A Probiotic Lozenge May Help Protect the Mouth During Head and Neck Cancer Radiotherapy

Radiation therapy for head and neck cancers is effective at targeting tumors, but it often damages healthy tissue lining the mouth and throat along the way. The result, known as oral mucositis, can range from mild redness to painful ulcers that make it difficult to eat, speak, or swallow. In its severe form, it's one of the most common reasons treatment gets interrupted or delayed. A newly published randomized controlled trial in Nature Communications looked at whether a probiotic lozenge containing the bacterium Limosilactobacillus reuteri could reduce how often this severe reaction occurs.

What the researchers tested

The study, conducted at West China Hospital of Sichuan University between March 2024 and January 2025, enrolled 160 patients undergoing radiotherapy for malignant head and neck tumors. Participants were randomly assigned, in double-blind fashion, to take either L. reuteri lozenges or an identical-looking placebo twice a day throughout their radiation treatment. Neither the patients nor the research team knew who was receiving which product until the data were analyzed, a design that helps guard against bias in reporting symptoms.

The main outcome the researchers tracked was the incidence of severe oral mucositis, defined using the World Health Organization's grading system as grade 3 or higher. They also looked at secondary measures: how quickly severe mucositis appeared, how long it lasted, and the rates of any-grade mucositis, not just the severe cases.

What they found

Patients who took the probiotic lozenge fared notably better. Severe oral mucositis occurred in 37.5% of the L. reuteri group compared with 61.3% of those on placebo. Statistically, this translated to a relative risk of 0.61, meaning the probiotic group had roughly 39% lower risk of developing severe mucositis compared with placebo. In absolute terms, the difference amounted to about 24 fewer cases per 100 patients treated, a gap the researchers considered both statistically and clinically meaningful (P = 0.004).

Beyond simply reducing how often severe mucositis appeared, the lozenge also seemed to change its course. Onset was delayed in the treatment group, and when severe mucositis did develop, it tended to resolve faster than in patients who received placebo. Rates of any-grade oral mucositis, encompassing milder cases as well, were also assessed as secondary measures, adding to the overall picture of a protective association between the probiotic and mouth tissue damage from radiation.

Safety monitoring throughout the trial did not turn up meaningful concerns, suggesting that the lozenge was reasonably well tolerated by this patient group during the study period.

Why the microbiome might matter here

L. reuteri is a bacterial species naturally found in the human gut and, in smaller amounts, in the mouth. Researchers have been interested in certain lactobacillus strains for their potential to influence local immune responses and support the balance of microbial communities on mucosal surfaces. The idea behind this trial was that supplementing the oral cavity with a beneficial strain during radiation treatment, when tissue is under heavy oxidative and inflammatory stress, might help the mucosal lining hold up better or heal more efficiently. The trial results are consistent with that idea, though the study itself was designed to test clinical outcomes rather than pin down the precise biological mechanism at play.

Reading the results with the right caveats

This was a phase II trial with 160 participants at a single hospital, which is a meaningful sample for early-stage evidence but still limited in scope. Larger, multi-center trials would help confirm whether the benefit holds across different patient populations, radiation protocols, and cancer types. The study also focused specifically on patients with malignant head and neck tumors undergoing radiotherapy, so the findings shouldn't be assumed to extend automatically to other groups or other triggers of mucositis.

A lower incidence of severe mucositis in this trial reflects a statistical association between probiotic use and clinical outcomes under controlled study conditions, not a guarantee of benefit for any individual patient. Factors like radiation dose, cancer stage, overall health, and concurrent chemotherapy can all influence how someone's mouth responds to treatment.

What this could mean going forward

If these findings are replicated in larger studies, probiotic lozenges could become a simple, low-cost addition to supportive care protocols for people undergoing head and neck radiotherapy, potentially reducing treatment interruptions and improving quality of life during a physically demanding course of treatment. For now, this remains a single trial, and decisions about supplements or interventions during cancer treatment should be made in conversation with an oncologist or treating physician who understands a patient's full medical picture. Anyone currently undergoing radiotherapy who is curious about this approach should bring it up with their own care team rather than starting a new supplement independently.

Source: Nature Communications, DOI: 10.1038/s41467-026-78088-6, based on a randomized controlled trial by Gao, Li, Pei, and colleagues.