What Is Tirzepatide? The Dual GIP/GLP-1 Peptide Explained

The short version

Tirzepatide is a prescription medicine given as a once-weekly injection under the skin. What makes it different from earlier weight-loss medicines is that it activates two receptors, not one. GIP and GLP-1 are hormones your gut naturally releases after eating; they signal your pancreas to release insulin and tell your brain you've had enough food. Tirzepatide — a single engineered peptide — mimics both signals at once. Early research suggested this dual approach produces greater weight loss than activating GLP-1 alone, and the phase 3 trials confirmed it. The FDA first approved it for type 2 diabetes in May 2022, then for chronic weight management in adults with obesity in November 2023. It has since been approved for obstructive sleep apnea in adults with obesity. Tirzepatide is a prescription drug — this page summarizes the published science about what it is and how it works.

Tirzepatide peptide: structure and half-life

Tirzepatide is a 39-amino-acid synthetic peptide built on the backbone of the native GIP hormone sequence. A C20 fatty diacid (eicosanedioic acid) is attached via a glutamic acid linker and two AEEA (2-(2-aminoethoxy)ethoxy)acetic acid) spacer units to a lysine side chain at position 20 ^[1]. This fatty-diacid arm binds reversibly to albumin — the abundant protein that circulates in blood — which slows renal clearance and protects the molecule from enzymatic degradation. The result is an elimination half-life of approximately five days, enabling once-weekly subcutaneous dosing ^[1].

The molecular formula is C225H348N48O68; molecular weight is 4,813.53 Da. The ATC classification is A10BX16 (other blood-glucose-lowering drugs, excluding insulins). CAS number: 2023788-19-2.

In vitro signalling studies have characterized tirzepatide as an "imbalanced" dual agonist: it engages the GIP receptor more fully than the GLP-1 receptor ^[2]. At the GLP-1 receptor specifically, it shows biased agonism — favouring cyclic AMP (cAMP) generation over beta-arrestin recruitment ^[2]. This means it activates insulin-stimulating pathways while producing less of the receptor internalization (the process by which cells reduce their own surface receptor count after repeated stimulation) that typically follows GLP-1 activation. In primary pancreatic islet experiments, beta-arrestin1 limited the insulin response to GLP-1 but not to GIP or tirzepatide — suggesting biased agonism at the GLP-1 receptor may enhance insulin secretion beyond what a selective GLP-1 agonist achieves ^[2].

Tirzepatide mechanism of action

The dual receptor mechanism works through several parallel pathways. GLP-1 receptor (GLP-1R) activation stimulates glucose-dependent insulin secretion (insulin release is amplified when blood glucose is already elevated — the incretin effect), suppresses glucagon (a hormone that raises blood sugar), slows gastric emptying (the rate at which the stomach empties into the intestine), and reduces appetite via central nervous system signalling ^[7]. GIP receptor (GIPR) activation enhances insulin secretion and may modulate fat metabolism and energy homeostasis through pathways distinct from GLP-1R alone ^[1].

Engaging both receptors with a single molecule is the key innovation. When tested against a selective GLP-1 receptor agonist at equipotent glycemic doses in mice, tirzepatide produced greater reductions in body weight and food intake, suggesting the GIP receptor engagement adds a real additive or synergistic effect — though the precise mechanism of that enhancement remains an active area of investigation ^[1].

Slowed gastric emptying — one of the shared GLP-1R-mediated effects — underpins both the appetite reduction (food stays in the stomach longer, prolonging the feeling of fullness) and the most common side effects (nausea and gastrointestinal discomfort). The gastric-emptying effect attenuates with continued dosing, which explains why gastrointestinal side effects are most pronounced during the escalation phase and generally improve with time ^[16].

FDA approval and approved indications

The FDA approved tirzepatide in May 2022 for the treatment of adults with type 2 diabetes mellitus, as an adjunct to diet and exercise ^[7]. A second approval in November 2023 expanded the indication to chronic weight management in adults with obesity (BMI of 30 or greater) or with overweight (BMI 27 or greater) with at least one weight-related comorbidity, adjunct to reduced-calorie diet and increased physical activity ^[17]. A third approval followed for moderate-to-severe obstructive sleep apnea in adults with obesity ^[12].

The drug is not approved for type 1 diabetes. It carries a boxed warning regarding thyroid C-cell tumours observed in rodent studies; the relevance to humans is unestablished, but the prescribing information contraindicates use in people with a personal or family history of medullary thyroid carcinoma or Multiple Endocrine Neoplasia syndrome type 2 ^[17].

Approved formulations are prescription-only. The clinical development program also included trials in heart failure with preserved ejection fraction (SUMMIT) and metabolic dysfunction-associated steatohepatitis (SYNERGY-NASH); data from those programmes have been published and contributed to the broader evidence base ^[13][14].