by Jason Wasserman MD PhD FRCPC
April 10, 2026
Acute mucositis is inflammation of the mucosa — the tissue that lines the inside of the oral cavity. The oral cavity includes the lips, tongue, gums, inner cheeks (buccal mucosa), floor of the mouth, and palate. The word acute means the inflammation is active or ongoing. Another name for this condition is stomatitis.
Acute mucositis is not cancer. A biopsy of the oral cavity is often taken when a patient or doctor notices a sore, patch, or area of tissue that is not healing normally — the biopsy is performed to confirm that the finding is inflammatory rather than precancerous or cancerous. A result of acute mucositis is, therefore, a reassuring one: it tells you that what was seen in your mouth is a reaction to injury or infection, not an abnormal growth. If the biopsy result had shown dysplasia or cancer, your report would say so specifically.
What causes acute mucositis?
Acute mucositis develops when the cells lining the oral cavity are damaged or irritated. There are several common causes:
- Radiation therapy. When radiation is used to treat head and neck cancers, it can damage the cells of the oral mucosa as it passes through the treatment area. Radiation-related mucositis typically begins within the first few weeks of treatment and can range from mild redness and soreness to severe ulceration that significantly affects eating and swallowing.
- Chemotherapy. Certain chemotherapy drugs damage rapidly dividing cells throughout the body — including the cells that line the mouth. The severity depends on the specific drugs, the dose, and how sensitive the individual is to their effects. Mucositis from chemotherapy typically develops within a few days of treatment and usually resolves within two to three weeks of the last dose.
- Stem cell transplantation. Before a stem cell transplant, patients receive high-dose chemotherapy or total body irradiation to prepare the immune system. This conditioning treatment often causes severe mucositis, which can make eating and drinking very difficult during the recovery period.
- Infection. Viral, bacterial, and fungal infections can all cause acute mucositis. The most common viral cause is herpes simplex virus (HSV), which produces painful ulcers on the lips and inside the mouth. The most common fungal cause is Candida, which typically appears as white patches or a burning sensation. Both can be identified in biopsied tissue and are important to recognise because they require specific treatment.
- Trauma and physical injury. Burns from hot food or liquids, poorly fitting dental appliances, biting the cheek, or dental procedures can all cause acute inflammation in the oral mucosa. This type of mucositis is usually localised to the area of injury and resolves as the tissue heals.
What are the symptoms?
Symptoms of acute mucositis depend on the severity of the inflammation and what is causing it. They may include:
- Redness or swelling of the oral lining.
- White or yellow patches, or ulcers (open sores) in the mouth.
- Pain or burning in the mouth, which may make eating, drinking, or swallowing difficult.
- Bleeding from the gums or ulcerated areas.
- A dry or sticky feeling in the mouth.
Mucositis related to cancer treatment can be severe. In its worst form it may make eating nearly impossible and significantly affect nutrition, hydration, and quality of life. Oncology teams routinely monitor for and manage this complication as part of treatment.
How is the diagnosis made?
The diagnosis is made by a pathologist who examines a small tissue sample — called a biopsy — under a microscope. A biopsy is usually taken when a doctor or dentist finds an area in the mouth that does not look normal, particularly one that is not healing after two to three weeks. The main reason for performing a biopsy is to rule out dysplasia or cancer rather than to diagnose mucositis directly — mucositis is often the result that confirms the lesion is benign.
The pathologist examines the tissue, identifies the features of inflammation present, and reports whether any worrying changes such as dysplasia are present. If an infection is suspected, special stains or immunohistochemistry tests may be used to identify the specific organism.
What does the pathology report describe?
The oral cavity is lined by squamous cells that form a protective surface layer called the epithelium. Beneath the epithelium is a layer of supportive tissue called the lamina propria. In acute mucositis, the pathologist typically describes the following features:
- Neutrophils in the epithelium and lamina propria. Immune cells called neutrophils are the hallmark of acute (active) inflammation. Their presence in the surface lining and underlying tissue indicates that the inflammation is current and ongoing, not long-standing.
- Reactive atypia. The squamous cells in inflamed tissue often look abnormal under the microscope — they may be larger, more irregular, or more deeply stained than usual. Pathologists call this reactive atypia. It is an expected response to inflammation and injury, not a sign of cancer or precancerous change. Reactive atypia is important because it can sometimes look worrying at first glance, but a pathologist experienced in oral tissue can reliably distinguish it from true dysplasia. If your report mentions reactive atypia, it means the cells looked abnormal due to inflammation, not because they were becoming cancerous.
- Ulceration. In more severe cases, the surface layer of the epithelium may be lost entirely, leaving an ulcer — an open sore that exposes the underlying tissue. This is a sign of significant active injury.
- Microorganisms. If an infection is the cause, the pathologist may see fungal organisms such as Candida within or on the surface of the epithelium. Viral infections such as herpes simplex virus (HSV) produce characteristic changes in the cells that the pathologist can recognise, and immunohistochemistry testing may be performed to confirm the specific virus.
What happens next?
Management of acute mucositis depends on the underlying cause and the severity of symptoms.
- Treatment-related mucositis (radiation or chemotherapy). There is no treatment that completely prevents mucositis in patients undergoing cancer therapy, but it can be managed effectively. Good oral hygiene — gentle brushing, regular rinsing with bland or salt-water mouth rinses — helps keep the mouth clean and reduces the risk of secondary infection. Pain can be managed with topical anaesthetics or systemic pain relief, depending on severity. Adequate nutrition and hydration are important; in severe cases, a temporary feeding tube may be needed. Most treatment-related mucositis resolves within two to four weeks after cancer therapy ends.
- Infective mucositis. Fungal mucositis caused by Candida is treated with antifungal medications, typically an oral rinse containing nystatin or a systemic antifungal such as fluconazole. HSV mucositis is treated with antiviral medications such as aciclovir or valaciclovir, which reduce the duration and severity of the outbreak. Treatment is most effective when started early.
- Trauma-related mucositis. If mucositis results from a physical injury — such as a poorly fitting dental appliance — removing the source of irritation is the primary step. The mucosa typically heals on its own once the injury is no longer ongoing.
Because a persistent or non-healing mouth sore can be a sign of a more serious condition, any area that does not improve after the expected healing time — usually two to three weeks — should be re-evaluated by your doctor or dentist, even if the initial biopsy showed only acute mucositis.
Questions to ask your doctor
- What caused the mucositis in my case — is it related to my cancer treatment, an infection, or something else?
- Does the biopsy show any signs of dysplasia or cancer?
- What is the best way to manage the pain and discomfort?
- Do I need treatment for an infection such as Candida or herpes simplex virus?
- How long should it take for the mucositis to heal?
- What should I do if the area does not heal or gets worse?
- Are there any mouth rinses or products I should use or avoid?
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