The mechanical difference
Semaglutide is a GLP-1 receptor agonist. It mimics one gut hormone, GLP-1, which signals fullness to the brain and slows gastric emptying. It is sold as Wegovy for weight management and Ozempic for type 2 diabetes, both weekly injections, plus Rybelsus as a daily oral tablet for type 2 diabetes.
Tirzepatide is a dual agonist. It acts on the GLP-1 receptor and also on the GIP receptor, a second gut hormone pathway. It is sold as Zepbound for weight management and Mounjaro for type 2 diabetes, both weekly injections.
The practical significance of that second receptor is still being worked out, but the clinical results consistently favour the dual agonist on weight outcomes.
What the individual trials showed
In the STEP 1 trial, published in the New England Journal of Medicine in 2021, participants on semaglutide 2.4mg weekly lost a mean of about 14.9% of body weight over 68 weeks, against about 2.4% on placebo. Everyone in the trial also received lifestyle intervention.
In SURMOUNT-1, published in 2022, participants on tirzepatide lost a mean of roughly 15% at the 5mg dose, 19.5% at 10mg and 20.9% at 15mg over 72 weeks, against about 3.1% on placebo.
Comparing across separate trials is a genuinely unreliable exercise — different populations, different durations, different protocols. Which is what makes the direct comparison worth paying attention to.
The head-to-head result
SURMOUNT-5 compared the two drugs directly in adults with obesity and without diabetes, over 72 weeks. Tirzepatide produced a mean weight reduction of about 20.2% against about 13.7% for semaglutide.
That is a meaningful margin and it is the strongest evidence available on the question. It is also one trial, in one population, at maximum tolerated doses — and a mean is not a prediction for any individual. Plenty of people in the semaglutide arm outperformed plenty of people in the tirzepatide arm.
Side effects are broadly similar
Both drugs are dominated by gastrointestinal effects: nausea, vomiting, diarrhoea and constipation, most pronounced during dose escalation. In the trials these were the leading reason participants stopped treatment.
Both carry a boxed warning regarding thyroid C-cell tumours observed in rodent studies, and both are contraindicated in people with a personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia syndrome type 2.
Tolerance is individual. Some people who cannot tolerate one manage the other perfectly well, which is a conversation worth having with a prescriber rather than abandoning the class entirely.
Why the bigger number may not be your answer
Several things can reasonably outrank efficacy. Cost and insurance coverage differ between the two and can differ by thousands of dollars a year. Supply has been inconsistent for both at various points. Individual tolerability varies. And your prescriber may have a clinical reason specific to your history.
The medication you can actually obtain, afford, and keep taking will outperform the theoretically superior one you stop after two months. That is not a consolation prize — adherence is the single largest determinant of outcome in this entire category.
What neither drug does
Neither is a course of treatment you finish. Weight regain after discontinuation is well documented for both, because the appetite signal returns when the drug suppressing it is withdrawn.
Neither preserves muscle on its own. Rapid weight loss from any cause includes lean tissue, and adequate protein intake and resistance training matter more than usual while taking these drugs.
And neither was studied in isolation. Every one of these trials included structured lifestyle support alongside the medication. The results describe the combination.