Probiotics and Seasonal Allergies

Spring shows up, the trees do their thing, and for a lot of people the air stops being neutral. Can a probiotic help? Here is what the research actually says.

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There is a tidy story that gets told about this: gut to immune system to nose, one clean arrow. The arrow is not imaginary. It is also not as short or as straight as it looks.

So here is the evidence, laid out — the trials that found something, the trials that did not, what happens when researchers pool them, and what the allergy guidelines have to say. Some of it is genuinely interesting. Some of it is thinner than it first appears.

This article is about research, not treatment. Flourish is a dietary supplement, not a medicine. Nothing here is intended to diagnose, treat, cure or prevent any condition. Seasonal allergies are a medical condition — please talk to your doctor about managing yours.

What is actually happening in allergy season

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Grass, tree and weed pollens are, biologically speaking, extremely boring. They are plant reproductive material. They are not infectious, not toxic, and for most of the planet they pass through without incident.

In seasonal allergic rhinitis, the immune system files them under threat anyway. It produces IgE antibodies against specific pollen proteins, and those antibodies take up residence on mast cells lining the nose and eyes. The next time that pollen arrives, it cross-links the antibodies and the mast cells release histamine and other mediators within minutes. Hours later a second, slower wave of inflammatory cells arrives. That two-stage sequence is why symptoms start fast and then linger.^1,2

The immunology underneath is usually described as a Th2-skewed response — one arm of the immune system running louder than the other, with too little of the regulatory signaling that normally says this is pollen, stand down.^2,3

The problem is not the pollen. It is the filing decision.

Where the gut comes into it

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Here is the observation that started all of this. In 1989, an epidemiologist noticed that hay fever was less common in children with more older siblings, and floated the idea that early infections passed around a large family might be protective.⁴ That became the hygiene hypothesis, and it has been quoted ever since — usually in a form its author never quite proposed.

The modern version is better and more interesting. It is not about being dirty. It is about the range of microbes a developing immune system meets. A study of more than 6,800 European children found that the diversity of microbial exposure — not the quantity — was inversely related to asthma and atopy.⁵

That distinction does a lot of work. The environmental-exposure evidence is strong. Whether taking a probiotic later in life reproduces any part of it is a separate question, and it has a separate and much thinner answer.⁷

What the trials actually found

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There are dozens of randomized trials. Three are worth knowing in detail, because they show the shape of the whole field.

The best-designed adult trial

A trial at the University of Florida gave 173 adults with self-reported seasonal allergies either a three-strain combination — Lactobacillus gasseri KS-13, Bifidobacterium bifidum G9-1 and B. longum MM-2, 3 billion CFU a day — or a placebo, for eight weeks through spring pollen season. It was triple-masked and registered in advance.⁸

The result: quality-of-life scores improved more in the probiotic group, and the difference was statistically significant (p = 0.0092). The probiotic group improved by 0.68 points on a 0–6 scale; the placebo group by 0.19. So the difference attributable to the product was 0.49 points.⁸

The children’s trial

Forty Italian children with pollen-driven allergic rhinitis and intermittent asthma received either a three-strain Bifidobacterium mixture or placebo, one sachet a day for four weeks. Symptoms and quality of life both improved significantly in the probiotic group, and both worsened in the placebo group over the same period — which is what a pollen season does if nothing intervenes. The between-group comparison favored the probiotic on every measure.¹⁰

One detail from that trial deserves to be repeated, because it gets left out a lot: the use of rescue medication was, in the authors’ word, overlapping between the two groups. The children on the probiotic did not use less of it.¹⁰

The eyes-but-not-the-nose trial

A Japanese trial gave 40 people with cedar pollinosis yogurt containing Bifidobacterium longum BB536, 200 g a day for fourteen weeks. Eye symptoms improved significantly (odds ratio 0.31, 95% CI 0.10–0.97, p = 0.044). Nasal and throat symptoms did not reach significance — the paper describes them as tending to improve. Blood eosinophils rose in both groups over the season, significantly so only in the placebo group.¹¹

An eye-only result is a real pattern in this literature rather than a fluke. The largest single trial in the field — 425 adults on Lactobacillus paracasei LP-33 alongside their antihistamine — found the same split: overall quality of life improved by 0.286 points (p = 0.026), ocular symptoms improved, and nasal symptoms did not change at all (p = 0.13).¹²

And when you pool them all

This is where it gets harder. A 2022 meta-analysis of 28 studies found symptoms improved (standardized mean difference −0.29) and quality of life improved (−0.64) — but with heterogeneity of 89% and 97% respectively. At that level, the individual trials are measuring meaningfully different things, and a pooled average is a weak summary of them. The authors’ own conclusion is that clinicians “should be cautious when recommending probiotics in treating allergic rhinitis.”¹³

And the frequently repeated line that “eight of nine probiotics were effective” comes from a 2022 systematic review that did say exactly that — about nine probiotic types, counting any trial that improved at least one symptom. The same paper ran the actual numbers on the two strains with enough trials to pool, and neither reached significance.¹⁶

What the guidelines say

None of that makes the trials fake. It means the field has produced a lot of small, varied, mostly short studies of different organisms, and the bodies whose job is to weigh evidence have looked at the pile and declined to make a recommendation either way.

What conventional treatment looks like

Worth setting out accurately, since it is the comparison anyone weighing a probiotic is really making.

Intranasal corticosteroids

These are the most effective single class for allergic rhinitis and the first-line recommendation in the major guidelines.^18,19 The labeled adverse reactions occurring in more than 3% of patients are headache, sore throat, nosebleed, nasal burning or irritation, nausea or vomiting, asthma symptoms and cough — and the placebo rates sit close behind most of them. For fluticasone propionate, headache is 16.1% on drug against 14.6% on placebo, and cough 3.8% against 2.8%. Nosebleed is the one with a clearer separation, 6.9% against 5.4%. The label itself notes that adverse reactions were reported at approximately the same frequency by people taking placebo, and that fewer than 2% of trial participants stopped because of them.²²

Antihistamines, and a distinction that gets muddled

There are two generations, and they are not interchangeable on safety.

If you have read that a common allergy tablet causes long-term cognitive problems, it is worth checking which generation the claim was about. The anticholinergic research points at the older drugs.^23,25,26

Decongestants

Oral and nasal decongestants work quickly and are the class with the most restrictions. Labels direct people with heart disease, high blood pressure, thyroid disease, diabetes or prostate enlargement to ask a doctor first, and contraindicate use with MAOI antidepressants. The FDA recommends against over-the-counter cough and cold medicines in children under two, and manufacturers advise against them under four.²⁷ In 2024 the FDA proposed removing oral phenylephrine from the over-the-counter monograph — on effectiveness grounds, not safety.²⁸

Where Flourish fits

Straight answer: Flourish has not been tested for seasonal allergies, and we are not going to imply otherwise by standing next to somebody else’s trial.

Our cultures are identified at the species level — Lactobacillus acidophilus, Bifidobacterium longum and so on — without individual strain designations. Given how strain-specific this evidence is,^20,21 that means the results above belong to the strains that were studied, and none of those strains is ours.

If you want to take a probiotic through allergy season, take it because you want the digestive and microbiome support, and take it consistently — that is the part that actually matters. Laboratory work on gastric transit suggests taking a non-coated probiotic with a meal, or about 30 minutes before one, and that a meal containing some fat helps more than one without.²⁹

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A few questions we hear a lot

Will a probiotic help my seasonal allergies?

Nobody can tell you that, and anyone who does is ahead of the evidence. Several trials of specific named strains have reported improvements in quality-of-life scores, mostly modest, and often in eye symptoms more than nasal ones.^8,11,12 Pooled analyses are highly inconsistent,¹³ the objective markers are null,¹⁴ and no major allergy guideline recommends probiotics for this.^7,17,18 Talk to your doctor.

Does it matter which probiotic?

Enormously. Efficacy in this literature is strain-specific, and within one species some strains perform and others do not for the same use.^20,21 A result for a named, deposited strain is a result for that strain only.

Has Flourish been tested for allergies?

No. We have not run that trial, and none of the studies described here used Flourish, any Flourish culture at strain level, or a liquid probiotic at all.

Should I stop my allergy medication and take a probiotic instead?

No — and please do not change any medication without talking to your doctor. Intranasal corticosteroids are the most effective class available and are first-line in the guidelines.^18,19 Notably, the probiotic trials in this area were run in people who were already on their usual treatment, as an addition to it, not a replacement.^10,12

Is the hygiene hypothesis why I have hay fever?

The current framing is about the diversity of microbes a developing immune system encounters rather than cleanliness as such,^5,30 and the strongest evidence concerns childhood environment — farm dust, animals, the air in the house.⁶ It is a compelling body of work about early life. It has not been shown to translate into a benefit from taking a supplement as an adult.

When should I start if I want to try one?

The trials that reported benefits generally started before the season and ran through it — eight weeks in the adult trial, four in the children’s trial, fourteen in the yogurt trial.^8,10,11 Consistency mattered more than timing in every case.

Sources

1. Bousquet, J., Anto, J. M., Bachert, C., et al. (2020). Allergic rhinitis. Nature Reviews Disease Primers, 6(1), 95 — the current authoritative review of allergic rhinitis: IgE sensitization, mast cell activation, early and late phase responses, and treatment. Source
2. Eifan, A. O., & Durham, S. R. (2016). Pathogenesis of rhinitis. Clinical & Experimental Immunology, 46(9), 1139–1151 — mechanistic account of the Th2 response, IgE and regulatory T cells in allergic rhinitis. Source
3. Wheatley, L. M., & Togias, A. (2015). Clinical practice: allergic rhinitis. New England Journal of Medicine, 372(5), 456–463 — clinical overview of diagnosis and management. Source
4. Strachan, D. P. (1989). Hay fever, hygiene, and household size. BMJ, 299(6710), 1259–1260 — the original observation linking hay fever prevalence to number of older siblings. Source
5. Ege, M. J., Mayer, M., Normand, A. C., et al. (2011). Exposure to environmental microorganisms and childhood asthma. New England Journal of Medicine, 364(8), 701–709 — in 6,843 children, the diversity of microbial exposure was inversely related to asthma and atopy. Source
6. Stein, M. M., Hrusch, C. L., Gozdz, J., et al. (2016). Innate immunity and asthma risk in Amish and Hutterite farm children. New England Journal of Medicine, 375(5), 411–421 — asthma prevalence was about four times lower among Amish schoolchildren; house dust extracts from Amish homes protected mice from airway hyperreactivity, and Hutterite dust did not. Source
7. Fiocchi, A., Pawankar, R., Cuello-Garcia, C., et al. (2015). World Allergy Organization–McMaster University guidelines for allergic disease prevention (GLAD-P): probiotics. World Allergy Organization Journal, 8(1), 4 — three conditional recommendations, all at very low quality of evidence, with benefit confined to eczema; “there is lack of evidence that probiotics prevent any other allergy”, and no effect on development of allergic rhinitis. Source
8. Dennis-Wall, J. C., Culpepper, T., Nieves, C. Jr., et al. (2017). Probiotics (Lactobacillus gasseri KS-13, Bifidobacterium bifidum G9-1, and Bifidobacterium longum MM-2) improve rhinoconjunctivitis-specific quality of life in individuals with seasonal allergies. American Journal of Clinical Nutrition, 105(3), 758–767 — 173 adults, 3 billion CFU/day for 8 weeks; MRQLQ global score improved −0.68 vs −0.19 on placebo (p = 0.0092); serum total IgE and regulatory T cell percentage rose in both groups with no difference between them; the authors concluded the mechanism “is still unclear.” Source
9. Juniper, E. F., Guyatt, G. H., Willan, A., & Griffith, L. E. (1994). Determining a minimal important change in a disease-specific quality of life questionnaire. Journal of Clinical Epidemiology, 47(1), 81–87 — establishes the minimal important difference at approximately 0.5 per item for this family of instruments. See also Juniper, E. F., et al. (2000), Clinical & Experimental Allergy, 30(1), 132–140, for the mini Rhinoconjunctivitis Quality of Life Questionnaire itself. Source
10. Miraglia Del Giudice, M., Indolfi, C., Capasso, M., Maiello, N., Decimo, F., & Ciprandi, G. (2017). Bifidobacterium mixture (B. longum BB536, B. infantis M-63, B. breve M-16V) treatment in children with seasonal allergic rhinitis and intermittent asthma. Italian Journal of Pediatrics, 43(1), 25 — 40 children, one sachet daily for 4 weeks; symptoms and quality of life improved on probiotic (p < 0.005 and p < 0.001) and worsened on placebo; intergroup analysis favored probiotic on all parameters; “the use of rescue medications was overlapping in the two groups.” Source
11. Xiao, J. Z., Kondo, S., Yanagisawa, N., et al. (2006). Effect of probiotic Bifidobacterium longum BB536 in relieving clinical symptoms and modulating plasma cytokine levels of Japanese cedar pollinosis during the pollen season. Journal of Investigational Allergology and Clinical Immunology, 16(2), 86–93 — 40 subjects, 200 g yogurt daily for 14 weeks; eye symptoms improved (odds ratio 0.31, 95% CI 0.10–0.97, p = 0.044), nasal and throat symptoms did not reach significance; blood eosinophil rates rose in both arms, significantly only in placebo. The authors were employees of the manufacturer. Source
12. Costa, D. J., Marteau, P., Amouyal, M., et al. (2014). Efficacy and safety of the probiotic Lactobacillus paracasei LP-33 in allergic rhinitis: a double-blind, randomized, placebo-controlled trial (GA²LEN study). European Journal of Clinical Nutrition, 68(5), 602–607 — 425 subjects, all taking loratadine; RQLQ global score improved by 0.286 points (95% CI 0.035–0.536, p = 0.0255) and ocular symptoms by 0.409 (p = 0.0029), while the rhinitis total symptom score did not differ (p = 0.1288). Source
13. Luo, C., Peng, S., Li, M., Ao, X., & Liu, Z. (2022). The efficacy and safety of probiotics for allergic rhinitis: a systematic review and meta-analysis. Frontiers in Immunology, 13, 848279 — 28 studies; symptoms SMD −0.29 (I² = 89%), RQLQ SMD −0.64 (I² = 97%), total IgE SMD 0.09 (I² = 0%, not significant), specific IgE SMD −0.03 (p = 0.72); the authors advise that “clinicians should be cautious when recommending probiotics in treating allergic rhinitis.” Source
14. Yan, S., Ai, S., Huang, L., et al. (2022). Systematic review and meta-analysis of probiotics in the treatment of allergic rhinitis. Allergologia et Immunopathologia, 50(3), 24–37 — 30 randomized trials, 2,708 patients; quality-of-life scores improved, while eosinophil count (mean difference −0.09, p = 0.82), total IgE, specific IgE, total symptom score and eye symptom score were all non-significant. Source
15. Zajac, A. E., Adams, A. S., & Turner, J. H. (2015). A systematic review and meta-analysis of probiotics for the treatment of allergic rhinitis. International Forum of Allergy & Rhinology, 5(6), 524–532 — 23 studies, 1,919 patients; quality of life improved (SMD −2.23, p = 0.02) while the rhinitis total symptom score did not (SMD −0.36, p = 0.13) and total IgE did not (p = 0.94). Source
16. Farahmandi, K., Mohr, A. E., & McFarland, L. V. (2022). Effects of probiotics on allergic rhinitis: a systematic review and meta-analysis of randomized clinical trials. American Journal of Rhinology & Allergy, 36(4), 440–450 — 13 trials, 1,591 participants; “8 of 9 probiotic types alleviated at least one clinical symptom”, but in the pooled analysis neither L. paracasei Lp33 (SMD −1.61, 95% CI −4.67 to 1.45) nor L. rhamnosus GG (SMD −1.00, 95% CI −3.01 to 1.00) showed significant efficacy on nasal symptom scores. Source
17. Wang, H. Z., Hayles, E. H., Fiander, M., Sinn, J. K., & Osborn, D. A. (2025). Probiotics in infants for prevention of allergic disease. Cochrane Database of Systematic Reviews, 6(6), CD006475 — 24 studies, 7,077 mother–infant pairs; allergic rhinitis risk ratio 0.89 (95% CI 0.45–1.77), low-certainty evidence; “there is insufficient evidence to make conclusions about the effect of probiotics and synbiotics on preventing the development of allergic diseases.” Source
18. Dykewicz, M. S., Wallace, D. V., Amrol, D. J., et al. (2020). Rhinitis 2020: a practice parameter update. Journal of Allergy and Clinical Immunology, 146(4), 721–767 — the US practice parameter; intranasal corticosteroids are recommended as first-line for allergic rhinitis. The document does not address probiotics. Source
19. Brożek, J. L., Bousquet, J., Agache, I., et al. (2017). Allergic Rhinitis and its Impact on Asthma (ARIA) guidelines — 2016 revision. Journal of Allergy and Clinical Immunology, 140(4), 950–958 — addresses oral and intranasal H1-antihistamines, intranasal corticosteroids and leukotriene receptor antagonists; probiotics are outside its scope. Source
20. McFarland, L. V., Evans, C. T., & Goldstein, E. J. C. (2018). Strain-specificity and disease-specificity of probiotic efficacy: a systematic review and meta-analysis. Frontiers in Medicine, 5, 124 — 228 trials; efficacy is both strain-specific and disease-specific, and within the same species some strains showed efficacy while others did not for the same indication. Source
21. Hill, C., Guarner, F., Reid, G., et al. (2014). Expert consensus document: the ISAPP consensus statement on the scope and appropriate use of the term probiotic. Nature Reviews Gastroenterology & Hepatology, 11(8), 506–514 — establishes strain-level identification as the basis of probiotic characterization. Source
22. Fluticasone propionate nasal spray, FDA-approved prescribing information, section 6.1 (Adverse Reactions), via DailyMed — adverse reactions above 3% and more common than placebo in patients aged 4 and over with allergic rhinitis, with the corresponding placebo rates; the label notes that adverse reactions “were reported with approximately the same frequency by subjects treated with placebo” and that fewer than 2% of subjects discontinued because of them. Source
23. Fein, M. N., Fischer, D. A., O’Keefe, A. W., & Sussman, G. L. (2019). CSACI position statement: newer generation H1-antihistamines are safer than first-generation H1-antihistamines and should be the first-line antihistamines for the treatment of allergic rhinitis and urticaria. Allergy, Asthma & Clinical Immunology, 15, 61 — first-generation agents such as diphenhydramine and hydroxyzine carry sedation and “impairment with decreased cognitive function”; newer-generation agents are recommended first-line. Source
24. By the 2023 American Geriatrics Society Beers Criteria Update Expert Panel (2023). American Geriatrics Society 2023 updated AGS Beers Criteria for potentially inappropriate medication use in older adults. Journal of the American Geriatrics Society, 71(7), 2052–2081 — first-generation antihistamines are listed as “avoid” in adults 65 and over, moderate quality of evidence, strong recommendation. Cetirizine, loratadine and fexofenadine are not listed. Source
25. Gray, S. L., Anderson, M. L., Dublin, S., et al. (2015). Cumulative use of strong anticholinergics and incident dementia: a prospective cohort study. JAMA Internal Medicine, 175(3), 401–407 — 3,434 adults aged 65 and over; the most commonly used anticholinergic classes were tricyclic antidepressants, first-generation antihistamines and bladder antimuscarinics; hazard ratio for dementia at the highest cumulative exposure 1.54 (95% CI 1.21–1.96). Source
26. Coupland, C. A. C., Hill, T., Dening, T., Morriss, R., Moore, M., & Hippisley-Cox, J. (2019). Anticholinergic drug exposure and the risk of dementia: a nested case-control study. JAMA Internal Medicine, 179(8), 1084–1093 — 58,769 dementia cases and 225,574 matched controls; significant associations were found for anticholinergic antidepressants, antiparkinson drugs, antipsychotics, bladder antimuscarinics and antiepileptics. Antihistamines were not among the classes reaching significance. Source
27. Over-the-counter Drug Facts labeling for pseudoephedrine and for cetirizine, via DailyMed — pseudoephedrine directs consumers to ask a doctor before use with heart disease, high blood pressure, thyroid disease, diabetes or enlarged prostate, and not to use with an MAOI; cetirizine directs adults 65 and over, and children under 6, to ask a doctor. The FDA recommends against over-the-counter cough and cold medicines in children under 2 years of age. Source
28. US Food and Drug Administration (7 November 2024). FDA proposes ending use of oral phenylephrine as OTC monograph nasal decongestant active ingredient after extensive review — a proposed order based on effectiveness, not safety; only a final order would affect what products can be marketed. Source
29. Tompkins, T. A., Mainville, I., & Arcand, Y. (2011). The impact of meals on a probiotic during transit through a model of the human upper gastrointestinal tract. Beneficial Microbes, 2(4), 295–303 — survival was best when the probiotic was given with a meal or 30 minutes before one, and better in milk and oatmeal-milk than in apple juice or spring water; the authors judged the meal’s fat content more important than its protein content. Source
30. Haahtela, T. (2019). A biodiversity hypothesis. Allergy, 74(8), 1445–1456 — the current successor to the hygiene hypothesis: reduced contact with natural environments and loss of microbial diversity, rather than cleanliness itself, is proposed as the driver of rising allergic disease. Source