Tirzepatide does nothing anywhere without a receptor. This maps every human cell type known to carry GLP-1R or GIPR, what happens when the drug lands there, and what you can actually do about it. Rodent-only findings are flagged, because most of the popular story rests on them.
This page describes mechanism. It is not medical advice, it is not a treatment plan, and reading it creates no clinical relationship. Dosing, safety, and treatment decisions belong with your own clinician, who knows things about you that this page cannot.
Half of understanding this drug is knowing which effects arrive without a receptor. Everything below is a real effect with no direct target - which changes what you can do about it.
Tirzepatide was built by engineering GLP-1 activity into the GIP backbone. It is not a balanced dual agonist, and it does not signal at GLP-1R the way GLP-1 does. Both facts are load-bearing.
At GIPR it behaves like the real hormone. At GLP-1R it drives cAMP while barely recruiting β-arrestin - so the receptor internalises less and stays available on the cell surface. In primary islets, β-arrestin1 limits the insulin response to GLP-1 but not to tirzepatide.
Tirzepatide binds GIPR with affinity matching native GIP, but binds GLP-1R roughly 5× weaker than native GLP-1. Receptor-occupancy modelling at clinical doses confirms greater GIPR engagement.
Why this is practical: GLP-1R dose escalation is limited by nausea and vomiting. GIPR engagement isn't. Leaning the molecule toward GIPR is how you get more total incretin effect per unit of gastrointestinal misery.
Tirzepatide is far less potent at mouse GIPR than at human GIPR. Any mouse study of this drug systematically underweights the GIP arm - the half that most distinguishes it from a GLP-1-only agonist.
How to use this: when you meet a mechanistic claim about tirzepatide, check the species first. Mouse work here is a weak proxy in a specific, predictable direction.
Delayed gastric emptying shows clear tachyphylaxis - pronounced after the first dose, substantially attenuated after about four weekly doses. Appetite suppression, glucose lowering and weight loss do not fade the same way.
Consequence: nausea and reflux are worst early and after each escalation, and genuinely improve. Constipation, driven by enteric neurons along the whole gut, is more persistent - manage it as an ongoing condition rather than waiting it out.