Mouth sores


Mouth sores, also called mucositis, are a side effect of some cancer treatments that inflame the lining of the digestive system. Mouth sores often appear as ulcers on the lips or inside the mouth. Sometimes they can appear inside the esophagus, the tube that runs from your throat to your stomach.

Not everyone undergoing breast cancer treatment gets mouth sores, and there are ways to reduce the risk. If mouth sores do develop, your care team will help you with pain relief and ways to keep them from getting worse.


What are the symptoms of mouth sores?

Symptoms of mouth sores may include:

  • Red, shiny, or swollen gums and mouth
  • Small sores on the soft tissues of your mouth or lips
  • A yellow or white film on your tongue or inside your mouth
  • Pain in your mouth or throat
  • Bleeding gums or mouth
  • White patches or increased mucus in your mouth
  • Pain, dryness, or burning when you eat

Mouth sores can be uncomfortable and distressing. Pain from the sores can make it difficult to chew, swallow, talk, eat, and drink. If it's hard to eat and drink, mouth sores can lead to poor nutrition, weight loss, and dehydration. If they are severe, they may become infected or bleed.

For some people, mouth sores are mild and more of a nuisance. But for others, mouth sores can be painful and interfere with daily life. It's important to let your care team know if you start to develop mouth sores.


What causes mouth sores?

Mouth sores are caused by some chemotherapy, targeted therapy, and immunotherapy medicines used to treat breast cancer. They can also be caused by infection, vitamin or protein deficiency, dehydration, poor oral or dental health, and alcohol or tobacco use.

If you get mouth sores during chemotherapy for early-stage breast cancer, they are likely to develop within the first 2 weeks of treatment. It usually takes 2 to 4 weeks for mouth sores to heal after treatment ends.

Any type of chemotherapy medicine can cause mouth sores, and some targeted therapies can cause mouth sores. These include:

The immunotherapy pembrolizumab (Keytruda), given with chemotherapy, can also cause mouth sores.



How can I reduce my risk of developing mouth sores?

Here are some ways you can try to prevent mouth sores:

  • Visit a dentist before you get treatment to make sure your teeth and gums are healthy.
  • Practice good oral hygiene.
    • Use a soft toothbrush to brush your teeth several times a day. Rinse your toothbrush in hot water and store it in a cool place.
    • Floss every day. Avoid mouthwash that contains alcohol, as it can be drying. There are other mouthwash options that can reduce the risk of developing mouth sores or pain from mouth sores:
      • Ask your care team about whether prescription steroid mouthwash might be helpful for you. A 2017 study in women diagnosed with hormone receptor-positive breast cancer taking exemestane (Aromasin) and everolimus (Afinitor) showed that when they used a prescription mouthwash containing the steroid dexamethasone, they had fewer and less serious mouth sores. Participants used the mouthwash for 2 minutes, four times a day.
      • Make a homemade mouthwash using either a half teaspoon of table salt or baking soda mixed in 8 ounces of water. Swish and spit before and after meals and before bed.
  • Eat healthy, nutritious food including plenty of fruits and vegetables.
  • Drink plenty of fluids. Try to drink 8-10 glasses of non-alcoholic, non-caffeinated beverages daily.
  • Keep lips moisturized.
  • Talk to your doctor if you have a history of mouth sores. Your doctor may be able to prescribe a medicine to prevent them during treatment.
  • If you smoke, it's important to try to stop. Talk with your care team about getting help for quitting smoking.
  • Some women report success using cold therapy, such as swirling ice chips around in your mouth before treatment, to prevent mouth sores.

Your doctor may also recommend medicines to improve the health of your mouth cells.


How can I manage mouth sores?

If you are being treated for early-stage breast cancer, mouth sores will start to heal after you've finished the treatment that caused them. If you do develop mouth sores during treatment, there are ways to prevent them from getting worse, reduce your risk for infection, and relieve pain.

Here are some strategies:

  • Ask your care team about topical prescription medicines that can reduce the pain and protect your mouth.
  • Ask your care team about prescription steroid mouthwash that might help, or use a homemade mouth rinse (a mix of 8 ounces of water with half a teaspoon of salt or baking soda).
  • Avoid foods that are acidic, alcoholic, salty, or spicy, because they can cause pain.
  • Avoid smoking or tobacco products.
  • Keep your mouth clean.
  • Eat small mouthfuls of chilled, soft food.
  • Use a straw to drink.

It can also help to keep a symptom journal. Write down:

  • How long you've had mouth sores
  • How severe they are
  • When you have symptoms
  • What medicines or mouthwashes you use to manage them
  • How well any medicines or mouthwashes are working

You can also rate symptoms on a scale of 1 (not at all uncomfortable) to 10 (the most uncomfortable you have ever been).

Be as specific as possible when tracking symptoms in your journal. Jot down questions that may come up, and take your journal with you to medical appointments so you can share the information with your team. When you talk with your nurses or doctors, let them know how much mouth sores are impacting your life.


Mouth sores and metastatic breast cancer treatment

Mouth sores can happen with certain treatments for any stage of breast cancer. Treatments approved for metastatic breast cancer that may cause mouth sores include:

Having chemotherapy and radiation at the same time can also increase the risk of developing mouth sores.

Symptoms of mouth sores can slowly build over time as a person continues to take a treatment that causes them. Always let your care team know if you have mouth sores and if they are interfering with your daily life.

Managing mouth sores with ongoing treatment

There are many ways to manage mouth sores, including the strategies mentioned earlier on this page, such as:

  • Avoiding acidic or spicy foods
  • Using a straw to drink
  • Asking your care team about prescription medicine and steroid mouthwash that can help reduce pain and protect your mouth

It can help to keep a symptom journal to share with your team at appointments. Keep a log of how long you've had mouth sores, how severe they are, any medicine you are taking to treat them, and how well the medicine is working.

It's important to know that if mouth sores become severe, they can cause complications:

  • If your immune system is compromised due to breast cancer or treatment, there is a higher risk for serious infection. Infection can happen if germs come into contact with the sores and travel further into your body. Reduce your risk by practicing good oral hygiene, including alcohol-free toothpaste and mouthwash.
  • Ongoing chemotherapy can cause increased risk of mild to severe bleeding from mouth sores.
  • Sometimes, mouth sores can make it difficult to eat or drink, which can cause rapid weight loss or dehydration. If you are losing too much weight, your care team may recommend meeting with a dietician or taking oral supplements such as Ensure, Boost, or Carnation Instant Breakfast to add more calories.

If mouth sores begin to interfere with your daily activities, there are options. Your care team may:

  • Lower the dose of your medicine
  • Delay your treatment by an extra week
  • In some cases, stop treatment for a short time

It might also be possible to switch to a different treatment that doesn't cause mouth sores.

Most treatments have recommendations built in to lower the dose or change the timing of the treatment if side effects are causing difficulty. Many treatments are still effective against the cancer if the dose is adjusted. Talk with your care team about how they will monitor the impact of any changes to your treatment.


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Reviewed and updated: August 14, 2022

Reviewed by: Evelyn Robles-Rodriguez, DNP, APN, AOCN


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