How Does Zepbound Work? GIP and GLP-1, Explained Simply

Zepbound works by activating two receptors at once: the GIP (glucose-dependent insulinotropic polypeptide) receptor and the GLP-1 (glucagon-like peptide-1) receptor ^[1]. Those are the same receptors the body’s natural incretin hormones lock onto, and tirzepatide is built to mimic both hormones rather than just one ^[3]. In studies, once-weekly dual GIP/GLP-1 agonism has produced greater weight loss than GLP-1 receptor agonism alone ^[8][9].

The FDA label classifies Zepbound as a GIP receptor and GLP-1 receptor agonist, with an initial U.S. approval year of 2022 ^[1]. It is a subcutaneous injection indicated, in combination with a reduced-calorie diet and increased physical activity, to reduce excess body weight and maintain that reduction long term in adults with obesity, or adults with overweight plus at least one weight-related comorbid condition, and to treat moderate to severe obstructive sleep apnea (OSA) in adults with obesity ^[1]. The label states that coadministration with other tirzepatide-containing products or with any GLP-1 receptor agonist is not recommended ^[1].

What are GIP and GLP-1?

GLP-1 stands for glucagon-like peptide-1, and GIP stands for glucose-dependent insulinotropic polypeptide ^[1][8]. Both are incretin hormones, part of the same signaling family that drug developers have targeted with incretin receptor therapies ^[3].

The GLP-1 side of this picture is the better understood one. The mechanisms by which GLP-1 receptor agonism contributes to chronic weight management and glycemic control are described as well understood ^[9]. GLP-1 receptor agonist use is also associated with gastrointestinal tolerability problems, including nausea and vomiting, which can limit how well people stick with treatment ^[9].

GIP biology, by contrast, is described as multimodal, meaning it acts through several different pathways rather than one clean mechanism ^[8]. That complexity is why researchers are still working out exactly what the GIP half of Zepbound contributes.

Why does targeting both receptors matter?

The development of dual agonists for the GLP-1 and GIP receptors has been described as a landmark moment in the treatment of type 2 diabetes and obesity ^[8]. Once-weekly injectable tirzepatide, a dual agonist in this class, is described as highly effective for treating people living with obesity and type 2 diabetes, resulting in greater weight loss than GLP-1 receptor agonism alone ^[8]. A phase 1 research letter on GIP receptor agonism and GLP-1 tolerability opens by noting the same pattern: tirzepatide has shown greater reductions in HbA1c and body weight compared with placebo or with selective GLP-1 receptor agonists ^[9].

The interesting wrinkle is that GIP is not a simple “more is better” target. Both activating the GIP receptor (agonism, which is what Zepbound does) and blocking it (antagonism) have been shown to add weight loss when combined with GLP-1 receptor agonism, and the reasons are incompletely understood ^[8].

How was the tirzepatide molecule designed?

Tirzepatide was developed by Eli Lilly under the code name LY3298176 ^[3]. It is a fatty acid–modified peptide with dual GIP and GLP-1 receptor agonist activity, designed for once-weekly subcutaneous administration ^[3].

In laboratory cell assays, the molecule activated signaling at both the GIP and GLP-1 receptors ^[3]. In mice, it produced glucose-dependent insulin secretion and improved glucose tolerance by acting on both receptor types ^[3]. With chronic dosing in mice, it decreased body weight and food intake, and those effects were significantly greater than the effects of a GLP-1 receptor agonist given on its own ^[3].

The first human testing was a Phase 1 program registered as NCT02759107, with three parts: a single-ascending dose study (0.25–8 mg) and a 4-week multiple-ascending dose study (0.5–10 mg) in healthy subjects, followed by a 4-week Phase 1b proof-of-concept study (0.5–15 mg) in people with type 2 diabetes ^[3]. Across the program, a total of 142 human subjects received at least one dose of LY3298176, the positive control dulaglutide, or placebo ^[3]. In the Phase 1b diabetes trial, the 10 mg and 15 mg doses significantly reduced fasting serum glucose compared with placebo, with least-squares mean differences of −49.12 mg/dL and −43.15 mg/dL, respectively ^[3].

What does the GLP-1 side of Zepbound do?

GLP-1 receptor agonism is the better-characterized half of the mechanism. The mechanisms by which GLP-1 receptor agonism contributes to chronic weight management and glycemic control are described as well understood ^[9].

The trade-off is tolerability. GLP-1 receptor agonist use is associated with nausea and vomiting that can limit efficacy and compliance ^[9]. Researchers note that both the beneficial and the adverse effects of GLP-1 receptor agonism are ascribed to signaling in hindbrain regions — and those same regions also express GIP receptors, which is one reason scientists suspected the two systems interact ^[9].

Separately, as a drug-interaction fact about Zepbound as a whole, the label states that Zepbound delays gastric emptying and has the potential to affect the absorption of oral medications taken at the same time ^[1].

What does the GIP side add in the brain?

Most of what is known about the GIP half comes from animal research, and it points to specific brain circuits. In mice, GIP receptor agonists decrease food intake and body weight through a pathway involving Gipr-expressing GABAergic neurons, a population of which sits in the area postrema (AP), a structure in the brainstem ^[8][11]. This appetite effect works independently of the GLP-1 receptor in mice ^[11]. Separately, chemogenetic activation of Gipr-expressing neurons in either the hypothalamus or the hindbrain decreases food intake in mice, suggesting more than one central GIPR population can regulate appetite ^[8].

A 2026 study in Nature Metabolism tested the roles of these two brain regions directly by knocking out Gipr in either the area postrema (GiprAP-KO mice) or the hypothalamus (Giprhypo-KO mice) ^[8]. The two knockouts behaved very differently. GiprAP-KO mice showed partial protection against diet-induced obesity and a reduced response to the appetite-suppressing effects of acyl-GIP ^[8]. Giprhypo-KO mice showed normal appetite suppression from acyl-GIP but enhanced weight loss on the GLP-1 receptor agonist liraglutide, and knocking out hypothalamic Gipr abolished the extra weight loss that a GIP receptor antagonist normally adds to liraglutide ^[8].

The study’s conclusion: the area postrema mediates the appetite-suppressing effects of GIP receptor agonism (Zepbound’s approach), while GIP receptors in the hypothalamus underlie the ability of GIP receptor antagonism to enhance weight loss from GLP-1 and amylin receptor agonists ^[8]. The same paper presents a contrasting line of evidence: Gipr knockout mice, and humans with impaired-function GIPR gene variants, are protected against obesity — a finding the authors set against the appetite-reducing actions of GIPR activation, not alongside them ^[8].

Does activating GIP help with nausea?

There is a scientific reason to think it might, but the human evidence is still early. In animals, GIP is anti-emetic: it reduces nausea-related behaviors triggered by a variety of agents, an effect attributed to Gipr-expressing cells in the brainstem and area postrema ^[8]. In animal experiments, GIP receptor agonism reduced gastrointestinal adverse effects induced by chemotherapy agents or by GLP-1 receptor agonism ^[9].

A Phase 1 crossover trial (NCT05444569) was run to test the idea in people. It took place at a single site in Singapore and enrolled healthy volunteers aged 21–60 with body weight of at least 55 kg and a BMI of 23–40 kg/m² ^[9]. Each participant received a single subcutaneous 25 mg dose of the selective long-acting GIP receptor agonist LY3537021 (half-life approximately 11–14 days) or placebo, followed by daily liraglutide escalated from 0.6 mg to 2.4 mg and held at 2.4 mg through day 9, with an 8-week washout between periods ^[9]. The hypothesis being tested was that liraglutide plus a GIP receptor agonist produces fewer gastrointestinal adverse events than liraglutide alone ^[9].

Two cautions apply. LY3537021 is a different molecule from tirzepatide, and the participants were healthy volunteers, not people taking Zepbound ^[9]. The published description covers the design and hypothesis; detailed results are not reported in the documents reviewed here.

Why do some drugs block GIP while Zepbound activates it?

In both preclinical and clinical studies, adding either a GIP receptor agonist or a GIP receptor antagonist to GLP-1 receptor agonism causes additional weight loss, and the reasons remain incompletely understood ^[8]. A 2025 review in the Journal of Clinical Medicine, titled “The Premise of the Paradox: Examining the Evidence That Motivated GIPR Agonist and Antagonist Drug Development Programs,” frames this debate directly ^[2].

The antagonist camp has real evidence behind it. A 2025 counterpoint in Diabetes notes that both global and central nervous system knockout of GIP receptors protects mice fed a high-fat diet from obesity and insulin resistance, and that genome-wide association studies correlate lower BMI with reduced-function GIPR gene variants in humans ^[10]. Pharmacology studies in mice and monkeys confirmed that GIP receptor antagonism enhances GLP-1-induced weight reduction, and a Phase 1 study provided proof of principle that beneficial effects extend to humans ^[10]. The authors state plainly that the mechanism behind the agonist/antagonist paradox is unknown ^[10].

Mouse genetics adds a layer. The body weight and food intake effects of GIP receptor antagonism are eliminated in mice with global loss of either Gipr or Glp-1r, but preserved in mice lacking Gipr in central nervous system GABAergic neurons or peripherin-expressing peripheral neurons ^[11]. Single-nucleus RNA sequencing shows that agonism and antagonism have opposing effects in the dorsal vagal complex, with antagonism — but not agonism — closely resembling a GLP-1 signature ^[11].

Does GIP act outside the brain, in fat tissue?

Possibly. A 2024 paper in Cell Metabolism is titled “Tirzepatide modulates the regulation of adipocyte nutrient metabolism through long-acting activation of the GIP receptor” ^[5]. The underlying data from that paper were not available for review here. On the other side of the debate, GIP’s physiologic role in fat deposition is part of the rationale reviewers cite for pursuing GIP receptor antagonism instead ^[10].

How is Zepbound dosed?

The label lays out a fixed escalation path for all indications ^[1].

Step	Dose (subcutaneous, once weekly)	Notes
Starting dose	2.5 mg for 4 weeks	For initiation only; not approved as a maintenance dosage [1]
First increase	5 mg	After the first 4 weeks [1]
Further increases	2.5 mg increments	Only after at least 4 weeks on the current dose [1]
Maintenance (weight)	5 mg, 10 mg, or 15 mg	For weight reduction and long-term maintenance [1]
Maintenance (OSA)	10 mg or 15 mg	For obstructive sleep apnea [1]
Maximum	15 mg	For all indications [1]

The escalation schedule exists specifically to reduce the risk of gastrointestinal adverse reactions, and the label instructs prescribers to consider treatment response and tolerability when selecting a maintenance dosage, including considering a lower maintenance dosage if the current one is not tolerated ^[1]. Decisions about which dose is right belong with a prescriber.

Zepbound comes as a single-dose pen or vial in strengths of 2.5, 5, 7.5, 10, 12.5, or 15 mg per 0.5 mL, and as a multi-dose vial or single-patient-use KwikPen holding four doses per device — for example, 10 mg/2.4 mL for four 2.5 mg/0.6 mL doses, up to 60 mg/2.4 mL for four 15 mg/0.6 mL doses ^[1]. A KwikPen should never be shared between patients, even with a new needle ^[1].

What side effects come with how Zepbound works?

The label carries a boxed warning: in rats, tirzepatide causes thyroid C-cell tumors, and it is unknown whether Zepbound causes thyroid C-cell tumors, including medullary thyroid carcinoma, in humans ^[1]. Zepbound is contraindicated in people with a personal or family history of medullary thyroid carcinoma, in people with Multiple Endocrine Neoplasia syndrome type 2, and in anyone with known serious hypersensitivity to tirzepatide or its excipients ^[1].

The most common adverse reactions, reported in ≥5% of patients, are nausea, diarrhea, vomiting, constipation, abdominal pain, dyspepsia, injection site reactions, fatigue, hypersensitivity reactions, eructation, hair loss, and gastroesophageal reflux disease ^[1]. Warnings include severe gastrointestinal reactions (the drug is not recommended in severe gastroparesis), acute kidney injury from volume depletion, acute gallbladder disease, acute pancreatitis, hypersensitivity, hypoglycemia when combined with insulin or an insulin secretagogue, diabetic retinopathy progression in people with type 2 diabetes, and pulmonary aspiration during general anesthesia or deep sedation ^[1]. The label also states Zepbound may cause fetal harm and should be discontinued when pregnancy is recognized, and advises oral contraceptive users to switch to a non-oral method or add a barrier method for 4 weeks after initiation and for 4 weeks after each dose escalation ^[1].

The pattern was visible from the first human trials: the most frequent side effects with tirzepatide in Phase 1 were gastrointestinal — vomiting, nausea, decreased appetite, diarrhea, and abdominal distension — and all were dose-dependent and mild to moderate in severity ^[3].

What is still not known?

The contributions of GIP receptor agonism to tirzepatide’s clinical efficacy are described as less well known than the GLP-1 receptor contributions ^[9]. The reasons why both GIP receptor agonism and antagonism add weight loss remain incompletely understood ^[8][10]. And much of the brain-circuit evidence — the regional Gipr knockouts, the acyl-GIP and liraglutide experiments — comes from mice, so it should not be read as proven human biology ^[8][11].

On the nutrition question, a post hoc analysis of the SURMOUNT-1 through SURMOUNT-4 randomized trials (N = 4726) found that malnutrition-related treatment-emergent adverse events were uncommon: abnormal weight loss in 0.03%, underweight in 0.06%, and hypoalbuminemia in 0.03% of tirzepatide-treated participants, with 0.38% of tirzepatide participants reaching a BMI below 18.5 kg/m² versus 0.06% on placebo ^[4]. The authors note that vitamin and mineral levels were not routinely collected during the trials, which limited what could be assessed ^[4].

Sources

1. Eli Lilly and Company. ZEPBOUND (tirzepatide) Injection, for subcutaneous use. Prescribing information.
2. MDPI. The Premise of the Paradox: Examining the Evidence That Motivated GIPR Agonist and Antagonist Drug Development Programs.
3. PMC. LY3298176, a novel dual GIP and GLP-1 receptor agonist for the treatment of type 2 diabetes mellitus: From discovery to clinical proof of concept.
4. Obesity Pillars. Nutritional status with tirzepatide in obesity: A post hoc analysis of the SURMOUNT-1-4 randomized clinical trials. March 2026.
5. Cell Metabolism. Tirzepatide modulates the regulation of adipocyte nutrient metabolism through long-acting activation of the GIP receptor.
6. MDPI. The Premise of the Paradox: Examining the Evidence That Motivated GIPR Agonist and Antagonist Drug Development Programs.
7. Cell Metabolism. The glucose-dependent insulinotropic polypeptide (GIP) regulates body weight and food intake via CNS-GIPR signaling.
8. Nature Metabolism. Distinct brain regions mediate regulation of food intake in response to GIPR agonism or antagonism.
9. Diabetes, Obesity and Metabolism. A long-acting glucose-dependent insulinotropic polypeptide receptor agonist improves the gastrointestinal tolerability of glucagon-like peptide-1 receptor agonist therapy. 2024.
10. Diabetes. GIP Receptor Antagonists in the Pharmacotherapy of Obesity: Physiologic, Genetic, and Clinical Rationale. August 2025.
11. Nature Metabolism. GIPR agonism and antagonism decrease body weight and food intake via different mechanisms in male mice.
12. Diabetes. Therapeutic Targeting of the GIP Receptor—Revisiting the Controversies. August 2025.
13. PubMed Central. Tirzepatide is an imbalanced and biased dual GIP and GLP-1 receptor agonist.
14. US Food and Drug Administration. Clinical pharmacology review for NDA 215866 (Mounjaro, tirzepatide).
15. Diabetes, Obesity and Metabolism. Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study of adults with obesity or overweight. 2025.
16. PubMed. Tirzepatide for maintenance of bodyweight reduction in people with obesity in the USA (SURMOUNT-MAINTAIN): a multicentre, double-blind, randomised, placebo-controlled trial.
17. American College of Cardiology. SURMOUNT-MAINTAIN: Continuing Tirzepatide Maintains Weight Loss. May 18, 2026.
18. Eli Lilly and Company. Zepbound (tirzepatide), the most prescribed weight management medication in 2025, now available in multi-dose KwikPen. February 23, 2026.