Nasal Probiotics for Allergic Rhinitis: Trial Finds No Relief

Spraying probiotics into the nose did not relieve allergic rhinitis in the only human randomized trial to test the idea. The result matters because the treatment targeted the inflamed tissue directly: rather than swallowing bacteria and hoping for an effect through the gut, participants sprayed live bacteria onto the nasal lining. After three weeks, symptoms, quality of life, nasal airflow, and exhaled nitric oxide did not improve versus placebo. Cultures also found that the bacteria did not establish themselves in the nose.

That leaves nasal probiotic sprays as a proof of concept with an appealing theory, not a treatment for hay fever. The trial was small, and its challenge model cannot reproduce an entire pollen season. But a probiotic that does not persist at its intended site has a basic delivery problem before it has an immune problem.

A clever route met an inhospitable nose

In Mårtensson et al.'s 2022 randomized, double-blind, placebo-controlled crossover trial, 24 adults with seasonal allergic rhinitis used either a probiotic assemblage or a matched placebo for three weeks, then crossed over after a four-week washout. Most participants (95.8%) were sensitized to timothy grass pollen. Their median age was 27 years, and their median total IgE was 89.9 kU/L.

The active spray combined Lactobacillus rhamnosus SP1, L. paracasei 101/37, and Lactococcus lactis L1A in equal proportions. Each dose contained 1.9×10¹⁰ colony-forming units. Participants delivered 200 μl into each nostril twice daily.

The rationale was straightforward. Allergic rhinitis is largely driven by type-2 immune activity in the nasal mucosa. Some lactic acid bacteria can stimulate immune pathways that might counter that pattern, so placing them directly on the tissue could, in theory, produce a local effect. It also skips the long and biologically indirect trip through the gut.

The nasal cavity, however, clears material efficiently through mucus and ciliary movement. Middle-meatus cultures taken with ESwabs found no persistent colonization by the probiotic assemblage. The study therefore tested repeated, temporary bacterial exposure rather than a resident nasal probiotic community.

No benefit on the outcomes patients notice

During each treatment period, participants underwent a standardized nasal allergen challenge daily for seven consecutive days. The investigators pre-specified four clinical outcomes: the Mini-Rhinoconjunctivitis Quality of Life Questionnaire, Total Nasal Symptom Score, peak nasal inspiratory flow, and fractional exhaled nitric oxide.

The probiotic arm did not produce a statistically significant improvement on any of them. The publication does not provide an effect estimate and confidence interval in the evidence summarized here, so the trial cannot tell us precisely how large a small benefit or harm it may have missed. With only 24 participants, it was poorly equipped to detect modest effects. It does show that this formulation did not generate a detectable clinical signal under the conditions tested.

The one immunologic difference was a modest increase in IL-17A in nasal lavage fluid during probiotic treatment. IL-17A is a biomarker, not symptom relief. The finding could fit transient innate recognition of bacterial cell-wall components, including signaling through pattern-recognition receptors. It does not demonstrate lasting Th1 or regulatory T-cell reprogramming, and the trial found no accompanying improvement in nasal symptoms or airflow.

The challenge model could detect allergic activity

A null result can mean either that a treatment failed or that the test could not see a real effect. The latter explanation looks less persuasive here. Linton et al.'s 2025 characterization of nasal allergen challenge found that the model reliably produces early allergic mediators—including histamine, tryptase, prostaglandins, and leukotrienes—within minutes. Nasal symptoms became pronounced within 15 minutes, while type-2 cytokines in nasal secretions remained elevated for up to 24 hours.

That does not make a seven-day challenge identical to life during pollen season. It does mean the model can provoke and measure the allergic response that an effective intervention would need to change. In the probiotic trial, neither patient-facing outcomes nor most measured immune outcomes moved in the desired direction.

There is still a specific limitation worth keeping. Zemelka-Wiacek et al. in 2025 compared nasal allergen challenge and an allergen exposure chamber with in-field symptoms in people allergic to timothy grass pollen. The controlled models diverged meaningfully from natural exposure, which is cumulative, variable, and prolonged. A treatment that required weeks of natural allergen co-exposure to build regulatory immune responses might look weaker in a short, out-of-season challenge protocol.

That caveat justifies better trials of a genuinely colonization-capable product, if one exists. It does not explain away the present result. The bacteria failed to persist in the nasal cavity, which undercuts the proposed mechanism before questions about season-long exposure arise.

The preclinical rationale did not match the route

The trial's biological premise drew partly on mouse studies in which probiotics reduced allergic immune signals, including IL-5 and IL-13. But those experiments used oral or systemic administration, not intranasal lactobacilli. Moving from an oral or systemic mouse intervention to a nasal spray in humans is not a minor formulation change; it is a different exposure route, tissue environment, and clearance problem.

Human evidence also suggests that microbiome effects on allergic rhinitis may operate through broader systemic pathways. In a prospective 2025 cohort study by Huo et al., 300 adults with allergic rhinitis eligible for allergen immunotherapy and 150 healthy controls provided stool and blood samples. Baseline gut microbiome composition and diversity predicted immunotherapy response. That is an association, not proof that changing gut bacteria improves treatment response. Still, it points toward gut-derived immune signaling—such as short-chain fatty acids, dendritic-cell education, and regulatory T-cell activity—rather than a brief layer of lactobacilli on the nasal surface.

What successful immune treatment looks like

Allergen immunotherapy has a more established rationale because it repeatedly exposes the immune system to the relevant allergen and can promote allergen-specific tolerance over time. In a 121-person randomized trial of cat-allergic patients, Corren et al. tested tezepelumab, subcutaneous cat allergen immunotherapy, their combination, or placebo for 52 weeks, followed by 52 weeks of observation. At week 52, the combination reduced nasal allergen challenge-induced Total Nasal Symptom Scores during the first hour more than the comparators.

Tezepelumab blocks TSLP, an epithelial cytokine involved in initiating and sustaining allergic responses. Pairing that blockade with allergen immunotherapy aims at both upstream type-2 signaling and allergen-specific tolerance. It is a useful contrast with the nasal probiotic result: an isolated IL-17A increase is not comparable to sustained changes that translate into lower challenge symptoms.

Other immunotherapy research also uses nasal allergen challenge to track response. A 2024 randomized trial of German cockroach subcutaneous immunotherapy in urban children with asthma and a 2026 prospective study of ultra-rush dust-mite immunotherapy in 57 adults illustrate how investigators use challenge outcomes alongside symptoms, quality of life, and immune measures. These interventions differ substantially from a three-week probiotic spray in population, allergen, duration, and mechanism, so they do not prove that a nasal probiotic should work.

What the trial can—and cannot—settle

• It can rule against this tested approach: standard lactic acid bacteria sprayed twice daily for three weeks did not improve the trial's pre-specified clinical outcomes and did not colonize the nasal mucosa.
• It cannot exclude every possible effect: 24 participants leave wide uncertainty around small-to-moderate benefits, especially for secondary immune outcomes.
• The crossover design has a washout question: a four-week washout may be too short if a future formulation causes durable immune priming. In this trial, failed colonization makes meaningful carryover less plausible.
• Rescue medication could blur differences: participants could use desloratadine 5 mg as needed during challenge periods, although the investigators monitored its use.
• Baseline nasal microbiomes were not characterized: resident bacteria could affect how an introduced strain behaves, and the study could not assess that possibility.

A nasal probiotic spray for allergic rhinitis has one human randomized trial behind it, and that trial found no clinical benefit or persistent nasal colonization.

For now, do not buy nasal probiotic sprays expecting relief from hay fever or perennial allergic rhinitis. The larger hypothesis—that microbial ecology can influence allergic disease—remains plausible, particularly through gut-systemic immune pathways. The specific nasal-lactobacillus strategy failed its first human test because the organisms did not stay where the treatment needed them to work. A future study would first need to show durable, safe nasal persistence, then demonstrate symptom relief in a sufficiently large trial under natural seasonal exposure.