Tirzepatide headlines almost always say the same thing: it outperforms semaglutide. The underlying trials support that conclusion in a limited way, but the way the comparison is usually drawn overstates the certainty of the evidence. The two drugs were never tested head-to-head in a weight loss trial. What exists are two separate trials, with different patient populations, different time points, and different doses, whose results happen to show different numbers.

Getting the comparison right requires understanding what each trial actually measured.

The Two Trials

SURMOUNT-1 tested tirzepatide for weight loss in adults without diabetes. Published in the New England Journal of Medicine in 2022, the trial enrolled 2,539 adults with a BMI of 30 or higher, or 27 or higher with at least one weight-related comorbidity. Participants received 5 mg, 10 mg, or 15 mg of tirzepatide weekly, or placebo. At 72 weeks:

  • 5 mg group: average weight loss of 15.0%
  • 10 mg group: average weight loss of 19.5%
  • 15 mg group: average weight loss of 20.9%
  • Placebo group: average weight loss of 3.1%

STEP-1 tested semaglutide 2.4 mg weekly for weight loss in adults without diabetes. Published in the New England Journal of Medicine in 2021, the trial enrolled 1,961 adults with similar BMI criteria. At 68 weeks, the semaglutide group lost an average of 14.9% versus 2.4% in the placebo group.

The headline comparison: 20.9% for tirzepatide versus 14.9% for semaglutide.

Why That Comparison Is Incomplete

The two trials differ in ways that make direct comparison methodologically problematic.

Different durations: SURMOUNT-1 ran for 72 weeks. STEP-1 ran for 68 weeks. Weight loss generally continues over time on these medications, so an additional 4 weeks gives tirzepatide a structural advantage.

Different populations: The exact BMI distributions, diabetes rates (both excluded current diabetes, but differed in other conditions), and baseline weights varied between trials. Different starting populations affect the absolute amount of weight available to lose and the rate at which it is lost.

Different placebo arms: STEP-1’s placebo lost 2.4% of body weight. SURMOUNT-1’s placebo lost 3.1%. This suggests participants in the two trials were not fully comparable, which complicates interpreting the treatment arm results relative to each other.

No shared control: Without a single trial randomizing participants to both drugs, random variation in baseline characteristics, site selection, and placebo response rates cannot be ruled out as contributors to the different results.

A 2023 network meta-analysis published in The Lancet attempted to synthesize the available data across GLP-1 and dual-agonist trials. It found that tirzepatide at higher doses appeared more effective than semaglutide, but noted significant uncertainty given the indirect nature of the comparison. Network meta-analyses provide better evidence than single-trial comparisons, but they cannot substitute for a head-to-head trial.

What Makes Tirzepatide Different Mechanistically

Both drugs work through GLP-1 receptors. The difference is that tirzepatide also activates GIP (glucose-dependent insulinotropic peptide) receptors. This dual-agonist mechanism is why tirzepatide is described as more than just a second GLP-1 drug.

GIP receptors are found in adipose tissue, the gut, and the brain. When activated, they appear to enhance GLP-1’s appetite-suppressing effects and may have independent effects on fat metabolism. The relative contribution of GIP receptor agonism to tirzepatide’s clinical effect, versus enhanced GLP-1 potency from its molecular structure, is still being studied.

The FDA approved tirzepatide for weight loss under the brand name Zepbound in November 2023, following the SURMOUNT-1 data. It is the same molecule as Mounjaro, which was approved for type 2 diabetes management in 2022.

Side Effect Profiles

The side effect profiles of tirzepatide and semaglutide are similar. Both cause primarily gastrointestinal side effects, most commonly nausea, vomiting, diarrhea, and constipation, with symptoms most pronounced during dose escalation.

In SURMOUNT-1, gastrointestinal adverse events in the tirzepatide groups ranged from 20-30% for nausea across all doses. Comparable rates in STEP-1 for semaglutide were around 44%. This difference may reflect the specific populations enrolled, the dose escalation protocols used, or genuine pharmacological differences. It cannot be definitively attributed to tirzepatide being better tolerated without a controlled comparison.

Serious adverse events occurred at similar rates between treatment and placebo in both trials.

Cost and Availability

At the brand-name level, Zepbound (tirzepatide) and Wegovy (semaglutide) are both expensive without insurance, with list prices in the $1,000-1,300 monthly range.

Compounded versions of both drugs are available through telehealth programs at substantially lower prices. The regulatory situation for compounded tirzepatide parallels that of semaglutide: FDA shortage exemptions allowed compounding during shortage periods, and the legal status of compounded versions continues to evolve as manufacturers restore supply.

As of early 2025, compounded tirzepatide remains more widely available than compounded semaglutide in the US market, partly because the tirzepatide shortage has not officially resolved to the same extent. This situation is subject to change.

Which Should You Choose

The trial data shows tirzepatide producing higher average weight loss, but the comparison is indirect and the real-world difference is likely smaller than headline numbers suggest.

For many patients, the choice is dictated by availability, insurance coverage, cost, and which compounded form their telehealth provider works with. For patients paying out of pocket for compounded forms, the cost difference between the two is often minimal.

A prescribing clinician who knows your medical history and response to prior interventions is better positioned to make a recommendation than a population-level comparison of two different trials. If one medication does not produce adequate results after dose escalation, switching is a reasonable option.

For context on how both medications work through GLP-1 receptors, see What Is Semaglutide and How Does It Work for Weight Loss?. For guidance on evaluating telehealth programs that prescribe these medications, see How to Get Weight Loss Medication Online.