Evidence review · from the 250-topic plan
SURMOUNT-5, beyond the headline: what 20.2% vs 13.7% actually compared
SURMOUNT-5 randomized adults with obesity, without diabetes, to tirzepatide or semaglutide at each drug's maximum tolerated dose for 72 weeks; tirzepatide produced about 20.2% mean weight loss against semaglutide's 13.7%, met superiority, and won most secondary endpoints. Real caveats travel with the headline: the trial was open-label (participants knew their drug), the comparison is 15 mg-class dual agonism against 2.4 mg semaglutide by design, and averages are not promises. For shoppers, the honest translation is a price question: the efficacy gap is real, and so is the cost gap you pay to buy it.
What the trial actually set up
Head-to-heads are rare in obesity medicine because nobody funds a fair fight lightly; SURMOUNT-5 is the one we have. Adults with obesity (or overweight plus a weight-related condition), without type 2 diabetes, were randomized to once-weekly tirzepatide or once-weekly semaglutide, each titrated to its maximum tolerated dose — up to 15 mg for tirzepatide, up to 2.4 mg for semaglutide — for 72 weeks, on top of lifestyle counseling in both arms. Two design facts frame every number that follows. The doses are each drug's own ceiling, so this compares programs as prescribed, not milligram against milligram — which is the comparison a patient actually faces, and also the reason "tirzepatide is stronger per molecule" is not quite what was tested. And the trial was open-label: with two distinct injector pens, blinding wasn't practical, so participants and clinicians knew the assignment.
The results, decomposed
At 72 weeks, mean weight reduction was roughly 20.2% with tirzepatide versus 13.7% with semaglutide — about a 6.5-point gap, a relative difference near 47%, meeting superiority. The distribution matters more than the mean for an individual: substantially more tirzepatide participants reached the deep-response thresholds — the 15%, 20% and 25% club — while both arms comfortably beat what lifestyle-plus-placebo produces in this population (2-3% in prior trials). Secondary endpoints moved the same direction, including waist circumference. The clinical reading is consistent with the mechanism story — adding GIP agonism to GLP-1 buys additional average effect — and with each drug's own placebo-controlled record: SURMOUNT-1's ~20.9% at 15 mg and STEP 1's ~14.9% at 2.4 mg landed almost exactly where this head-to-head did.
The open-label problem, taken seriously
Knowing your drug can change behavior: expectation effects on eating, adherence, even dropout. So how much of the 6.5 points could be psychology? Probably some, plausibly not much — the result reproduces the gap already visible across the blinded, placebo-controlled SURMOUNT and STEP programs, where expectation was controlled; objective endpoints like waist circumference moved concordantly; and the effect size dwarfs typical expectation effects in obesity trials. The intellectually honest position: treat 20.2-vs-13.7 as a robust estimate with soft edges, not a constant of nature. Anyone selling you certainty about your personal outcome from any trial mean is selling.
Side effects: the same family, similar bills
Both arms paid the class's usual toll — nausea, constipation, diarrhea, vomiting — mostly mild to moderate, clustering during dose escalation, with discontinuation for adverse events in the mid-single digits and broadly comparable between arms. The practical translation for a shopper deciding between molecules: tolerability is unlikely to be your tiebreaker at the class level, though individual variation is real and unpredictable in advance. The escalation-clustering fact is also the entire (unproven) premise of gentler-ramp protocols — covered in full here.
What it means at an actual checkout
Priced with this site's audited figures, the trial's gap becomes a clean decision. The audited semaglutide year is $1,428; the audited tirzepatide year is $1,668. That is $240 a year — twenty dollars a month — for the molecule that averaged roughly half again as much weight loss in the only head-to-head we have. For most shoppers whose budget clears both, the trial makes the extra $20 the easiest purchase on this site. Semaglutide keeps three honest lanes: budgets where $110-$119 audited is the ceiling, patients whose clinicians prefer its longer safety history or cardiovascular-outcomes data, and individuals who simply tolerate it better. Molecule choice remains a licensed clinician's call — this page prices the trade; it doesn't prescribe it.
FAQ
Did SURMOUNT-5 prove tirzepatide is better than semaglutide?
It showed superior average weight loss at maximum tolerated doses over 72 weeks — about 20.2% vs 13.7% — in an open-label design consistent with both drugs' blinded records. "Better on average for weight loss" is supported; "better for every person and purpose" is not what trials can say.
Was the comparison fair if the doses differ?
Each arm used its own approved maximum — the comparison patients actually face. It tests the prescribed programs, not potency per milligram.
Were side effects worse on tirzepatide?
Both arms showed the class's GI profile, mostly mild-moderate during escalation, with broadly comparable discontinuation. Class-level tolerability is a weak tiebreaker; individual response varies.
What does the difference cost?
On this site's audited prices, $240/year — $1,668 vs $1,428 — for the higher-averaging molecule.
Sources
- SURMOUNT-5 head-to-head results, tirzepatide vs semaglutide, 2025 — primary publication and presentations.
- Jastreboff AM, et al. SURMOUNT-1, NEJM 2022.
- Wilding JPH, et al. STEP 1, NEJM 2021.
- Zepbound and Wegovy prescribing information — dose ceilings and titration.
- Audited plan pricing — the open dataset, confirmed 2026-08-14.