Cold sores during chemo are now more tolerable due to treatment advancements. Sadly, there is no guaranteed way to get rid of severe cold sores during cancer treatment. The good news is that there are ways to reduce pain, redness, and swelling.

Many issues arise during chemotherapy. Toxic drugs can cause blisters and sores inside the mouth. Chemo kills many healthy cells, and mouth sores thrive due to a weakened immune system.[1] The issue is made worse if you also experience an HSV-1 outbreak. It’s also important to note that people with poor immune health are more vulnerable to Herpes Esophagitis (a form of herpes that affects the gullet[2]).

While cold sores heal within 2 weeks, chemo-induced sores often take longer to go away. Let’s look at why this is the case and explore things that can make life easier.

What is Chemo-Induced Oral Mucositis?

Oral mucositis, commonly referred to as stomatitis, is an inflammatory reaction that affects the mucous linings of the mouth. Chemo-induced stomatitis is quite common with various forms of chemotherapy.[3]

The result of stomatitis is painful ulcers and potential infection of the mouth. These ulcers and sores can last only a few days or for several weeks. The condition can impact the ability to eat, chew, taste, talk, and even swallow. HSV-1 carriers can suffer numerous outbreaks as a result of stomatitis. Additionally, general blisters and sores that are not related to HSV-1 are also likely to appear.

HSV-1 carriers can suffer numerous outbreaks as a result of stomatitis. Additionally, general blisters and sores that are not related to HSV-1 are likely to appear.

The broad condition is known as mucositis. This can impact the lips, mouth, and throat. It currently has no cure. While stomatitis impacts the mouth, esophagitis leads to inflamed soft tissue linings within the esophagus.

To recap:

  • Chemotherapy drugs can cause stomatitis. There is no proven cure.
  • The result of stomatitis is an outbreak of severe blisters, ulcers, and general sores. Those with HSV-1 are more likely to experience an outbreak of cold sores in addition to mouth irritation.
  • Stomatitis can make routine mouth functions quite difficult. This includes chewing and talking. In severe cases, the act of swallowing can also become hindered.

Why Does Chemotherapy Cause Cold Sores?

Chemo causes cold sores (and general mouth sores) to form because this style of treatment kills rapidly growing cells. Unfortunately, chemo often kills fast growing and healthy cells. Many of those cells tend to grow inside of your mouth.

While on a chemo treatment, your body is naturally being pushed to the limit. To kill potent cancer cells, your body is weakened. This is why sores that appear within the mouth typically remain for a long while.

Compromised healthy cells that reside in the mouth are unable to fend off germs and infection. The result is stomatitis caused sores failing to heal while new sores are created. This typically culminates in a host of blisters, sores, and infection.

HSV-1 carriers are often faced with an even more taxing issue. Unfortunately, the sores created by chemo and fever blisters are not the same. HSV-1 sufferers often have to cope with ulcers inside the mouth while simultaneously dealing with cold sores on the mouth and lips.

Which Chemo Drugs are Likely to Trigger HSV-1?

While it is possible that any form of chemo can cause mouth sores, some are known contributors. Specific drugs do play a huge role, but also the frequency of the treatments. The more chemo is required, the more outbreaks you are likely to endure.

Most cold sore outbreaks caused by chemo form within days of initial treatment. The outbreaks begin to diminish roughly 14-21 days after chemo has been stopped. Blisters caused by chemo are usually at their worst 6 to 7 days after cancer treatment has been concluded.

Listed below are chemo drugs that can trigger HSV-1[4]:

  • Capecitabine (Xeloda)
  • Cisplatin
  • Cytarabine (Depocyt)
  • Doxorubicin (Doxil)
  • Etoposide (Etopophos)
  • Fluorouracil
  • Methotrexate (Trexall)

Can Other Medical Issues Be Caused by Chemotherapy?

The side effects associated with the introduction of chemo treatment can be quite harsh. While a “typical” outbreak can be painful and annoying, blisters caused by chemo can become serious. This is due in large measure to a weakened immune system that is unable to fight germs and bacteria within the mouth and lips.

While symptoms can certainly vary regarding intensity, most severe side effects brought on by chemo are rather universal.

fever blisters due to chemo treatment

You are encouraged to seek medical attention if any of the symptoms noted below begin to arise:

  • If your body temperature rises above 100 degrees and you are unable to get it under control.
  • If your mouth becomes so devastated by blisters and sores that you are unable to eat, drink, or sleep without discomfort. It is not uncommon for stomatitis and cold sore blisters to develop within centimeters of each other.
  • Pain and visible signs of infection are a red flag. If your mouth and lips are in a state of constant pain, this is a sign of infection. Additionally, changes in the color or coating of the tongue is also a symptom of a complication.
  • Symptoms continue to increase rather than pause or slowly decline. If you have been off chemo for weeks and your blisters are getting worse this is cause for concern and medical attention.

How Do I Prevent Cold Sores During Chemotherapy?

The best way to prevent cold sores from occurring during chemo is essentially no different than normal prevention practice. It all begins with keeping your lips and your mouth as moist as possible.

Here are some quick tips:

  • Rinsing your mouth out with water, roughly every 2 hours, can serve to keep your mouth fresh and keep germs at bay. Adding a touch of salt or baking soda to the water can also play a positive role.
  • Another way to keep your lips and mouth moist is through the use of lip balm. Most notably balm that specializes in moisturizing.
  • Sucking on hard candy can also be beneficial for the inside of your mouth.
  • Brushing your teeth on a regular basis and using mouthwash can potentially prevent a cold sore outbreak during chemo.

Finally, as a means of prevention, avoid known cold sore triggers. The same foods and beverages you always knew to avoid before can still help you during your cancer treatment.

To recap:

  • Take care of your lips and mouth. Keep them both moisturized as much as possible. Water, hand candy, lip balm, and regular teeth brushing can all potentially prevent a major cold sore issue.
  • Continue to avoid known triggers. If you have had prior cold sore outbreaks, you are likely familiar with what sets off a problem. Remembering those triggers, and avoiding them accordingly, can help you immensely during chemo.
  • Inform your doctor if you have a history of mouth sores. This includes the disclosure of HSV-1. In many cases, antiviral medication can be administered in advance of chemo to help with a potential issue.[5]


The treatment of oral mucositis and cold sores has more to do with pain prevention than actual healing. Until healthy cells can be restored, total healing will be quite difficult. The main goal of treatment is to ease the existing issues.

One way to treat the problem is a topical painkiller. This type of medication numbs the area to a certain degree. While total numbing can create a new host of issues, topical painkiller numbs the mouth just enough for comfort purposes.

Cold sores due to chemopherapy

Various types of coating agents are also used as a form of treatment. These types of medications coat the lining of your mouth and allow you to do basic functions without pain. The ability to eat and chew without issue can be helped by a coating agent.

Protection from the existing blisters is just as good as a treatment. As long as chemotherapy is in full force, the blisters will not go away. However, various medications can provide a shield of protection as a means to combat discomfort.

To summarize:

  • The treatment of chemo-induced blisters and cold sores come in the form of protection. While the blisters are not likely to be healed, they can be quelled in terms of pain.
  • You can still use an OTC medication to ease the discomfort. However, it is important to know that OTC remedies might not work as well as they do when you have strong and healthy immune system.

Chemo can trigger a cold sore outbreak. If you suffer from HSV-1 and are undergoing chemo, it is important to understand that oral issues could become a major problem. Your best course of action is to inform your doctor if you have a history of lip or mouth sores.

Because there is no proven treatment for chemo-related cold sores, the best recommendation is to manage the pain from cold sores.


  1. Chen, YK., Hou, HA., Chow, JM. et al. Eur J Clin Microbiol Infect Dis (2011) 30: 753.
  2. Wang H-W, Kuo C-J, Lin W-R, et al. Clinical Characteristics and Manifestation of Herpes Esophagitis: One Single-center Experience in Taiwan. Kantarceken. B, ed. Medicine. 2016;95(14):e3187. doi:10.1097/MD.0000000000003187.
  3. Santos de Faria AB, Silva IH, de Godoy Almeida R, Silva SP, Carvalho AT, Leão JC. Seroprevalence of herpes virus associated with the presence and severity of oral mucositis in children diagnosed with acute lymphoid leukemia. J Oral Pathol Med. 2014 Apr;43(4):298-303. doi: 10.1111/jop.12138. Epub 2013 Dec 10.
  5. Sandherr M, Hentrich M, von Lilienfeld-Toal M, et al. Antiviral prophylaxis in patients with solid tumours and haematological malignancies—update of the Guidelines of the Infectious Diseases Working Party (AGIHO) of the German Society for Hematology and Medical Oncology (DGHO). Annals of Hematology. 2015;94(9):1441-1450. doi:10.1007/s00277-015-2447-3.