CT-388 and Zepbound aim at the same two receptors, and that is where the similarity ends. Zepbound has 20.9% weight loss over 72 weeks in 2,539 people. CT-388 has 8% over 29 days in a phase 1. The comparison people want cannot be made yet — but the reason CT-388 exists at all is a specific criticism of how drugs like Zepbound work.
Side by side
| CT-388 | Zepbound | |
|---|---|---|
| Status | Investigational, phase 3 | FDA approved |
| Targets | GLP-1 and GIP, signalling-biased | GLP-1 and GIP |
| Weight loss | up to 8.0% at day 29 (phase 1) | 20.9% at 72 weeks (phase 3) |
| Participants | Phase 1 cohort | 2,539 |
| Side effect data | "Mostly mild or moderate" — no figures | Full label: 28% nausea, 13% vomiting |
| Can you get it | No | Yes |
| Maker | Roche | Eli Lilly |
The comparison cannot be made, and here is why that matters
Twenty-nine days against seventy-two weeks is not a difference in scale, it is a difference in what was measured. A four-week figure tells you a drug works quickly; it tells you nothing about where it finishes. Some drugs front-load their effect and plateau early; others keep going.
So any statement of the form "CT-388 is more or less effective than Zepbound" is unsupported in both directions. What can be said is that 8% in four weeks is a strong early signal, and that Roche has committed to phase 3 on the strength of data the public has not seen.
What CT-388 is actually arguing
This is the part worth understanding, because it is a specific technical criticism rather than "ours is stronger".
When tirzepatide activates a GLP-1 or GIP receptor, the cell responds by pulling that receptor inside itself — internalisation. It is a normal self-protective mechanism and it takes the receptor temporarily out of service. Over a course of treatment, that is part of why the body adapts.
CT-388 is engineered to signal strongly while causing minimal internalisation. Its published characterisation shows it activating both receptors with far less internalisation than the natural hormones produce. The claim is that receptors left on the surface keep responding, so the same targets can be worked harder for longer.
If that is right, it is a criticism of every dual agonist including tirzepatide, and it would show up not in a four-week trial but in a long one — which is precisely what has not been published.
What Zepbound has
- Phase 3 evidence at scale: 2,539 participants over 72 weeks, against a phase 1 cohort over four.
- A full safety profile: every reaction over 5%, by dose, with placebo comparison. CT-388 has one sentence describing its adverse events as mostly mild or moderate.
- An approval and a second indication, for obstructive sleep apnoea, which can open insurance coverage.
- Years of post-marketing data across a large treated population.
CT-388 cannot be prescribed at all.
Which, and what to watch
- Treatment today: Zepbound, or another approved option — the best GLP-1 for weight loss.
- Which is the better molecule: unknown, and any answer is invented.
- What would change it: a phase 3 efficacy readout from Roche, or a registered head-to-head. Neither exists.
- The interesting question: whether biased agonism does what its designers claim over a full course. That is a question about the whole drug class, not just this molecule.
The other comparisons
CT-388 vs Wegovy separates the two departures this drug makes, and CT-388 vs Ozempic explains why that pairing has no overlapping evidence.
Scientific References
3 sources- 1
Chakravarthy MV, Rodriguez R, Hergarden A, et al.
Effects of CT-388, a once-weekly signaling-biased dual GLP-1/GIP receptor agonist, on weight loss and glycemic control in preclinical models and participants with obesity
Molecular Metabolism · 103 · 2026PMID: 41319798
NIH - 2
Jastreboff AM, Aronne LJ, Ahmad NN, et al.
Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1)
New England Journal of Medicine · 387(3) · 2022PMID: 35658024
NEJM - 3
Eli Lilly and Company
ZEPBOUND (tirzepatide) injection — Prescribing Information, §6.1
DailyMed, U.S. National Library of Medicine · 2026
NIH
References open in a new tab. Content is reviewed against peer-reviewed literature as part of our editorial policy.
About the author
Editor, Modern Weight Science
Claudiu Gheorghe is the editor of Modern Weight Science. He is not a physician. His role is to synthesize peer-reviewed studies, clinical-trial data, and FDA prescribing information into clear, plain-language explanations, and to make sure every factual claim on the site traces back to a cited source. Any decision about starting, changing, or stopping a medication belongs with a licensed clinician who knows your history.
Every claim is checked against peer-reviewed research through our review process and fact-checking policy.
Frequently Asked Questions
Is CT-388 better than Zepbound?
There is no evidence either way. CT-388's only published clinical figure is up to 8% weight loss at day 29 in a phase 1 trial; Zepbound's 20.9% comes from 2,539 people over 72 weeks. A four-week measurement cannot be compared with a seventy-two-week one, and no head-to-head trial is registered.
How is CT-388 different from Zepbound if they hit the same receptors?
By how they activate them. When an ordinary agonist switches on a receptor, the cell pulls that receptor inside itself — internalisation — taking it briefly out of service. CT-388 is engineered to signal strongly while causing minimal internalisation, so the receptors stay available. It is a criticism of how existing dual agonists work rather than a different target.
Which has better side effects, CT-388 or Zepbound?
Unknown, because CT-388 has no published figures. Its phase 1 report describes adverse events as mostly mild or moderate with a profile consistent with other incretin drugs — one sentence. Zepbound has a full label listing every reaction above 5% by dose, including 28% nausea and 13% vomiting.
Will CT-388 replace Zepbound?
Far too early to say. It is in phase 3 and recruiting, with no phase 3 results published and no approval date. Roche committing to a phase 3 obesity programme signals confidence based on data the public has not seen, but many phase 3 programmes fail.
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Where to read next
Not medical advice. This guide is for general education only. GLP-1 medications, dosing, and treatment suitability are decisions for you and a licensed clinician who knows your full medical history.