L-Theanine

There is plausible, evidence-backed reason to try L-theanine as an adjunct for IBS because it reduces stress/anxiety and modulates the gut–brain axis, but there are no high-quality randomized clinical trials showing L-theanine directly improves core IBS symptoms. Use it only as a complement to standard IBS care and after discussing with your clinician.

• IBS is a disorder of the gut–brain axis: psychological stress, anxiety and central nervous system signalling strongly influence IBS symptoms (pain, bowel habit change, visceral hypersensitivity). Treatments that reduce stress/anxiety often improve IBS outcomes. The Lancet
• L-theanine (an amino acid in tea) has well-documented central effects: it increases markers of relaxation (raises GABA, can modulate serotonin/dopamine and glutamatergic signalling), increases alpha EEG activity, and reduces self-reported stress/anxiety in multiple human trials and systematic reviews. Those central effects are the mechanism most relevant to IBS (i.e., it may reduce stress-driven symptom flares via the gut–brain axis). SpringerLink MDPI
• Animal and preclinical gut studies show L-theanine can reduce intestinal inflammation and markers of gut injury in rodent models (useful background but not proof for human IBS). These are mechanistic findings (anti-inflammatory, modulation of gut signalling) rather than clinical proof. SpringerLink ScienceDirect

Bottom line: L-theanine’s likely benefit for IBS is indirect — by reducing anxiety/stress and modulating neurochemical pathways of the gut–brain axis — not because it has robust, direct evidence to relieve stool frequency or abdominal pain in large IBS trials. The Lancet SpringerLink

These are pragmatic, evidence-informed suggestions that reflect doses used in human trials and product monographs — not formal clinical guidelines.

• Typical starting dose: 100–200 mg once daily (many people start 100–200 mg in the morning or mid-day). Drugs.com Healthline
• Common therapeutic range: 200–400 mg per day. Many clinical trials use 200 mg once or twice daily (so 200–400 mg/day). Higher single doses (up to 400 mg) have been used acutely in labs; up to 900 mg/day has been administered in some studies for limited times without major safety signals — but long-term safety at very high doses is not well established. MDPI WebMD
• Duration: try an initial trial of 4–8 weeks while tracking symptoms (IBS symptoms can fluctuate; 4–8 weeks is a reasonable window to see benefit tied to anxiety/sleep improvements). Many L-theanine trials for stress/sleep used 4 weeks or longer. MDPI University of Canberra Research Portal
• When to take: with or without food; if it causes mild stomach upset taking with food can help. If using twice daily, spacing ~12 hours apart (e.g., morning and early evening) is common. Avoid taking it with alcohol and be cautious if you already take sedatives. WebMD Brain Fuel
• Formulation: choose pharmaceutical-grade L-theanine (often labeled “Suntheanine®” or simply “L-theanine, 100–200 mg capsule”). Prefer products with third-party testing (USP, NSF, or similar) to avoid contaminants. Suntheanine
• How to judge effect: track IBS symptom severity (pain, stool frequency/consistency, bloating) and anxiety/sleep scores (e.g., simple daily diary, or validated scales if you have them). If anxiety and sleep improve and IBS flares reduce, the supplement may be helpful as an adjunct.

Important: these are common practice patterns based on existing human trials and supplement guidance — discuss personalization (dose, duration, interactions) with your prescriber.

Lancet commentary — gut–brain neuromodulators for IBS — explains central role of gut–brain signalling in IBS and why neuromodulatory agents (including interventions that target stress/anxiety) can affect IBS. (Useful background that explains why L-theanine could plausibly help). The Lancet

Systematic review — L-theanine (green tea amino acid) and stress/anxiety outcomes (Springer / Nutrition reviews / similar reviews): multiple randomized trials in humans show L-theanine reduces subjective stress/anxiety and improves relaxation/sleep markers. This is the principal human evidence we have. (Not IBS trials, but directly relevant to the gut–brain mechanism.) SpringerLink

Randomized trials of L-theanine in humans (examples) — e.g., the Nutrients 2019 randomized, double-blind crossover trial (healthy adults) showing effects on stress-related symptoms at commonly used doses (200 mg/day used in several trials). These trials document human safety and anxiety/stress benefits. MDPI

Animal / preclinical gut studies — e.g., rodent DSS colitis model: L-theanine reduced intestinal inflammation and disease activity index in rats (mechanistic evidence of local gut protective/anti-inflammatory effects). These support plausibility but don’t prove benefit in human IBS (which is not the same as inflammatory bowel disease). SpringerLink

Recent systematic reviews / meta-analyses on L-theanine and sleep/anxiety (2023–2024) — several reviews (and a 2024 systematic review on psychiatric outcomes) summarize RCTs showing L-theanine’s anxiolytic/sleep benefits and safety profile; none provide high-quality RCT evidence for direct IBS symptom relief. BioMed Central University of Canberra Research Portal