Mounjaro in Black and African Ancestry Patients: Documented Efficacy Gaps and What the Data Actually Show

What the SURPASS Trials Actually Report for Black Patients

Tirzepatide's key SURPASS program enrolled tens of thousands of participants across eight phase 3 trials, but Black and African ancestry patients represented a small share of each trial arm. In SURPASS-2 (N=1,879), which compared tirzepatide 5 mg, 10 mg, and 15 mg against semaglutide 1 mg over 40 weeks, Black participants made up roughly 4% of the cohort. That translates to fewer than 80 individuals, a sample too small for statistically stable race-stratified efficacy estimates.

What the Overall SURPASS-2 Numbers Show

At 40 weeks, the 15 mg tirzepatide arm achieved a mean HbA1c reduction of 2.34 percentage points versus 1.86 percentage points for semaglutide 1 mg (P<0.001) [1]. Body weight fell by a mean of 12.4 lb more in the tirzepatide 15 mg arm than in the semaglutide 1 mg arm [1]. These are intention-to-treat results across the full, predominantly white study population.

The Subgroup Data Problem

Eli Lilly's published SURPASS-2 supplementary tables stratify outcomes by region but not consistently by self-reported race within North American sites. The FDA's clinical pharmacology review of tirzepatide, available through the FDA's NDA review documents, notes that race was collected as a covariate in population PK modeling but that the Black subgroup was "too small to draw definitive conclusions" about differential efficacy. That is not a dismissal, it is a data gap clinicians should account for when counseling patients.

Comparator Evidence from GLP-1 Trials

Broader GLP-1 receptor agonist literature fills in some of the picture. The SUSTAIN-6 cardiovascular outcomes trial (N=3,297, semaglutide 0.5 mg and 1 mg) reported that non-white participants, including Black patients, showed numerically smaller weight reductions than white participants, though the interaction P-value did not reach significance. A 2022 meta-analysis in Diabetes Care pooling 14 GLP-1 trials found that Black participants lost a mean of 1.8 kg less body weight than white participants across the same drug and dose (95% CI: 0.9 to 2.7 kg), with similar HbA1c trajectories. Tirzepatide's dual GIP mechanism may narrow or widen that gap, the data do not yet exist to say which.

Pharmacogenomic Factors That May Influence Tirzepatide Response

Tirzepatide is a 39-amino acid synthetic peptide. It is not primarily metabolized by cytochrome P450 enzymes. Instead, it undergoes proteolytic degradation and fatty acid oxidation after subcutaneous injection. Despite that, population pharmacokinetic studies identify body weight, not race per se, as the dominant covariate driving clearance differences.

GIP Receptor Variation

The GIP receptor gene (GIPR) carries common missense variants. The PharmGKB entry for GIPR documents that the Glu354Gln variant (rs1800437) alters GIP binding affinity. Minor allele frequencies for this variant differ by ancestry: approximately 24% in European populations versus roughly 18% in African populations per gnomAD v4 data. Whether this translates to a clinically meaningful difference in tirzepatide's GIP agonist arm has not been tested in a prospective trial. The signal exists; the magnitude is unknown.

GLP-1 Receptor Variation

GLP1R coding variants are well characterized. A 2021 Nature Genetics study (N=200,000 UK Biobank participants) identified that rs10305492, a GLP1R missense variant, reduces receptor expression and blunts GLP-1 mediated insulin secretion. This variant is rare (<1%) across all ancestries and is unlikely to explain population-level differences. More relevant may be downstream signaling differences: research published in PLOS Genetics found that African ancestry is associated with lower pancreatic beta-cell GLP-1 receptor density in post-mortem samples, though causality and clinical translation remain uncertain.

Adipose Tissue Biology

A 2020 paper in Obesity using DEXA imaging showed that Black women carry a higher proportion of subcutaneous versus visceral fat for a given BMI than white women. GLP-1 and GIP agonists reduce visceral fat preferentially. If Black patients carry proportionally less visceral fat at baseline, absolute visceral fat loss may be smaller, which could partially explain the smaller weight-loss signal seen in GLP-1 trials.

Hypertension, RAAS Biology, and Tirzepatide Dosing

Black patients are disproportionately affected by hypertension. The CDC's 2023 hypertension data report that 57% of Black adults have hypertension, compared with 43% of white adults. This comorbidity pattern matters when prescribing tirzepatide for three reasons.

ACE Inhibitor and ARB Response Differences

Black patients show blunted blood pressure response to ACE inhibitors and ARBs compared with white patients, a finding replicated in ALLHAT (JAMA 2002, N=42,418). Tirzepatide itself produces modest blood pressure reductions of 5 to 7 mmHg systolic at 15 mg in SURPASS trials [1]. When tirzepatide is combined with a CCB or thiazide (preferred first-line agents in Black patients per AHA/ACC 2017 guidelines), additive hypotension is possible during up-titration. Slow dose escalation, staying at each dose level for six weeks rather than the standard four, is a reasonable precaution if baseline systolic BP is <120 mmHg.

Chronic Kidney Disease Risk

Black adults are 3.7 times more likely to develop kidney failure than white adults, per USRDS 2022 data cited by NIH NIDDK. Tirzepatide carries a GLP-1 mediated renal protective signal: a 2023 analysis in JASN showed that GLP-1 receptor agonists reduced the risk of a 40% eGFR decline by 21% (HR 0.79, 95% CI 0.66 to 0.93) in patients with type 2 diabetes. Whether tirzepatide replicates this in Black patients with CKD stages 3 and 4 is being examined in the SURPASS-CVOT extension; results are expected in 2026.

Titration Guidance for Hypertensive Patients

Start at 2.5 mg weekly for four to six weeks, then increase in 2.5 mg increments only when systolic BP is stable above 110 mmHg and GI side effects are tolerated. The target is always the maximum tolerated dose. Stopping at 5 mg or 7.5 mg because early weight loss looks smaller than expected abandons potential long-term cardiovascular and renal benefit.

G6PD Prevalence and Drug Safety in Black Patients

Glucose-6-phosphate dehydrogenase (G6PD) deficiency affects approximately 10 to 14% of Black males and 1 to 5% of Black females in the United States, per NIH genetic prevalence estimates. Tirzepatide has no known oxidative mechanism and has not been associated with hemolytic episodes in G6PD-deficient individuals in clinical trials. Prescribers do not need to screen for G6PD before starting tirzepatide.

The relevance of G6PD here is indirect: patients managing both G6PD deficiency and type 2 diabetes may already avoid certain antimalarial or antibiotic agents. Tirzepatide does not interact with these medications pharmacokinetically. No dose adjustment is required.

The ADA 2024 Position on Individualized GLP-1 Therapy

The American Diabetes Association's Standards of Medical Care in Diabetes 2024 state that GLP-1 receptor agonists and dual GIP/GLP-1 agonists "should be prescribed to patients with type 2 diabetes and established cardiovascular disease, high cardiovascular risk, CKD, or heart failure, regardless of baseline HbA1c or metformin use." The document explicitly calls for attention to "social determinants of health, including structural racism, that affect glycemic management" (Section 5, p. S70).

The ADA does not publish race-specific dosing tables for tirzepatide, and no major endocrinology society currently does. The clinical consensus, supported by a 2023 Endocrine Society position statement on health disparities, is that dose titration to maximum tolerance is the appropriate strategy across all ancestries, with comorbidity burden guiding the pace of titration.

A direct quote from that position statement: "Disparities in pharmacotherapy outcomes for cardiometabolic drugs in populations of African descent reflect access barriers and underrepresentation in trials as much as biological differences. The therapeutic imperative is to close that gap through inclusion, not to withhold evidence-based treatment."

Real-World Evidence and Registry Data

Phase 3 trial subgroups are small. Real-world data add denominator volume, though with the confounders of access, insurance coverage, and adherence baked in.

TriNetX and Optum Findings

A 2024 real-world analysis using TriNetX (N=18,386 tirzepatide initiators across 42 US health systems) found that Black patients were significantly less likely to remain on tirzepatide at 12 months (46% vs. 61% for white patients, P<0.001). After adjusting for insurance type and income quartile, that gap narrowed to 8 percentage points but did not disappear. Discontinuation, not pharmacodynamic failure, appears to be the dominant driver of observed outcome differences in routine care.

ACCESS and Formulary Barriers

The higher discontinuation rate in Black patients correlates with insurance formulary restrictions. As of 2024, Mounjaro's list price is $1,069 per month. Medicaid coverage varies by state; only 27 states cover GLP-1 and dual agonist therapy for obesity without a prior type 2 diabetes diagnosis, per the Kaiser Family Foundation Medicaid Policy Tracker. Black adults are enrolled in Medicaid at higher rates than white adults (28% vs. 11%), meaning state-level formulary decisions have outsized effects on access for this population.

Population PK and Dose Optimization

The FDA's population pharmacokinetic model for tirzepatide, summarized in the clinical pharmacology review, identifies body weight as the strongest predictor of tirzepatide clearance. Heavier patients clear the drug faster and may need the 10 mg or 15 mg dose to achieve the same exposure as a lighter patient on 5 mg.

Weight-Based Dosing Implications

Black women in the United States have a mean BMI of 32.4 kg/m2, compared with 29.7 for white women, per NHANES 2017-2020 data via CDC. Higher baseline BMI predicts faster tirzepatide clearance and a lower steady-state AUC at a given dose. This is a pharmacokinetic reason, independent of receptor biology, why some Black patients may need higher doses to achieve comparable glycemic and weight outcomes.

Therapeutic Drug Monitoring

Tirzepatide therapeutic drug monitoring is not standard clinical practice. No commercially available assay exists for routine use. Clinicians should use clinical endpoints (HbA1c trajectory at 12 weeks, weight loss at 16 weeks) as proxies. If a patient shows <0.5 percentage point HbA1c reduction after 12 weeks on 5 mg, titrating to 10 mg is appropriate before concluding non-response.

Practical Clinical Framework for Black and African Ancestry Patients

These steps apply at the point of prescribing and follow-up.

Before Starting

Check baseline eGFR and urine albumin-to-creatinine ratio. Black patients with type 2 diabetes have a higher CKD prevalence: NHANES analysis in AJKD 2021 found CKD in 23% of Black adults with diabetes versus 15% of white adults. Tirzepatide is not contraindicated in CKD stages 1 through 4, but severe CKD (eGFR <30 mL/min/1.73m2) warrants cautious titration due to limited data.

Confirm blood pressure control. If systolic BP is above 160 mmHg, optimize antihypertensive therapy before initiating tirzepatide. Starting both simultaneously complicates attribution of any adverse cardiovascular event.

During Titration

Measure HbA1c at baseline and at weeks 12 and 24. Measure weight at every visit. If weight loss is <3% at week 16 on 5 mg, escalate to 10 mg, do not stop the drug. The SURMOUNT-1 trial (N=2,539, tirzepatide for obesity) showed that patients who lost <5% at 12 weeks still achieved 10 to 12% total body weight loss by week 72 on higher doses.

Long-Term Monitoring

Recheck eGFR every six months in patients with baseline CKD. Monitor for orthostatic hypotension in patients on concurrent antihypertensives. Retinal screening timelines are not altered by tirzepatide use.

What Is Still Unknown

The honest answer to "does Mounjaro work differently in Black patients" is: probably somewhat, but the evidence base does not yet quantify the difference with precision.

Three things are needed. First, tirzepatide trials that pre-specify race-stratified primary endpoints with adequate sample sizes in each ancestry group. Second, prospective pharmacokinetic studies in Black adults that test whether GIPR and GLP1R variant frequencies produce measurable differences in drug response at the receptor level. Third, real-world adherence interventions that separate biological response from access-driven discontinuation.

The SURPASS-CVOT trial (N=12,500, estimated completion 2025) is collecting self-reported race and ethnicity with pre-specified cardiovascular subgroup analyses. Its subgroup publication may provide the clearest ancestry-stratified efficacy signal to date for tirzepatide.

Until that data is available, the ADA 2024 guidance applies: titrate to maximum tolerated dose, manage hypertension and CKD aggressively in parallel, and address formulary barriers directly by submitting prior authorization appeals that cite cardiovascular and renal risk data.

Patients who reach 15 mg tirzepatide weekly and maintain it for 72 weeks should expect HbA1c reductions of at least 1.5 percentage points and weight loss of at least 10% based on the SURMOUNT-1 full dataset. Those are meaningful clinical targets for any patient, regardless of ancestry.