How tirzepatide works: GIP and GLP-1 dual agonism

GLP-1 receptor activation drives several effects: it stimulates glucose-dependent insulin release (so insulin rises mainly when blood glucose is high), suppresses glucagon (the hormone that raises blood sugar), slows the rate at which the stomach empties, and acts on appetite centers in the hypothalamus and brainstem to increase satiety. Because insulin release is glucose-dependent, the risk of hypoglycemia from GLP-1 action alone is low.

By engaging both receptors with one molecule, tirzepatide lowers blood glucose and reduces body weight to a degree that, in trials, exceeded a leading GLP-1-only therapy (see SURPASS-2). In the obesity program, mean weight reductions reached roughly a fifth of body weight at the highest dose (see SURMOUNT-1). The dual mechanism is the leading explanation for these results, though weight loss also depends on dose, duration and lifestyle.

Even as the clinical results are clear, the mechanism is not fully mapped. Scientists are still quantifying how much of tirzepatide's effect comes from the GIP arm versus the GLP-1 arm, why combined agonism appears to outperform either alone, and how central (brain) versus peripheral (gut, pancreas, fat) actions each contribute. There is also active debate about whether GIP-receptor agonism or, paradoxically, partial antagonism best explains the weight effect, since both hypotheses have experimental support. Receptor signaling is complex: the same receptor can trigger different downstream pathways depending on how a drug binds it (so-called biased agonism), which may explain why tirzepatide behaves differently from native GIP. For readers, the practical point is humility — the drug works in trials, but “we know exactly why” would overstate the current science. This is one reason next-generation molecules are tested empirically in humans rather than designed purely from theory.