Probiotic Lozenges Cut Severe Mouth Sores in Head and Neck Radiotherapy Patients

For the hundreds of thousands of people diagnosed with head and neck cancer each year, radiation therapy offers a real chance of cure, but it exacts a brutal toll on the very tissues it passes through. Among the most feared consequences is oral mucositis, a condition in which the delicate lining of the mouth becomes inflamed, ulcerated, and in severe cases, covered with confluent pseudomembranes that make eating, drinking, and even speaking agonizing. Severe mucositis forces patients onto liquid diets and strong opioid analgesics, and it frequently interrupts or delays radiation treatment itself, potentially compromising cancer control. Now, a randomized controlled trial published in Nature Communications reports that a simple, inexpensive intervention, a probiotic lozenge containing the bacterium Limosilactobacillus reuteri, substantially reduced the incidence of severe oral mucositis in patients undergoing radiotherapy for malignant head and neck tumors.

The trial, conducted at West China Hospital of Sichuan University between March 2024 and January 2025, enrolled 160 patients and randomly assigned them to receive either L. reuteri probiotic lozenges or an indistinguishable placebo twice daily throughout their course of radiotherapy. The study was double-blind, meaning neither the patients nor the treating clinicians knew who received the active product, and it was registered as a phase II trial under the identifier NCT06285591. The primary endpoint was straightforward and clinically meaningful: the proportion of patients who developed severe oral mucositis, defined as grade 3 or higher according to the World Health Organization Oral Toxicity criteria, during or shortly after treatment.

The results were striking. Severe oral mucositis occurred in 37.5 percent of patients in the L. reuteri group compared with 61.3 percent of those receiving placebo. That difference corresponds to a relative risk of 0.61, with a 95 percent confidence interval of 0.44 to 0.85, and an absolute risk reduction of 23.8 percentage points, with a confidence interval spanning 8.7 to 38.8 percentage points. The primary endpoint was met with a P value of 0.004, indicating that the observed difference was unlikely to have arisen by chance. In practical terms, for roughly every four patients who received the probiotic lozenges rather than placebo, one additional patient avoided the most debilitating form of radiation-induced oral injury.

Beyond simply preventing severe disease, the probiotic appeared to reshape the entire trajectory of the condition. Patients in the L. reuteri arm experienced a delayed onset of severe mucositis, meaning they remained free of the worst toxicity for longer during their treatment course, and the duration of severe episodes was shortened compared with the placebo group. Secondary endpoints also examined the incidence, onset, and duration of oral mucositis of any grade, and safety outcomes were monitored throughout the study period. No significant safety concerns were observed, an important finding given that immunocompromised patients undergoing cancer treatment are sometimes considered a population in which live bacterial products warrant particular caution.

The biology behind this result is grounded in a growing understanding of the oral microbiome and its role in radiation injury. Radiotherapy to the head and neck disrupts the ecological balance of microorganisms living in the mouth, often favoring pathogenic species that colonize damaged epithelium, form biofilms, and amplify inflammatory signaling. L. reuteri is a commensal lactic acid bacterium that has been studied for decades for its immunomodulatory and antimicrobial properties. It produces reuterin, a broad-spectrum antimicrobial compound, and can modulate Toll-like receptor signaling and reduce pro-inflammatory cytokine production in epithelial and immune cells. By delivering the bacterium directly to the oral cavity in lozenge form, the trial’s designers aimed to reinforce a protective microbial presence precisely where the radiation damage occurs.

The mechanism of radiation-induced mucositis unfolds in predictable phases, and understanding them clarifies why a probiotic might intervene effectively. Within hours of the first radiation fractions, epithelial cells begin to die through apoptosis and mitotic catastrophe, triggering an inflammatory cascade involving tumor necrosis factor alpha, interleukin-1 beta, and interleukin-6. As fractions accumulate, the mucosal barrier breaks down, ulceration develops, and opportunistic bacteria invade the wounds, sustaining inflammation and pain through innate immune activation. A probiotic that limits pathogenic overgrowth, strengthens epithelial integrity, and dampens inflammatory signaling could plausibly interrupt this self-reinforcing cycle at multiple points, which is consistent with the reductions in both incidence and duration observed in the trial.

The clinical significance of the findings is difficult to overstate. Current international guidelines for the management of oral mucositis offer few truly preventive options. Basic oral care protocols, saline and baking soda rinses, and pain management form the backbone of standard practice, while more advanced interventions such as low-level laser therapy require specialized equipment and trained personnel that many centers lack. Palifermin, a keratinocyte growth factor, is approved only for narrow indications in hematopoietic stem cell transplantation and is costly. Against this backdrop, a twice-daily lozenge that is easy to administer, well tolerated, and inexpensive represents an unusually accessible addition to the supportive care arsenal, particularly for radiotherapy patients in resource-limited settings where severe mucositis often goes undertreated.

The trial’s design deserves attention as well. A sample size of 160 patients is respectable for a phase II study, and the choice of a hard, clinically defined primary endpoint, WHO grade 3 or worse mucositis, reduces the risk of subjective scoring inflating the result. The double-blind, placebo-controlled structure guards against expectation effects in both patient-reported symptoms and clinician assessments. That said, as a single-center trial conducted at one hospital in China, the findings will need replication in larger, multicenter populations before probiotic lozenges become a universal standard of care. Differences in radiotherapy techniques, concurrent chemotherapy regimens, baseline oral health, and dietary habits across populations could all influence how well the intervention generalizes.

There are also broader questions that follow naturally from this work. Which strains of L. reuteri were used, and whether strain-specific effects matter, will be of keen interest to microbiome researchers, since probiotic effects are notoriously strain-dependent. The trial’s supplementary materials and transparent peer review file, published alongside the article, will allow the scientific community to scrutinize the randomization, blinding, and statistical analyses in detail. The study was supported by a range of Chinese national and provincial research programs, and the authors declared no competing interests, factors that strengthen confidence in the independence of the reported results.

For patients facing head and neck radiotherapy, the message from this trial is cautiously hopeful. A live microorganism, taken as a simple lozenge during treatment, cut the risk of one of oncology’s most painful toxicities by nearly a quarter in absolute terms, delayed its arrival, and shortened its course, all without detectable harm. If larger trials confirm these results, L. reuteri could join the short list of evidence-based mucositis preventives and change routine supportive care for head and neck cancer worldwide. The study is a vivid illustration of a theme running through modern medicine: that the trillions of microbes sharing our bodies are not passive bystanders but active partners whose careful management can meaningfully alter the course of even the harshest cancer treatments.

Subject of Research: Probiotic prevention of radiation-induced oral mucositis in head and neck cancer radiotherapy

Article Title: Limosilactobacillus reuteri for oral mucositis in patients undergoing radiotherapy for malignant head and neck tumors: a randomized controlled trial

Article References: Gao, K., Li, Y.-T., Pei, Y.-Y., Li, Z.-H., He, J.-G., Jiang, Z., Liu, Z.-R., Min, Y., Li, Z.-X., Ai, P., Ji, S.-M., Guo, W.-Y., & Peng, X.-C. (2026). Limosilactobacillus reuteri for oral mucositis in patients undergoing radiotherapy for malignant head and neck tumors: a randomized controlled trial. Nature Communications. https://doi.org/10.1038/s41467-026-78088-6

Image Credits: AI Generated

DOI: 10.1038/s41467-026-78088-6

Keywords: Limosilactobacillus reuteri, oral mucositis, head and neck cancer, radiotherapy, probiotics, randomized controlled trial, oral microbiome, supportive care, cancer therapy, mucositis prevention, clinical trial, Nature Communications