L - Reuteri
Lactobacillus reuteri (L. reuteri) is a probiotic bacterium naturally found in the human gut and in some fermented foods. It’s commonly taken as a supplement to support digestive health and may help with issues like diarrhea, constipation, and gut inflammation in some people. Some strains produce compounds (like reuterin) that can inhibit certain harmful microbes, which is why it’s also studied for oral and immune-related benefits. Effects are strain-specific, so different L. reuteri products can have different outcomes.
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This page summarizes anecdotal reports and community observations, not medical evidence. Reports may be incomplete, biased or inaccurate and are not medical advice or recommendations. “Risk” here refers to how frequently severe or prolonged symptom worsening is reported, not to proven causation or population-wide probability. Individual responses vary widely, and absence of issues in some users does not rule out significant reactions in others.
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Within PFS/PSSD/PAS communities, L. reuteri is discussed in relation to its potential interactions with tryptophan metabolism and serotonergic signaling pathways. L. reuteri metabolizes dietary tryptophan through specific enzymes (araT, fldH, amiE) to produce various tryptophan-derived metabolites, including indole acetate, indole-3-glyoxylic acid, tryptamine, p-cresol, and imidazole derivatives that can act as aryl hydrocarbon receptor (AhR) agonists and antagonists. A key mechanism is that L. reuteri metabolites, particularly ethanol, upregulate the serotonin transporter (SERT) in intestinal epithelial cells, which helps regulate serotonin availability within the intestine by controlling 5-HT reuptake. L. reuteri treatment has been shown to increase enzyme expression involved in serotonin biosynthesis while suppressing enzymes involved in the kynurenine pathway of tryptophan metabolism in both the colon and prefrontal cortex, effectively shifting tryptophan metabolism toward serotonin production rather than alternative degradation pathways. Studies have demonstrated that administration of L. reuteri can significantly increase blood and colon serotonin (5-HT) levels. Serotonin functions as a key mediator in the microbiota-gut-brain axis, with intestinal microbiota playing a crucial role in serotonin signaling. Most serotonin in the body is produced in the gut (primarily by enterochromaffin cells), and microbial metabolites can influence serotonin signaling, vagal/autonomic tone, and even blood–brain barrier and microglial activity. These mechanisms may interact with pathways involving gut–brain signaling, serotonin metabolism, tryptophan partitioning, or serotonergic tone that are often discussed in relation to PFS / PSSD / PAS.
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Mixed Results Temporary Improvement and Flares:
Lactobacillus reuteri (L. reuteri) is a probiotic that shows mixed results in PFS/PSSD-style communities: some people report temporary improvements, while others report temporary worsening or “crash-like” symptoms, especially early on or when changing dose/strain. Because the microbiome can shift quickly, responses may be highly strain-specific and sensitive to factors like diet, histamine sensitivity, and existing gut dysbiosis. Homemade L. reuteri yogurt/ferments can be much more potent than standard capsules (higher CFU and different fermentation byproducts), so it’s often reported as more “powerful” — for better or worse. Overall, many longer-term improvement stories involve some form of gut-focused work (diet, antimicrobials, probiotics/prebiotics, motility support), but L. reuteri itself should be viewed as a mixed-risk tool rather than a reliable fix.
Evidence Basis: Anecdotal reports (online forums, self-reports); mixed mechanistic hypotheses; no controlled human studies examining PFS/PSSD/PAS outcomes.
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Public comments reflect individual experiences and opinions. They are not medical advice and may not be accurate or representative.