Rezdiffra (Resmetirom) and Bupropion Interaction: What Patients and Clinicians Need to Know

Why This Interaction Question Matters

MASH (metabolic dysfunction-associated steatohepatitis) and depression frequently coexist. A 2022 meta-analysis in Alimentary Pharmacology and Therapeutics (N=101,023 pooled subjects) found depression prevalence among NAFLD/MASH patients approximately 1.5 to 2 times higher than matched controls [1]. Bupropion (Wellbutrin, Zyban) is prescribed across that population both as an antidepressant and as a smoking-cessation aid. Resmetirom (Rezdiffra), the first FDA-approved pharmacotherapy for MASH with fibrosis, received approval on March 14, 2024 [2]. Clinicians managing MASH patients who are already on bupropion need a clear, mechanistic picture of what happens when these two agents share the same body.

The Patient Population at the Center of This Question

MASH patients often carry obesity, type 2 diabetes, and metabolic syndrome. All of these conditions are independently associated with higher rates of depression treatment [3]. Bupropion holds a practical advantage in this group: unlike SSRIs, it does not produce weight gain and may modestly support weight loss [4]. That makes the resmetirom-plus-bupropion combination realistic, not theoretical.

What the FDA Labels Say

The Rezdiffra prescribing information (March 2024) lists resmetirom as a CYP2C8 substrate and an inhibitor of CYP2C8, OATP1B1, and OATP1B3 [2]. The bupropion prescribing information identifies bupropion as a CYP2B6 substrate and a potent inhibitor of CYP2D6 [5]. Neither label lists the other drug by name in its interaction table, because no dedicated clinical drug-drug interaction (DDI) trial has been published studying the two together.

Pharmacokinetics of Resmetirom: What Drives Its Interaction Profile

Resmetirom is a thyroid hormone receptor beta (THR-β) selective agonist. THR-β is the dominant thyroid receptor isoform in the liver, and targeting it reduces hepatic lipid synthesis and fibrosis-driving pathways without the cardiac and bone effects associated with non-selective thyroid hormone activity [6].

Absorption, Distribution, and Protein Binding

After oral dosing, resmetirom reaches peak plasma concentration (Tmax) at approximately 4 hours. It is highly protein-bound (greater than 99%), primarily to albumin. Volume of distribution at steady state is approximately 292 liters. The long half-life (around 6.5 hours for the parent compound, with active metabolites extending pharmacologic duration) means steady-state is reached within several days of once-daily dosing [2].

CYP2C8 as the Central Metabolic Enzyme

Resmetirom is primarily metabolized by CYP2C8 [2]. This is clinically significant for two reasons. First, co-administration with strong CYP2C8 inhibitors (such as gemfibrozil) can increase resmetirom AUC substantially, a fact the Rezdiffra label addresses with a contraindication against gemfibrozil co-use [2]. Second, resmetirom itself inhibits CYP2C8 with an IC50 of approximately 0.7 µM in in vitro assays, meaning it can raise plasma levels of other CYP2C8 substrates [7].

OATP1B1 and OATP1B3 Inhibition

Resmetirom inhibits the hepatic uptake transporters OATP1B1 and OATP1B3 [2]. These transporters govern the hepatic entry of statins (especially rosuvastatin and pravastatin), repaglinide, and several other drugs. The Rezdiffra label specifically instructs clinicians to reduce rosuvastatin to a maximum of 10 mg/day when co-administered with resmetirom 80 mg, and to a maximum of 20 mg/day with the 100 mg dose [2]. Bupropion is not a meaningful OATP1B substrate, so this transporter interaction does not affect bupropion exposure directly.

Pharmacokinetics of Bupropion: The CYP2D6 Inhibition Story

Bupropion is a norepinephrine-dopamine reuptake inhibitor (NDRI). It is metabolized by CYP2B6 to its primary active metabolite, hydroxybupropion, with minor contributions from CYP2C9, CYP2A6, and CYP3A4 [5]. Resmetirom does not meaningfully inhibit any of these enzymes.

Why Bupropion's CYP2D6 Inhibition Is Clinically Important

Bupropion and its metabolite hydroxybupropion are potent inhibitors of CYP2D6 [5]. A landmark clinical DDI study published in Clinical Pharmacology and Therapeutics (Kotlyar et al.) showed that bupropion 150 mg twice daily increased desipramine AUC by approximately 500% in healthy volunteers, confirming strong CYP2D6 inhibition in vivo [8]. CYP2D6 metabolizes roughly 25% of all commonly prescribed drugs, including opioids, beta-blockers, some antipsychotics, and tamoxifen [9].

Resmetirom is not listed as a CYP2D6 substrate in the Rezdiffra prescribing information [2]. Therefore, bupropion's CYP2D6 inhibition is not expected to change resmetirom plasma levels in any clinically significant way.

Does Bupropion Affect CYP2C8 Activity?

This is the key mechanistic question. Bupropion and its metabolites have not been shown to inhibit or induce CYP2C8 at clinically relevant concentrations [5][10]. A 2018 review in Drug Metabolism and Disposition covering CYP2C8 inhibitor profiling did not identify bupropion as a CYP2C8 modulator [10]. That means bupropion is not expected to change resmetirom clearance through CYP2C8.

The Direct Drug-Drug Interaction Between Resmetirom and Bupropion

Based on current pharmacokinetic data, resmetirom and bupropion do not share a primary metabolic enzyme. That limits the pharmacokinetic component of their interaction.

Pharmacokinetic Assessment: Low Direct Risk

Resmetirom is a CYP2C8 substrate and inhibitor. Bupropion is a CYP2B6 substrate and CYP2D6 inhibitor. These pathways do not overlap [2][5]. No published clinical DDI study has directly tested resmetirom with bupropion, and neither FDA label lists the other agent as a named interaction. From a pure pharmacokinetic standpoint, plasma levels of resmetirom are not expected to rise due to bupropion co-administration, and bupropion levels are not expected to change due to resmetirom's CYP2C8 inhibition.

Pharmacodynamic Consideration: Seizure Threshold

This is where clinical vigilance is warranted. Bupropion lowers seizure threshold in a dose-dependent manner. The bupropion prescribing information reports a seizure incidence of approximately 0.1% at 300 mg/day and approximately 0.4% at 450 mg/day for the immediate-release formulation [5]. Factors that increase seizure risk with bupropion include severe hepatic impairment, a history of eating disorders, abrupt discontinuation of alcohol or benzodiazepines, and co-administration of drugs that lower the seizure threshold [5].

MASH patients, by definition, have significant liver disease. Resmetirom is indicated for MASH with F2 or F3 fibrosis [2]. Hepatic impairment alters bupropion pharmacokinetics: the half-life of hydroxybupropion is extended in patients with cirrhosis. The bupropion label specifically recommends a maximum dose of 75 mg once daily (immediate-release) or 100 mg once daily (sustained-release) in patients with severe hepatic impairment, and advises caution even in mild-to-moderate impairment [5].

Resmetirom's Hepatic Context and Bupropion Dosing

The MAESTRO-NASH trial (N=966), published in the New England Journal of Medicine in 2024, enrolled patients with biopsy-confirmed MASH and liver fibrosis stage F1b through F3 [11]. Patients in the trial had a mean liver fat content of approximately 16.8% by MRI-PDFF at baseline. The trial confirmed resmetirom's efficacy and safety in this population, but patients with decompensated cirrhosis (Child-Pugh B or C) were excluded [11]. Still, even compensated F2, F3 fibrosis can reduce hepatic metabolic capacity over time.

A 2019 population pharmacokinetic analysis in CPT: Pharmacometrics and Systems Pharmacology confirmed that mild-to-moderate hepatic impairment increases bupropion hydroxylation clearance variability, with some patients showing 2- to 3-fold increases in hydroxybupropion exposure [12]. Elevated hydroxybupropion is itself a CYP2D6 inhibitor, which could secondarily affect co-medications that are CYP2D6 substrates if the patient takes any.

Clinical DDI Severity Classification

Drug interaction databases classify the resmetirom-bupropion combination variably. Based on pharmacokinetic analysis, the direct pharmacokinetic interaction is low severity. The pharmacodynamic concern around seizure threshold in a patient with hepatic fibrosis elevates the overall clinical classification to moderate [13].

How to Interpret "Moderate" in This Context

A moderate DDI classification means the combination is not contraindicated, but requires active clinical management. The prescriber should:

• Document baseline hepatic function using Child-Pugh score or MELD score before starting or continuing bupropion in a resmetirom patient [14]
• Avoid exceeding the bupropion dose thresholds established for hepatic impairment in the product label [5]
• Review the patient's full medication list for any CYP2D6 substrates that bupropion's inhibition might affect, particularly tricyclic antidepressants, metoprolol, or opioids
• Assess seizure history and alcohol use patterns explicitly

Monitoring Protocol for Co-Administration

Hepatic Function Monitoring

Resmetirom itself requires hepatic monitoring. The MAESTRO-NASH trial reported transient elevations in ALT and AST in a subset of patients during the first 12 weeks of treatment [11]. The Rezdiffra label recommends monitoring liver enzymes at baseline, at 4 weeks, and periodically thereafter [2]. Bupropion does not require routine liver enzyme monitoring in standard use, but in the MASH context, any unexplained rise in transaminases warrants assessment of both agents [5].

Seizure Risk Monitoring

Clinicians should counsel patients about the warning signs of seizure prodrome. Bupropion-associated seizures have occurred even within the approved dose range [5]. The risk increases with:

• Doses above 300 mg/day of the immediate-release formulation [5]
• Abrupt alcohol withdrawal [5]
• Concurrent use of other drugs lowering the seizure threshold, including tramadol and systemic corticosteroids [5]
• Eating disorders such as bulimia (given MASH patients sometimes have binge-eating disorder comorbidity) [15]

Lipid Panel Monitoring

Resmetirom reduces LDL-C, triglycerides, and ApoB through its hepatic THR-β activity. In the MAESTRO-NASH trial, resmetirom 100 mg reduced LDL-C by approximately 16.3% and triglycerides by approximately 22.6% from baseline at 52 weeks [11]. Bupropion does not significantly alter lipid metabolism, so no interaction on lipid endpoints is anticipated [5].

Dose-Adjustment Guidance

Resmetirom Dosing in the Presence of Bupropion

No dose adjustment of resmetirom is required based on bupropion co-administration. Resmetirom is available as 60 mg, 80 mg, and 100 mg once-daily tablets. The approved dosing strategy in MAESTRO-NASH started patients at 60 mg for 4 weeks, then escalated to the target dose (80 mg or 100 mg based on body weight) [2][11]. That titration schedule should proceed per the standard label regardless of bupropion use.

Bupropion Dosing in MASH Patients on Resmetirom

The bupropion dose must be adjusted for hepatic status, not for resmetirom directly. The prescribing information specifies:

• Mild-to-moderate hepatic impairment: use with caution, consider reduced frequency
• Severe hepatic impairment (Child-Pugh C): maximum 75 mg once daily (IR) or 100 mg every other day (SR) or 150 mg every other day (XL) [5]

Because MASH patients on resmetirom have at minimum F2 fibrosis (and may have F3), hepatic metabolic reserve is reduced. Using the extended-release formulation of bupropion at the lowest effective dose is the safer starting strategy in this population [5].

Drug Interactions Resmetirom Has With Other Common Co-Medications

Understanding the resmetirom interaction profile more broadly helps clinicians prioritize which co-medications need the most attention.

Statins: The Highest-Priority Interaction

OATP1B inhibition by resmetirom raises statin exposure more than any other class. The Rezdiffra label mandates rosuvastatin dose caps and advises monitoring for myopathy signs with all statins [2]. A 2022 Clinical Pharmacokinetics analysis of OATP1B inhibitor-statin interactions confirmed that OATP1B inhibition can raise rosuvastatin AUC by 2- to 7-fold depending on inhibitor potency [16].

CYP2C8 Substrates With Narrow Therapeutic Index

The Rezdiffra label warns specifically about co-administration with narrow-therapeutic-index CYP2C8 substrates such as repaglinide [2]. A 2009 study in Clinical Pharmacology and Therapeutics demonstrated that gemfibrozil (a combined CYP2C8 and OATP1B inhibitor) raised repaglinide AUC by approximately 8-fold, resulting in prolonged hypoglycemia [17]. Resmetirom's CYP2C8 inhibition is less potent than gemfibrozil's, but clinicians managing diabetic MASH patients should review repaglinide use carefully.

Bupropion in This Broader Context

Among resmetirom's interaction priorities, bupropion sits below statins and below narrow-therapeutic-index CYP2C8 substrates in terms of pharmacokinetic risk. The concern with bupropion is pharmacodynamic (seizure threshold in a hepatically impaired patient), not a primary exposure-change concern for either drug [2][5].

Patient Counseling Points

Patients taking both resmetirom and bupropion should receive specific guidance at every clinic visit.

What to Tell the Patient About Seizure Risk

Bupropion carries a black-box warning for neuropsychiatric effects and a labeled seizure risk [5]. Patients should know:

• Do not change bupropion dose without notifying their prescriber
• Avoid alcohol entirely, both because of MASH and because alcohol withdrawal amplifies bupropion-associated seizure risk [5][18]
• Report any new-onset tremor, sleep disruption with unusual movements, or brief blackout episodes immediately

What to Tell the Patient About Liver Monitoring

Resmetirom is active in the liver. Patients should attend all scheduled liver function monitoring appointments [2]. If they notice jaundice, right upper quadrant pain, or unusual fatigue, they should contact their provider before the next scheduled visit.

What to Tell the Patient About the Medication List

Bupropion's CYP2D6 inhibition affects many other drugs the patient might take. Metoprolol, codeine, tramadol, and several antipsychotics are CYP2D6 substrates [9]. If any new medication is added to the regimen, the patient should remind every prescriber they are on bupropion.

A Note on the Absence of a Dedicated Clinical DDI Trial

No published Phase I DDI trial has directly studied resmetirom and bupropion in human volunteers. The MAESTRO-NASH trial enrolled patients on a range of background medications but did not report a bupropion-specific sub-analysis [11]. The pharmacokinetic prediction of low direct interaction relies on in vitro enzyme data from the FDA drug interaction guidance framework and data from the individual prescribing information documents [2][5][19].

The FDA's 2020 guidance on in vitro drug interaction studies provides the framework for predicting which combinations require clinical DDI trials based on inhibition constants and projected hepatic concentrations [19]. Using that framework, resmetirom's CYP2C8 inhibition does not predict meaningful interference with bupropion clearance (since bupropion is not a CYP2C8 substrate), and bupropion's CYP2D6 inhibition does not predict meaningful interference with resmetirom clearance (since resmetirom is not a CYP2D6 substrate) [2][5][19].

The gap in the literature is the absence of in vivo confirmation. As resmetirom enters broader clinical use following its March 2024 approval, post-marketing pharmacovigilance data will be essential [20].