Off-Trial: Real-World Weight Loss on Tirzepatide and Semaglutide

Background. GLP-1 receptor agonist trials are tightly controlled: standardized titration, intensive dietary counseling, frequent in-person follow-up, and rigorous exclusion criteria. The real world is none of those things. In a U.S. telehealth GLP-1 program, diet engagement, exercise, medication choice, dose timing, and out-of-pocket cost vary substantially from patient to patient. Whether trial-level efficacy translates into the outcomes a patient and clinician will actually see is an open question, and the answer matters, because telehealth is now where most GLP-1 prescribing happens. Methods. We conducted a retrospective cohort study of 13,507 adults who used a single GLP-1 agent (tirzepatide or semaglutide) through the Mochi Health telehealth obesity program and had a documented six-month weight observation. The primary outcome was achievement of >=10% total body weight loss at six months. To address selection bias in the tirzepatide-semaglutide comparison, we used 1:1 nearest-neighbor propensity-score matching on age, sex, baseline BMI, baseline weight, and comorbid diabetes, hypertension, dyslipidemia, and prior bariatric surgery (recorded at intake), with a 0.25 SD caliper on the propensity logit. We drew a directed acyclic graph (DAG) with a clinical co-author to make the identifying assumptions explicit and to mark where unobserved variables (insurance, socioeconomic status, concomitant medications such as metformin) limit causal interpretation. We report multivariable predictors via logistic regression, compute an E-value for the matched contrast, and benchmark our point estimates against landmark RCT outcomes. Results. Overall, 59.1% of patients achieved >=10% loss at six months, with mean loss of 11.5% (median 11.3%). Threshold attainment was 86.6% at >=5%, 59.1% at >=10%, 27.5% at >=15%, and 9.1% at >=20%. The unadjusted tirzepatide-semaglutide response gap was +16.0 percentage points (68.8% vs 52.8%); after 1:1 propensity-score matching (3,480 pairs, all post-match |SMD| < 0.05) the gap was +18.1 percentage points (69.6% vs 51.6%, 95% CI +15.9 to +20.3). Matching on the measured covariates did not attenuate the advantage, indicating that selection on those characteristics does not explain it; the matched risk ratio was 1.35 (E-value 2.04). The gap was unchanged when a self-reported insurance indicator was added to the matching (+18.4 pp) and remained large (+14.2 pp) within patients who reached a therapeutic dose. Multivariable predictors of response were tirzepatide (OR 2.10, 1.95-2.26), female sex (OR 1.37, 1.20-1.56), and prior bariatric surgery (OR 1.36, 1.18-1.57); response was lower with comorbid diabetes (OR 0.84, 0.77-0.92) and, modestly, with higher baseline BMI per unit (OR 0.98, 0.97-0.99). Response varied by baseline BMI, from 58.0% in overweight patients (BMI <30) and a peak of 63.5% in Obese I to 52.4% in Obese III. Conclusions. Real-world response to GLP-1 therapy in a telehealth setting is meaningfully attenuated from RCT benchmarks but remains clinically substantial: roughly three in five patients reach the 10% threshold. The tirzepatide advantage over semaglutide is large and, notably, does not shrink under propensity-score matching on measured confounders, so it is not an artifact of the observed selection variables; an unmeasured confounder would need a risk-ratio association of about 2.0 with both drug choice and response to explain it away (E-value 2.04). The findings are observational, conditional on the DAG's identifying assumptions, and unmeasured confounders (insurance, socioeconomic status, concomitant medications) remain possible.