Review finds tirzepatide produces more weight loss than semaglutide, with more serious adverse events

A meta-analysis of 10 head-to-head studies concluded that tirzepatide (sold as Mounjaro and Zepbound) produces greater weight loss than semaglutide (sold as Ozempic, Wegovy and Rybelsus) in adults with overweight or obesity, but that the extra weight loss came with more serious adverse events ^[1]. The review was published online September 9, 2026, in the journal Clinical Obesity ^[1].

The analysis pooled three randomized controlled trials and seven retrospective cohort studies, covering 41,381 adults with a body mass index of 25 or higher ^[1]. Follow-up ranged from 24 to 72 weeks. The studies excluded people with type 1 diabetes, prior bariatric surgery or active cancer ^[1]. Both drugs were given as once-weekly injections under differing regimens — fixed-dose, dose-escalation, maximum tolerated dose or variable real-world dosing. Doses ranged from 2.5 mg to 15 mg for tirzepatide and from 0.25 mg to 2.4 mg for semaglutide ^[1].

On weight, tirzepatide came out ahead. The mean difference between the two drugs was 4.28 percentage points when weight change was measured as a share of body weight, across 10 studies, and 4.43 kg in absolute terms, across eight studies ^[1]. A seven-study analysis found that people on tirzepatide were more likely to lose at least 10% of their body weight (risk ratio 1.43), at least 15% (risk ratio 1.85) and at least 20% (risk ratio 2.03) than people on semaglutide. There was no significant difference between the drugs at the 5% weight-loss threshold ^[1]. Tirzepatide was also linked to a slightly larger drop in A1c, a marker of average blood sugar, with a mean difference of 0.29 percentage points (95% CI, −0.44 to −0.14; five studies) ^[1].

On safety, the picture was less favorable for tirzepatide. Serious adverse events were significantly more frequent with tirzepatide, reported in 5.7% of patients compared with 2.9% of those on semaglutide (risk ratio 1.83; three studies). However, the share of patients who stopped treatment because of adverse events did not differ significantly between the groups ^[1].

The authors framed the findings as a trade-off. "Among adults with overweight or obesity, tirzepatide is associated with greater weight reduction and modestly greater glycaemic improvement compared with semaglutide, although these benefits may be accompanied by a higher risk of serious adverse events," they wrote. "These findings highlight the need to balance efficacy and tolerability when selecting incretin-based therapies" ^[1].

The review has limits. Large observational cohorts made up much of the total sample, so residual confounding could not be ruled out, and the authors reported substantial heterogeneity across several efficacy outcomes. Maximum follow-up of 72 weeks left long-term weight-loss durability and rare safety events unevaluated ^[1]. The analysis also states that trial design and dose-escalation schedules varied between studies, which complicates direct comparisons of the two molecules ^[1]. Which specific serious adverse events occurred is not broken out in the results, and whether the two drugs perform differently in people who switch from one to the other has not been reported.

Why it matters for patients

Many people with overweight or obesity end up comparing these two injected drugs, sometimes because an employer or insurer covers only one of them. This review suggests the choice is not simply "the stronger one wins." On average, tirzepatide produced about four percentage points more weight loss, and far more patients on tirzepatide crossed the 15% and 20% weight-loss thresholds ^[1]. At the same time, the serious adverse event rate was roughly double with tirzepatide — 5.7% versus 2.9% — even though the rates of quitting treatment because of side effects were statistically similar ^[1].

The numbers also describe group averages across studies with different designs and dosing, not what any one person should expect. About 94% of tirzepatide patients and about 97% of semaglutide patients in the three-study safety analysis did not have a serious adverse event ^[1]. The review does not say whether the serious events were caused by the drugs, which events they were, or how they were managed, and it does not address insurance coverage rules or switching policies.