5-Fluorouracil and mouth ulcers (mucositis)
5-Fluorouracil (5-FU) is one of the most common causes of chemotherapy-induced oral mucositis. Mouth ulcers and lining breakdown often start within the first one to two weeks of treatment and can range from mild soreness to severe widespread ulceration that limits eating and drinking. The right time to work on the mouth is before chemo starts: any active dental infection, sharp tooth edge, or ill-fitting denture can become a much bigger problem once mucositis is active. During treatment, the focus is on gentle hygiene, pain management, and preventing infection of the ulcers.
5-FU is an antimetabolite chemotherapy drug that interferes with DNA synthesis in rapidly dividing cells. It is highly effective against many solid tumors (colon, breast, gastric, head and neck) because cancer cells divide rapidly. The same property is what causes its main side effects: the normal cells of the body that divide most rapidly (mouth and gut lining, hair follicles, bone marrow) are also affected.
The cells lining the mouth turn over every 7 to 14 days under normal conditions. When 5-FU slows that turnover, the lining cannot replace itself quickly enough. The protective layer breaks down, and the underlying tissue is exposed. The result is mucositis: red, raw areas that range from small isolated ulcers to widespread breakdown covering most of the mouth in severe cases.
Mucositis usually starts 5 to 10 days after the start of 5-FU treatment, peaks around day 10 to 14, and resolves over the following week or two if treatment is paused. With continuous 5-FU infusions or repeated cycles, mucositis can become a recurring issue that limits how much chemotherapy can be delivered. This is why oral side effects are one of the most common reasons for dose reductions in 5-FU treatment.
- Get a dental exam and any urgent dental work done before chemotherapy starts if possible. Sharp tooth edges, active infections, and ill-fitting prosthetics all worsen mucositis.
- Brush gently with a soft-bristle toothbrush and a mild fluoride toothpaste. Avoid alcohol-containing mouthwash, which burns and slows healing.
- Rinse with a saltwater or baking soda solution several times a day. One teaspoon of either in a cup of warm water is gentle and effective.
- Sip water frequently to keep the mouth moist. Dry mucositis hurts more than moist mucositis.
- Avoid spicy, acidic, sharp, or very hot foods during active mucositis. Smooth, room-temperature, high-protein foods are easier.
- Cryotherapy (sucking on ice chips) during certain 5-FU infusions has been shown to reduce mucositis severity in studies. Ask your oncology team if this fits your treatment plan.
- Tell your oncology team if pain is preventing you from eating or drinking. Pain control may need to be more aggressive, and severe mucositis sometimes requires a dose adjustment or treatment break.
- Mouth ulcers covering more than small isolated areas, especially in the first week of treatment.
- Severe pain that prevents you from eating, drinking, or taking your medications.
- Signs of infection on top of mucositis: white or yellow patches that look thick or coated, fever, or worsening pain.
- Bleeding from the ulcers that does not stop with gentle pressure.
- Difficulty swallowing, which can indicate mucositis extending into the esophagus.
General guidance is a starting point. Your specific dental plan depends on your medical history, your other medications, and what your mouth looks like in person. Schedule a consultation and we’ll walk through it.
Reviewed by Dr. Isaac Sun, DDS.
This page is general information, not medical advice. Do not start, stop, or change any medication based on what you read here. Talk to your prescribing physician and your dentist about your specific situation.