Tirzepatide vs Semaglutide: A Head-to-Head Look at SURMOUNT and STEP

Semaglutide (Wegovy) and tirzepatide (Zepbound) are the two injectable weight-loss medications most patients are choosing between in 2026. They're mechanistically similar — both are once-weekly subcutaneous injections that target the body's incretin pathways — but they aren't the same drug, and the trial evidence comparing them is now substantial enough to say meaningful things about which one works better, for whom, and at what cost.

This piece walks through the head-to-head and parallel trial data from the STEP family (semaglutide) and the SURMOUNT family (tirzepatide), then layers on the side effect profile, discontinuation rates, and the cost-per-pound calculation that most patients actually need to make a real decision.

Mechanism: a single agonist vs a dual agonist

Semaglutide is a glucagon-like peptide-1 (GLP-1) receptor agonist. It mimics the gut hormone GLP-1, which is released in response to eating and produces several effects relevant to weight: it slows gastric emptying, increases satiety signaling in the brain, and improves insulin secretion in response to glucose.^[1]

Tirzepatide is a dual agonist — it activates both the GLP-1 receptor and the glucose-dependent insulinotropic polypeptide (GIP) receptor. GIP is a sister incretin hormone with overlapping but distinct effects on appetite, energy expenditure, and adipose tissue metabolism. The combination produces measurably stronger effects on weight and glucose control in trials than GLP-1 agonism alone, and it's the reason tirzepatide outperforms semaglutide on most weight-loss endpoints in head-to-head data.^[2]

Whether GIP's contribution is purely additive or whether it enables novel mechanisms (changes in adipose tissue insulin sensitivity, increased energy expenditure) is still a live research question. What's clear from the trials: the dual agonist consistently produces more weight loss at any given dose equivalence point.

The headline trial results

The cleanest comparison is between STEP 1 (semaglutide 2.4 mg weekly, 68 weeks, n=1,961) and SURMOUNT-1 (tirzepatide 5/10/15 mg weekly, 72 weeks, n=2,539). Both enrolled adults with overweight or obesity but without type 2 diabetes, both used the same lifestyle intervention background, and both used the highest labeled doses for the weight-loss indication.^[3][4]

At the highest dose:

• Semaglutide 2.4 mg: mean weight loss of 14.9% at 68 weeks. About 32% of patients reached ≥20% body weight loss.
• Tirzepatide 15 mg: mean weight loss of 22.5% at 72 weeks. About 57% of patients reached ≥20% body weight loss.

That's roughly a 7-8 percentage point absolute difference, which translates to about 50% more weight loss on tirzepatide than semaglutide at maximum dose. The difference is large, it's statistically robust, and it has held up across subsequent trial replications.

SURMOUNT-5 — the first true head-to-head trial — confirmed this. Published in 2025, SURMOUNT-5 randomized 751 adults with obesity directly to tirzepatide or semaglutide at maximum tolerated doses for 72 weeks. Tirzepatide produced a mean weight loss of about 20.2% versus 13.7% for semaglutide. The 6.5 point absolute difference was significant for every secondary endpoint, including waist circumference, BMI category transitions, and fasting glucose.^[5]

Side effect profile

Both drugs share the GI side effect profile that's characteristic of GLP-1 agonism: nausea, vomiting, diarrhea, constipation. The pooled rates are higher than placebo for both, and meaningfully different between the two only in degree.

Across the STEP and SURMOUNT trials, side effect rates at the highest doses look roughly like this:

• Nausea: ~44% on semaglutide, ~33% on tirzepatide. Most events are mild to moderate and concentrated in the dose-escalation period.
• Vomiting: ~24% semaglutide, ~13% tirzepatide.
• Diarrhea: ~30% on both.
• Constipation: ~24% semaglutide, ~17% tirzepatide.
• Severe / serious adverse events: roughly equivalent, around 8–10% in both treatment arms versus 5–7% in placebo.

The somewhat surprising pattern: tirzepatide actually had lower rates of upper-GI symptoms (nausea, vomiting) than semaglutide in the trials, despite producing more weight loss. The leading hypothesis is that GIP receptor activation has a separate anti-emetic effect that partially counteracts the nausea induced by GLP-1 agonism. This is one of the more intriguing pharmacology stories in the field.^[6]

Discontinuation rates

Discontinuation is the metric that matters most in real clinical practice, because the trial completion rates — both around 75–80% — mask the fact that many of the people who finished did so on lower doses than planned, after dose reductions for tolerability. In the trials:

• STEP 1 semaglutide: 7.0% discontinued for adverse events, vs 3.1% on placebo.
• SURMOUNT-1 tirzepatide 15 mg: 6.2% discontinued for adverse events, vs 2.6% on placebo.

Real-world data is messier and shows much higher attrition. A large 2024 commercial-claims analysis of US patients starting either drug for obesity found that roughly 70% had stopped within 12 months — far higher than the trial dropout rates. The leading reasons in the real-world data were cost (in 38% of discontinuers), GI side effects (28%), and access problems (drug shortages, pharmacy issues, prior auth denials, ~22%).^[7]

Patients who can sustain therapy for at least 6 months tend to keep going, while patients who quit before 90 days are usually out for good. The first three months are the highest-risk window for discontinuation, and they're also the dose escalation window.

Cardiovascular and metabolic outcomes

Trial data on hard cardiovascular outcomes is more mature for semaglutide than for tirzepatide, simply because semaglutide has been on the market longer. The SELECT trial (2023) showed a 20% relative risk reduction for major adverse cardiovascular events (MACE) in patients with established cardiovascular disease and overweight/obesity but without diabetes. That's the strongest evidence in the field that the cardiovascular benefits of GLP-1 weight loss are real and durable.^[8]

Tirzepatide's cardiovascular outcomes trial — SURPASS-CVOT, a head-to-head against dulaglutide in patients with type 2 diabetes — reported topline results in 2024 showing non-inferiority for MACE. The dedicated tirzepatide-for-obesity cardiovascular outcomes trial (SURMOUNT-MMO) is ongoing, with results expected in 2027. Until that reads out, the assumption in the field is that the dual agonist will produce at least as much cardiovascular benefit as the GLP-1-only drug, but the formal evidence base is currently smaller.^[9]

Both drugs produce roughly equivalent improvements in fasting glucose, HbA1c, blood pressure, triglycerides, and liver enzymes. For patients with type 2 diabetes specifically, tirzepatide's edge on glucose control is more pronounced — the SURPASS trials showed superior HbA1c reductions head-to-head against semaglutide in diabetic populations.^[10]

The cost-per-pound calculation

The interesting question for most patients isn't which drug produces more weight loss in a trial — it's which one produces the best value, given what they'll actually pay out of pocket.

For a hypothetical 220-pound patient:

• Brand-name Wegovy at ~$1,349/month list price, producing 14.9% weight loss over 68 weeks: roughly $5,750 per 15% body weight reduction, or about $176 per pound lost.
• Brand-name Zepbound at ~$1,086/month list price (Lilly recently lowered cash-pay), producing 22.5% weight loss over 72 weeks: roughly $120 per pound lost.
• Compounded semaglutide at the median ~$150/month, same trial efficacy assumption: roughly $20 per pound lost.
• Compounded tirzepatide at the median ~$300/month: roughly $33 per pound lost.

The compounded forms reverse the cost ranking: semaglutide is actually cheaper per pound on a cash-pay basis, even though tirzepatide produces more total weight loss, because compounded tirzepatide remains roughly 2x the price of compounded semaglutide. For patients optimizing pure cost-per-result, that flip is the most important practical takeaway from this whole comparison.

Caveat: these calculations assume trial efficacy translates to real-world results, which is generous. Real-world weight loss is usually 60–75% of trial weight loss because patients don't complete the same dose escalation, miss more doses, and have less structured lifestyle support. But the relative ranking holds.

Who should pick which

Working through the trial data and the cost calculations, here's how the choice tends to shake out for different patient profiles:

• Higher BMI (40+) seeking maximum weight loss: tirzepatide. The 7-8 point absolute weight-loss advantage is larger in absolute terms when you have more weight to lose, and the cost differential matters less proportionally.
• Type 2 diabetes plus obesity: tirzepatide. The glucose control advantage in the SURPASS trials is meaningful and the FDA-approved obesity indication was extended to this population first.
• Lower BMI (27–32) needing 10–15% loss to hit a goal: semaglutide. You don't need the extra horsepower, and you'll save substantially on cost.
• Established cardiovascular disease without diabetes: currently semaglutide, on the strength of SELECT. This may change once SURMOUNT-MMO reads out for tirzepatide in 2027.
• History of severe GI sensitivity or prior GLP-1 intolerance: tirzepatide is worth trying given the lower nausea and vomiting rates — but expect to start slow regardless of which one.
• Cost is the deciding factor: compounded semaglutide remains the lowest cost-per-pound option in the market, by a meaningful margin.

What we'll be watching in 2026–2027

The two big readouts that should change how this comparison looks:

• SURMOUNT-MMO: the dedicated tirzepatide cardiovascular outcomes trial in patients with obesity but without diabetes. If positive (which seems likely), it eliminates the last major reason to default to semaglutide for patients with cardiovascular risk.
• Oral semaglutide in higher doses: oral semaglutide already exists as Rybelsus at low doses for diabetes. The OASIS trial showed that higher oral doses produce weight loss approaching (but not matching) injectable semaglutide. If approved at the higher dose for obesity, it could shift the convenience calculus dramatically.^[11]
• Triple agonists (retatrutide). Eli Lilly's triple GLP-1/GIP/glucagon agonist showed mean weight loss of 24.2% at 48 weeks in phase 2 — numerically higher than tirzepatide. If phase 3 results are consistent and approval comes through in 2027, the entire competitive picture resets.^[12]

Until those readouts, the practical answer for most patients remains the one above: tirzepatide for maximum effect, semaglutide for maximum value, and the compounded forms of either for meaningful cost savings against brand. The full comparison across every provider in our dataset is on our compare page.