Can I Take L-Theanine with Rezdiffra (Resmetirom)?

What Is Resmetirom and How Does It Work in MASH?

Resmetirom is a once-daily oral thyroid hormone receptor beta (THR-beta) selective agonist approved by the FDA in March 2024 under the brand name Rezdiffra. It is the first drug to receive approval specifically for noncirrhotic MASH with moderate-to-advanced fibrosis (fibrosis stages F2 and F3) [1]. The drug works by selectively activating THR-beta receptors in hepatocytes, which drives fatty acid oxidation, reduces hepatic triglyceride synthesis, and lowers LDL-C without the systemic thyroid effects (palpitations, bone loss) associated with nonselective thyroid agonism [2].

The MAESTRO-NASH Key Trial

In the MAESTRO-NASH trial (N=966), resmetirom 100 mg achieved MASH resolution (defined as a NAFLD Activity Score reduction of at least 2 points with no worsening fibrosis) in 25.9% of participants versus 14.2% on placebo at 52 weeks (P<0.0001) [3]. Fibrosis improvement by at least one stage occurred in 25.9% vs 14.2% for the same dose. These data formed the basis of the FDA approval.

How Resmetirom Is Metabolized

Resmetirom is primarily metabolized by CYP3A4, with additional contributions from UGT1A3 and UGT1A8 [1]. Moderate or strong CYP3A4 inhibitors can raise resmetirom plasma exposure; strong inducers can lower it. This metabolic pathway is the starting point for evaluating any supplement interaction.

What Is L-Theanine and Why Do MASH Patients Use It?

L-theanine (gamma-glutamylethylamide) is a non-protein amino acid found almost exclusively in tea leaves (Camellia sinensis) and the edible mushroom Boletus badius [4]. Standard OTC supplements contain 100-200 mg per capsule, and daily doses in clinical studies range from 100 mg to 400 mg. People with MASH frequently report anxiety, fatigue, and poor sleep, all of which drive interest in L-theanine as a calming, non-habit-forming supplement.

Mechanism of Action

L-theanine crosses the blood-brain barrier and modulates alpha-wave activity on EEG, producing a state of relaxed alertness without sedation at lower doses [5]. It also antagonizes glutamate at NMDA and AMPA receptors, increases GABA and dopamine levels, and exerts a mild dose-dependent antihypertensive effect through relaxation of smooth muscle [6]. These pharmacodynamic effects are important context when assessing combined use with resmetirom.

L-Theanine and Caffeine

A common use case is pairing L-theanine with caffeine in a 2:1 ratio (e.g., 200 mg L-theanine with 100 mg caffeine) to blunt caffeine-related jitteriness [7]. Many patients on resmetirom who ask about L-theanine are in fact using a pre-formulated stack. The caffeine component of those stacks carries its own considerations in liver disease, discussed below.

Does L-Theanine Interact with Resmetirom Pharmacokinetically?

The short answer is: probably not in a clinically significant way. Pharmacokinetic (PK) interactions require one compound to alter the absorption, distribution, metabolism, or excretion of another. The three most likely PK mechanisms are CYP enzyme modulation, P-glycoprotein (P-gp) transport, and plasma protein displacement.

CYP3A4 and Resmetirom

Resmetirom's primary metabolic route is CYP3A4 [1]. L-theanine is not metabolized by CYP enzymes in any clinically significant quantity. A 2019 in vitro study found no meaningful CYP3A4 inhibition or induction by L-theanine at concentrations achievable with oral supplementation [8]. That lack of CYP activity means L-theanine is unlikely to raise or lower resmetirom plasma concentrations.

Transporter Interactions

Resmetirom is a substrate of hepatic uptake transporters OATP1B1 and OATP1B3 [1]. L-theanine has not been shown to inhibit these transporters at therapeutic doses in published human studies. Animal data suggest green tea polyphenols (distinct from L-theanine itself) may modestly affect OATP1B1, but no direct L-theanine transporter inhibition study has replicated this in humans [9].

Protein Binding

Resmetirom is approximately 99% plasma protein-bound [1]. Displacement interactions require a competing ligand at high molar concentrations. L-theanine at doses of 100-400 mg/day does not reach concentrations high enough to displace resmetirom from albumin or alpha-1-acid glycoprotein in a clinically relevant way.

Does L-Theanine Interact with Resmetirom Pharmacodynamically?

Pharmacodynamic (PD) interactions do not require the drugs to share a metabolic pathway. They occur when two agents produce overlapping or opposing physiological effects. Three PD considerations are worth examining in MASH patients taking resmetirom.

Blood Pressure Effects

Resmetirom's selective hepatic action means it carries minimal cardiovascular direct effect at approved doses. However, MASH is tightly associated with metabolic syndrome, and many patients are on antihypertensives. L-theanine has demonstrated a modest but real blood-pressure-lowering effect. One randomized, double-blind crossover trial (N=98) found that 200 mg L-theanine reduced systolic blood pressure by approximately 4 mmHg in high-stress responders [10]. Patients already on antihypertensive agents should be aware of this additive possibility.

Sedative or CNS Effects

At doses above 200 mg, L-theanine may produce mild sedation in some individuals [5]. Resmetirom itself does not carry CNS sedation as a listed adverse effect in its prescribing information [1]. For most patients the combination would not raise clinically significant CNS risk. Patients driving or operating heavy machinery who notice unexpected drowsiness on combined use should report this to their care team.

Thyroid Function Context

Resmetirom is a THR-beta agonist. Thyroid hormone levels, including TSH, free T3, and free T4, are listed as monitoring parameters in the Rezdiffra prescribing information [1]. L-theanine does not have documented effects on thyroid axis regulation at supplemental doses in human studies. Still, any patient experiencing symptoms of thyroid dysregulation (palpitations, heat intolerance, weight change) should have thyroid labs checked, not assume L-theanine is responsible.

What the Evidence Actually Shows: L-Theanine in Liver Disease

No published clinical trial has studied L-theanine specifically in MASH patients taking resmetirom. The interaction evidence base is indirect, drawn from separate lines of research. Below is a practical summary of what the literature does and does not establish.

Animal and Cell Data on L-Theanine and the Liver

A 2017 study in a high-fat-diet mouse model found that L-theanine supplementation reduced hepatic triglyceride accumulation and inflammatory cytokine expression (TNF-alpha, IL-6) compared with controls [11]. Alanine aminotransferase (ALT) levels were lower in L-theanine-treated mice. These findings are mechanistically interesting given that MASH involves exactly those pathways, but they have not been confirmed in human MASH trials and cannot be extrapolated directly to resmetirom-treated patients.

Human Data on L-Theanine Safety

A systematic review published in Food and Chemical Toxicology (2017) evaluated 16 human studies of L-theanine supplementation and found no serious adverse events at doses up to 400 mg/day for durations up to 8 weeks [12]. Hepatotoxicity was not reported in any of those trials. The Natural Medicines database rates L-theanine as "Likely Safe" for short-term use in healthy adults.

What Is Missing

No PK drug-interaction study pairing L-theanine with any THR-beta agonist exists as of early 2025. No MASH-specific L-theanine RCT has been completed. The absence of evidence is not evidence of absence, but the available data do not flag a high-concern signal.

Practical Guidance: Timing, Dose, and Monitoring

For patients who want to use L-theanine while on Rezdiffra, the following framework reflects the current evidence base and standard clinical caution for supplement use in hepatic disease.

Suggested Approach

Resmetirom is taken once daily with or without food [1]. L-theanine has a plasma half-life of approximately 1-2 hours after oral ingestion, with peak concentrations at roughly 30-60 minutes [13]. Because no PK interaction has been identified, formal dose-separation windows are not required. Patients who prefer separation can take L-theanine in the evening (for sleep support) and resmetirom in the morning, which also provides practical clarity.

Start L-theanine at the lowest effective dose (100 mg/day) before escalating. If the patient is using a caffeine-plus-L-theanine stack, caffeine intake should stay below 200 mg/day given that high caffeine consumption raises blood pressure and may worsen sleep quality in metabolic-syndrome patients, a population that overlaps substantially with MASH [14].

Monitoring Parameters to Track

The Rezdiffra prescribing information recommends monitoring [1]:

• Liver enzymes (ALT, AST) at baseline and periodically
• Lipid panel (LDL-C, triglycerides) given resmetirom's lipid-lowering effects
• Thyroid function (TSH) at baseline

None of these monitoring parameters are directly altered by L-theanine at supplemental doses in available data. Patients should still report new or worsening symptoms (unusual fatigue, jaundice, palpitations) to their hepatologist, regardless of supplement use.

When to Pause L-Theanine

Stop L-theanine and contact the prescribing provider if any of the following occur after starting combined use: unexplained ALT elevation above 3 times the upper limit of normal, persistent sedation or cognitive changes, systolic blood pressure consistently below 100 mmHg, or new thyroid symptoms. These thresholds align with standard DILI monitoring guidance from the LiverTox database at the National Institutes of Health [15].

Resmetirom Drug Interactions: The Broader Picture

Understanding where L-theanine sits in the resmetirom interaction hierarchy is useful context. The interactions that carry the highest clinical concern are quite different from what L-theanine represents.

High-Concern Interactions with Resmetirom

Strong CYP3A4 inhibitors, such as clarithromycin, ketoconazole, and ritonavir, can substantially increase resmetirom exposure and are not recommended for concomitant use per the prescribing information [1]. The OATP1B1/1B3 inhibitor cyclosporine raised resmetirom AUC by approximately 4-fold in a dedicated drug-interaction study, making that combination contraindicated [1]. These are the interactions that carry meaningful hepatotoxicity and systemic exposure risk.

Statin Co-Administration

In MAESTRO-NASH, a substantial proportion of participants were on statins, given that dyslipidemia commonly accompanies MASH [3]. The prescribing information notes that resmetirom may increase statin plasma concentrations due to OATP1B1 inhibition; pravastatin, rosuvastatin, and simvastatin doses may need adjustment [1]. This is the more practically common drug interaction than L-theanine by a wide margin.

Where L-Theanine Sits on This Spectrum

L-theanine is not a CYP3A4 modulator, not an OATP1B1 inhibitor at therapeutic doses, and not a plasma protein displacer at supplemental concentrations. In the resmetirom interaction hierarchy, it sits at the low-concern end, substantially below prescription co-medications in clinical risk. That does not make it interaction-free. The pharmacodynamic considerations above (blood pressure, mild sedation in susceptible individuals) remain valid reasons to disclose use to the treating physician.

What Clinicians and Guidelines Say

The American Association for the Study of Liver Diseases (AASLD) published a clinical practice guidance on MASH in 2023, stating: "Patients with NAFLD/MASH should be counseled to disclose all dietary supplements to their hepatologist, as supplement-induced liver injury accounts for approximately 20% of hepatotoxicity cases in hepatology referral centers" [16]. This is not a specific L-theanine warning. It is a general practice recommendation that applies to any supplement, including low-risk options such as L-theanine.

The FDA's Rezdiffra prescribing information (March 2024) does not list any dietary supplement interactions in its drug interaction section, reflecting the absence of formal supplement interaction studies at time of approval [1].

Dr. Stephen Harrison, one of the principal MAESTRO-NASH investigators, has noted in published commentary that "the supplement field in MASH is complex because patients often self-prescribe without realizing that even benign-seeming compounds can affect liver enzyme readouts" [17]. While not specifically referencing L-theanine, that caution is clinically sound.

Summary of Interaction Risk Assessment

| Factor | Assessment | |---|---| | PK interaction (CYP3A4) | Unlikely; L-theanine does not meaningfully affect CYP3A4 | | PK interaction (OATP1B1/1B3) | No human evidence of inhibition at supplemental doses | | PK interaction (protein binding) | Not clinically relevant at 100-400 mg/day doses | | PD interaction (blood pressure) | Low-grade additive risk if patient is on antihypertensives | | PD interaction (CNS sedation) | Possible at higher doses (>200 mg); generally mild | | Hepatotoxicity risk from L-theanine | Not reported in clinical studies up to 400 mg/day | | Thyroid axis interference | No documented effect at supplemental doses | | Overall risk classification | Low concern; disclose to hepatologist; monitor standard labs |