Semaglutide vs tirzepatide

Both are approved, effective GLP-1-class weight-loss and diabetes drugs. The real difference is mechanism and magnitude: tirzepatide's added GIP action produces more weight loss on average, including in a real head-to-head trial against semaglutide — not just separate placebo-controlled ones. Insurance and public-system coverage for both remains patchy and inconsistent across countries, in ways that don't track the efficacy difference at all.

Side by side

Mechanism: one receptor vs two

Semaglutide is a GLP-1 (glucagon-like peptide-1) receptor agonist: it slows gastric emptying and increases satiety through one signalling pathway. Tirzepatide adds a second one — it also activates the GIP (glucose-dependent insulinotropic polypeptide) receptor. The two hormones have complementary effects on insulin secretion and appetite, which is the mechanistic reason tirzepatide's trials have consistently reported a larger effect size than semaglutide's. See each drug's own semaglutide and tirzepatide page for the full trial evidence and safety profile of each.

Efficacy: separate trials, then a real head-to-head

For years, the standard comparison between these two drugs was indirect: STEP 1 (Wilding et al., N Engl J Med 2021) tested semaglutide against placebo and found 14.9% mean weight loss at 68 weeks; SURMOUNT-1 (Jastreboff et al., N Engl J Med 2022) tested tirzepatide against placebo and found 15–20.9% depending on dose, at 72 weeks. Comparing two separate placebo-controlled trials run in different patient populations at different times is suggestive, not conclusive — cross-trial comparisons are exactly the kind of indirect evidence a single head-to-head trial is designed to replace.

That head-to-head trial now exists. SURMOUNT-5 (Aronne et al., N Engl J Med 2025, DOI 10.1056/NEJMoa2416394) randomised 751 adults with obesity or overweight and at least one weight-related comorbidity to either drug directly, at their respective highest approved/studied doses, for 72 weeks. Result: tirzepatide produced 20.2% mean weight loss versus 13.7% for semaglutide — a statistically significant difference, with tirzepatide also ahead on waist circumference and several secondary measures. This is the first trial that actually put both drugs in the same study rather than comparing across two separate placebo-controlled trials, and it confirms the pattern the separate trials had already suggested.

Insurance and public-coverage comparison

Coverage for both drugs is inconsistent in ways that track pricing negotiations and system capacity, not the efficacy difference above. In the UK, both are NICE-recommended, but on different timelines and through different eligibility tiers: semaglutide's NICE appraisal (TA875) dates to March 2023, tirzepatide's (TA1026) to December 2024 — over a year and a half later — and each drug's actual NHS rollout has its own BMI-and-comorbidity-tiered access schedule, detailed on each drug's own page. In Australia, as of this review, neither is PBS-listed for weight loss specifically: Wegovy (semaglutide) was PBAC-recommended in November 2025 but a listing still hasn't landed, and Mounjaro (tirzepatide) was PBAC-recommended in March 2026 only for Eli Lilly to reject the government's price and walk away in April 2026 — a harder stop than semaglutide's pricing delay. Canada's divergence is specific to semaglutide: a lapsed patent-maintenance fee let Health Canada authorize a generic semaglutide years ahead of the US, a story with no equivalent reported for tirzepatide. In the US, coverage for either drug for a weight-loss indication specifically depends heavily on the individual health plan's formulary — Medicare Part D has historically excluded anti-obesity drugs from coverage by statute, regardless of which GLP-1/GIP drug is prescribed, and this hasn't been consistently true for the diabetes-indication brands (Ozempic, Mounjaro) versus the obesity-indication ones (Wegovy, Zepbound) even though the underlying molecule is identical — check your own plan's formulary rather than assuming either drug is covered.

The compounded/grey-market version: same situation, same deadline

Neither drug's grey-market, non-pharmacy-dispensed version is on more solid legal ground than the other. FDA's 30 April 2026 proposal to exclude compounded versions from the 503B bulks list names semaglutide, tirzepatide and liraglutide together (Federal Register 2026-08552) — there is no meaningful difference in legal exposure between buying one or the other outside a licensed pharmacy's actual prescription.

Which one is "right"? Wrong question

The honest framing isn't "which drug is better" — it's "which drug is actually accessible to you, at what cost, prescribed by an actual clinician who can monitor you on it." The efficacy gap above is real and reproducible, but it's a population-average result from a 751-person trial, not a guarantee for any one individual; some people respond better to one mechanism than the other regardless of the average. If both are genuinely available to you through a licensed prescriber, the SURMOUNT-5 result is a legitimate reason to discuss tirzepatide specifically. If only one is covered by your insurance, in your public health system, or available at all where you live, that access question — not the average trial result — is usually what actually decides it.