# What Is Tirzepatide? The Dual GIP/GLP-1 Peptide Explained > What is tirzepatide? A 39-amino-acid synthetic peptide that activates both the GIP and GLP-1 receptors. FDA-approved for type 2 diabetes and obesity. Structure, mechanism, regulatory history, and clinical context. ## The short version What is tirzepatide? It is a man-made peptide — a short protein-like molecule — that mimics two gut hormones at once. Those hormones, GIP and GLP-1, are normally released after meals and tell the pancreas to make insulin in response to rising blood sugar. Most earlier drugs in this class only mimicked GLP-1. Tirzepatide mimics both. The FDA approved it first for type 2 diabetes, then for weight management in people with obesity, and then for a sleep-breathing condition. Clinical trials showed it reduces blood sugar and body weight more than other agents in its class. It is given as a once-weekly injection under the skin. It is not a weight-loss supplement or an over-the-counter product — it is a prescription medicine, and using it requires a physician's involvement. This page explains the structure, how it works, and what the regulatory record shows. ## Tirzepatide peptide: structure and pharmacokinetics Tirzepatide (INN, CAS 2023788-19-2, ATC A10BX16) is a 39-amino-acid linear peptide with a molecular weight of 4,813.53 Da and the molecular formula C225H348N48O68. It was engineered on the native GIP hormone backbone with a C20 fatty diacid side arm attached via a glutamic acid linker and two (2-(2-aminoethoxy)ethoxy)acetic acid (AEEA) spacers to a lysine residue. This acylation confers high albumin affinity and an elimination half-life of approximately 5 days — the pharmacokinetic property that makes once-weekly dosing viable [1]. The discovery paper (Coskun et al., Mol Metab, 2018) reported proof-of-concept in mice and a Phase 1 programme in 142 human subjects demonstrating the PK profile and dose-response [1]. A separate in vitro characterisation found that tirzepatide engages the GIP receptor to a greater degree than the GLP-1 receptor and shows biased GLP-1 receptor signalling — specifically, it favours cyclic AMP (cAMP) generation over beta-arrestin recruitment. Beta-arrestin1 limits the insulin secretion response to native GLP-1 but not to GIP or tirzepatide; this biased profile is proposed to enhance net insulin output [2]. ## Tirzepatide mechanism of action: how the dual agonism works The two receptors tirzepatide targets — the GIP receptor (GIPR) and the GLP-1 receptor (GLP-1R) — are expressed in the pancreatic beta cells that secrete insulin. Activating them in the presence of elevated blood glucose amplifies insulin secretion. This glucose-dependent mechanism means the insulin response is tied to the glucose signal — the drug does not stimulate insulin in the absence of elevated blood glucose, which is why hypoglycaemia risk is low when the drug is used alone [10]. Beyond insulin secretion, both pathways contribute to glucagon suppression (glucagon — a hormone that raises blood glucose — is suppressed in the fed state), slowed gastric emptying (the rate at which the stomach passes food into the small intestine is transiently delayed), and reduced appetite and food intake via central signals. The combination of appetite suppression and slowed gastric transit is what produces the pronounced weight reduction observed in clinical trials [1, 2]. A review of the compound's history and clinical profile describes it as 'a novel once-weekly dual GIP and GLP-1 receptor agonist' summarising the phase 3 evidence base [8]. A broader dual-incretin overview by Campbell et al. (Cell Metab, 2023) placed it in the context of decades of incretin science and the field of GIP/GLP-1 combination therapy [20]. ## Tirzepatide injection: route of administration and approved formulation Tirzepatide injection is the only administered route in the approved programme — subcutaneous injection (under the skin) once weekly. Oral formulations have not been developed for tirzepatide; unlike some molecules in adjacent classes, this compound's structure requires injectable delivery. The FDA prescribing information documents a starting dose of 2.5 mg once weekly, stepped up at four-week intervals through 5 mg, 7.5 mg, 10 mg, 12.5 mg, and up to a maximum of 15 mg [10]. The three maintenance doses studied in the phase 3 programme were 5, 10, and 15 mg. Dose escalation is the primary driver of gastrointestinal adverse effects; slowing the titration is the main clinical strategy for improving tolerability [10, 14]. Approved products are prescription-only, refrigerated, and administered at home or by a healthcare provider. Questions about the tirzepatide injection process — technique, site rotation, storage — are properly directed to the prescribing physician or pharmacist rather than an editorial digest. ## Tirzepatide cost and access Tirzepatide cost is a practical question outside this site's editorial scope — this is a research digest, not a pharmacy or vendor. What the published literature documents is the supply and access context. During a documented shortage period, compounded versions of tirzepatide proliferated outside the regulated drug supply. Regulatory agencies raised concerns about the quality, purity, and identity of non-FDA-approved compounded preparations. The shortage and subsequent resolution of compounded-product availability is a regulatory-record fact the literature records, distinct from the editorial summary of the clinical trial findings on this site [10]. For current access, formulary, and prescribing information, the appropriate sources are the FDA drug database, the prescribing information, and a licensed prescriber. ## FDA approvals: the regulatory timeline The FDA regulatory record for tirzepatide has three entries. First approval: May 2022, for the treatment of type 2 diabetes mellitus in adults, with a titration schedule from 2.5 mg to a maximum of 15 mg once weekly [10]. Second approval: November 2023, for chronic weight management in adults with obesity (BMI ≥30 kg/m²) or overweight (BMI ≥27 kg/m²) with at least one weight-related comorbidity [11]. Third approval: obstructive sleep apnea in adults with obesity, supported by the SURMOUNT-OSA trial data [11, 16]. The prescribing information also documents contraindications (personal or family history of medullary thyroid carcinoma or Multiple Endocrine Neoplasia syndrome type 2, based on rodent data) and notes that tirzepatide is not approved for type 1 diabetes [10]. For [Tirzepatide research](/research) into the clinical trial record that supported each approval, see the research page. --- Observational field notes on the tirzepatide trial record — each finding catalogued by evidence type, each gap left plainly unfilled; no clinic behind this desk, no prescription written here.