs. canker sores—Oral sex risks?

Dear Alice,

What's the difference between a canker sore and a cold sore? In your response to Oral sex with canker sores, you replied that it was safe and could not cause an infection of the genital region. My girlfriend had what we thought was a canker sore, and I happened to become infected with what we have assumed to this point is herpes. Would a cold sore cause this or did we just win the lottery with a canker sore?


Dear ignorant,

There is often confusion between canker sores and cold sores. Canker sores are painful ulcers that may appear on the lining of the mouth, gums, and tongue. They aren't caused by an infection and are therefore not contagious. Cold sores, on the other hand, are small red blisters that generally affect the mouth and facial areas, but usually appear on the lip and outer edge of the mouth. In contrast to canker sores, cold sores are extremely contagious and are most often caused by the herpes simplex virus type 1 (HSV-1). While both can cause irritation, their causes and how they spread are very different. 

While approximately 20 percent of people in the U.S. have reported experiencing canker sores, the precise reason some people are more likely to develop canker sores than others has yet to be determined. Common triggers of canker sores include:

  • Stress.
  • Injury to the inside of your mouth.
  • Acidic foods, such as citrus fruits.
  • Nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen.
  • Dental appliances, such as braces or ill-fitting dentures.

List adapted from Cleveland Clinic.

When it comes to cold sores, the outbreaks can be caused by a number of triggers. They can include things such as stress, infection or changes to the immune system, or exposure to the sun, among other things. For more information on cold sores, you may want to check out the Go Ask Alice! Q&A Cold sores. While cold sores are commonly associated with the lips, they may affect other parts of the body. When discharge from oral herpes sores or the sores themselves directly contact the genital area, a herpes infection can develop on or around the genitals.

Telling the difference between a canker sore and a cold sore by sight can be difficult. If you're unsure, it may be helpful to treat future sores as though they are herpes sores, which are contagious. If your partner has unknown mouth sores, abstaining from sex involving the mouth until it has healed completely can reduce the risk of transmission. However, herpes can still be transmitted when a sore isn't present through viral shedding. When engaging in sexual activity, using safer sex methods every time you have sex (oral or otherwise) can reduce the risk of transmission of sexually transmitted infections (STIs). These can include using an internal or external condom, a dental dam, or other barriers that completely cover areas with sores to help reduce the chances of transmission of the herpes virus or other STIs.

If you have not already done so, consider seeing a health care provider as soon as possible—a diagnosis of genital herpes is easier when symptoms (sores) are present. Doing this will allow you to be certain about your herpes status, give you time to ask additional questions, and receive appropriate treatment. If you're diagnosed with genital herpes or another STI, it's recommended that your girlfriend visit a health care provider for an examination as well. Check out the related Q&As to learn more about safer sex, oral hygiene, and managing herpes.

Last updated Jan 06, 2023
Originally published Sep 05, 1996

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