The trial enrolled 106 adults who were scheduled to receive either an autologous transplant, in which patients receive their own stem cells, or an allogeneic transplant, in which cells come from a donor. Participants were randomly assigned in equal numbers to two groups. Fifty-four patients received a mouthwash containing 600 milligrams of N-acetylcysteine, used every six hours for a total daily dose of 2,400 milligrams, while fifty-two received an identical-looking placebo. The rinsing regimen began on the first day of conditioning therapy, the high-dose chemotherapy phase that destroys the bone marrow in preparation for the transplant, and continued until day fourteen after the transplant or until neutrophil engraftment, the point at which new white blood cells begin to recover, whichever came first.

To assess the outcome, researchers examined each patient’s mouth daily using the World Health Organization Oral Toxicity Scale, a standard grading system that ranges from grade 0, indicating a healthy oral mucosa, to grade 4, indicating the most severe ulceration that prevents any oral intake. The primary endpoints were the incidence of mucositis of any grade and the incidence of severe mucositis, defined as grades 3 and 4 combined. The analysis followed the intention-to-treat principle, meaning that all randomized patients were included regardless of whether they completed the full course of treatment, a methodological choice that guards against bias from excluding patients who fare poorly.

The headline finding concerns severity rather than frequency. Mucositis of any grade developed in 61.1 percent of the NAC group, thirty-three of fifty-four patients, compared with 73.1 percent of the placebo group, thirty-eight of fifty-two patients, a difference that did not reach statistical significance. But when the researchers focused on severe disease, the picture changed dramatically. Only 5.6 percent of patients in the NAC group, three patients, developed grade 3 or 4 mucositis, compared with 25.0 percent of the placebo group, thirteen patients, a difference that was highly statistically significant. The overall distribution of WHO grades also favored the NAC group, indicating that even when mucositis occurred, it tended to be milder in patients who had used the antioxidant rinse.

The biology behind this result centers on reactive oxygen species. Conditioning chemotherapy generates a flood of free radicals in rapidly dividing tissues, and the oral epithelium, which renews itself every one to two weeks, is particularly vulnerable. This oxidative burst triggers a cascade of inflammatory signaling, tissue injury, and ulceration. N-acetylcysteine is a direct scavenger of free radicals and, crucially, a precursor for glutathione, the cell’s principal endogenous antioxidant. By replenishing glutathione stores in mucosal cells, NAC can theoretically blunt the oxidative damage that initiates mucositis before it spirals into deep ulceration. Laboratory studies in cell cultures and animal models of radiation-induced oral mucositis have supported this mechanism, and the new trial provides some of the strongest clinical evidence to date in the transplant setting.

Safety results were reassuring. The mouthwash was well tolerated, and gastrointestinal adverse effects, the most common complaints associated with NAC, were mild and infrequent. This tolerability profile matters because transplant recipients already take a heavy burden of medications, and any prophylactic intervention must avoid adding toxicity. The authors also note that NAC is inexpensive and widely available, which distinguishes it from palifermin, a recombinant growth factor used for mucositis prevention that carries a substantial cost, and from other agents with weaker or mixed evidence bases.

The trial’s multivariable analysis, which adjusted for several factors simultaneously, identified four predictors of higher odds of developing mucositis: undergoing an allogeneic rather than an autologous transplant, receiving myeloablative conditioning, the most intensive form of chemotherapy preparation, suboptimal adherence to the mouthwash regimen, and assignment to the placebo group. These findings align with established clinical experience. Donor transplants involve more aggressive conditioning and immune complications, myeloablative regimens inflict more tissue damage than reduced-intensity ones, and adherence is a perennial challenge for any intervention that requires patients to rinse every six hours for weeks while feeling profoundly unwell.

The study is not without limitations, and the authors are candid about them. It was conducted at a single center, which raises questions about generalizability to other populations, conditioning regimens, and formulations. The formulation of the NAC mouthwash itself was not standardized across a commercial platform, and compounded preparations can vary in stability and palatability, factors that may influence adherence. The severity endpoint, while clinically meaningful, was a secondary focus of the trial’s design, and the overall incidence result was null. The authors therefore call for multicenter trials with standardized formulations and, importantly, the inclusion of patient-reported outcomes, since mucositis pain and its impact on eating and quality of life are best understood from the patient’s own perspective. The trial was registered prospectively with ClinicalTrials.gov and the Iranian Registry of Clinical Trials, and it was reported according to CONSORT guidelines for randomized controlled trials.

Context matters for interpreting why this trial is generating attention. Oral mucositis affects a large majority of transplant recipients in published series, and its direct costs, encompassing pain management, nutritional support, and extended hospitalization, are substantial according to systematic reviews of mucositis management across cancer care. Current international clinical practice guidelines from the Multinational Association of Supportive Care in Cancer and the International Society of Oral Oncology recommend several preventive approaches, including oral cryotherapy and low-level laser therapy, but options remain limited, and many recommended interventions require equipment or expertise not available everywhere. An earlier Iranian trial published in Bone Marrow Transplant tested oral NAC for the same indication and suggested benefit, and the new study extends that line of evidence with a larger sample and a rigorous placebo-controlled design.

The practical takeaway is that a drug costing pennies per dose, best known to the public as a mucolytic agent used in respiratory conditions and as an antidote for acetaminophen overdose, may meaningfully reduce the most disabling complication of transplant conditioning. Severe mucositis is what forces patients onto feeding tubes and intravenous opioids, and cutting its rate from one in four to roughly one in twenty, as observed in this trial, would represent a meaningful improvement in supportive care if confirmed. The researchers emphasize that NAC mouthwash should be viewed as a practical, low-risk strategy rather than a complete solution, since it did not prevent mucositis from developing in the first place. Larger, multicenter studies will determine whether this simple rinse earns a place in standard transplant protocols worldwide, but for a patient population with few good options, the prospect of a cheap, safe, and effective preventive measure is a development worth watching closely.

Subject of Research: N-acetylcysteine mouthwash for prevention of oral mucositis in hematopoietic cell transplant recipients

Article Title: N-acetylcysteine mouthwash for prevention of oral mucositis in hematopoietic cell transplant recipients: a double-blind randomized controlled trial

Article References: Tahmasebi, M., Kaveh-Ahangaran, R., Barkhordar, M., Rad-Malekshahi, M., Vaezi, M., Sharifi Aliabadi, L., Zarabadi, M. S., & Shahrami, B. (2026). N-acetylcysteine mouthwash for prevention of oral mucositis in hematopoietic cell transplant recipients: a double-blind randomized controlled trial. Supportive Care in Cancer, 34(10), Article 1054. https://doi.org/10.1007/s00520-026-11286-6

Image Credits: AI Generated

DOI: 10.1007/s00520-026-11286-6

Keywords: hematopoietic cell transplantation, oral mucositis, N-acetylcysteine, mouthwash, randomized controlled trial, supportive care, conditioning chemotherapy, oxidative stress, glutathione, WHO Oral Toxicity Scale, stem cell transplant, cancer supportive care