EXPLAINER / MECHANISM + STRUCTURE
What Is Tirzepatide? The Dual GIP/GLP-1 Peptide Explained
One drug, two receptor targets, one consistent finding across seven large randomised trials.
The short version
Tirzepatide is a synthetic peptide — a short chain of amino acids — that mimics two natural gut hormones at once: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). Both hormones are released after you eat, and both signal the pancreas to release insulin. By activating both receptors with a single molecule (which is why it is sometimes called a 'twincretin' or dual incretin agonist), tirzepatide produces larger reductions in blood sugar and body weight than older drugs that only hit GLP-1 alone. It is not a natural compound — it is a drug engineered from the GIP hormone backbone and chemically modified to last in the body for about 5 days, which is long enough for once-weekly dosing. The FDA has approved it for type 2 diabetes (2022), obesity (2023), and obstructive sleep apnea (2024). It works as a subcutaneous injection — a short injection under the skin — given once per week.
The structure of tirzepatide
Tirzepatide (LY3298176, ATC code A10BX16) is a 39-amino-acid synthetic peptide built on the native GIP sequence, molecular formula C225H348N48O68, molecular weight 4813.53 Da [1].
The critical modification that makes it both long-acting and dual-acting is a C20 fatty-diacid arm — eicosanedioic acid attached via a glutamic acid linker and two aminoethoxyethoxyacetic acid units to a lysine side chain. This fatty-diacid arm binds tightly to albumin (a protein that travels through the blood), which is what produces the ~5-day elimination half-life supporting once-weekly dosing [1].
The molecule also incorporates GLP-1 receptor agonist activity through sequence modifications to the GIP backbone — the dual receptor engagement is engineered into the primary structure, not achieved by joining two separate molecules.
The two receptor targets and what they do
The GIP receptor (GIPR) and GLP-1 receptor (GLP-1R) are both found on pancreatic beta-cells (the cells that make insulin), in the central nervous system (where they regulate appetite), and in adipose tissue (fat cells).
GIP receptor activation: Enhances glucose-dependent insulin secretion, increases insulin sensitivity in fat tissue, and is involved in fat metabolism.
GLP-1 receptor activation: Enhances glucose-dependent insulin secretion, suppresses glucagon (a hormone that raises blood glucose), slows gastric emptying (the rate the stomach empties into the small intestine), reduces appetite and food intake, and is involved in weight regulation.
In vitro characterisation found tirzepatide is an 'imbalanced' dual agonist: it engages the GIP receptor to a greater degree than the GLP-1 receptor. It also exhibits biased GLP-1 receptor signalling that favours cAMP generation over beta-arrestin recruitment — a property proposed to enhance insulin secretion efficiency. In primary islet experiments, beta-arrestin1 limited the insulin response to GLP-1 but not to GIP or tirzepatide [2].
The 'imbalanced' characterisation matters: tirzepatide is not a 50/50 GIP/GLP-1 agonist. The specific receptor engagement profile may be responsible for its larger weight and glycaemic effects compared to selective GLP-1 agonists.
What makes it different from prior GLP-1 drugs
Prior approved incretin therapies were selective GLP-1 receptor agonists — they hit only the GLP-1 receptor. Tirzepatide is the first approved agent to simultaneously activate both GIP and GLP-1 receptors, which is why the FDA classification and the medical literature call it a novel drug class.
The clinical consequence was measured directly in SURPASS-2: head-to-head against semaglutide 1 mg in 1,879 adults with type 2 diabetes over 40 weeks. Tirzepatide was noninferior and superior at all three doses for HbA1c reduction, with treatment differences in body weight of -1.9, -3.6, and -5.5 kg at 5, 10, and 15 mg respectively [3]. In SURMOUNT-5, the head-to-head for obesity, tirzepatide produced -20.2% weight loss versus semaglutide's -13.7% over 72 weeks [5].
The practical difference: a larger average weight loss and glycaemic effect, but the same class of GI side-effect profile and the same class-wide thyroid C-cell boxed warning.
See Tirzepatide research for the full trial record, or what is tirzepatide alongside the tirzepatide injection page for the delivery picture.