GIP and GLP-1: What GIP Is and Why Tirzepatide Targets Both

GIP and GLP-1 are two gut hormones (incretins) that tell your pancreas to release insulin after you eat, and tirzepatide (Mounjaro, Zepbound) is a GIP/GLP-1 agonist that activates the receptors for both. That is why it's called a "dual agonist" rather than a GLP-1-only medicine, and why people ask what GIP adds; the short answer is that trials, not the number of targets, tell you how well it works. GIP itself stands for glucose-dependent insulinotropic polypeptide. It's released by K cells in the upper small intestine when you eat, and it helps tell the pancreas to release insulin when blood sugar is rising.

How is GIP different from GLP-1?

GIP and GLP-1 are both incretins, gut hormones that boost insulin release after a meal. Together they explain the "incretin effect": your body releases more insulin when glucose arrives by mouth than when the same glucose is given into a vein. But they come from different cells (GLP-1 is mostly made by L cells further down the gut), act on different receptors and do somewhat different jobs:

• GLP-1 stimulates glucose-dependent insulin release, lowers glucagon, slows stomach emptying and has well-studied effects on appetite and fullness.
• GIP also stimulates insulin release and is involved in how the body handles nutrients and fat. In type 2 diabetes, the pancreas responds much less to natural GIP than it does in people without diabetes.

Partly because of that, GIP was a less obvious drug target than GLP-1, and researchers are still working out exactly how much the GIP part of tirzepatide contributes to its effects.

Does the second target explain the results?

The mechanism helps explain why a GLP-1/GIP agonist like tirzepatide may behave differently from GLP-1-only medicines, but clinical trials, not the number of targets, tell us how well it works. The clearest comparison is SURMOUNT-5 (NEJM, 2025): 751 adults with obesity but without type 2 diabetes were randomized to the highest tolerated dose of tirzepatide (10 or 15 mg) or semaglutide (1.7 or 2.4 mg) weekly for 72 weeks. Average weight change was −20.2% with tirzepatide and −13.7% with semaglutide. The trial was open-label and funded by Lilly, and gastrointestinal side effects were the most common in both groups.

That's a meaningful result for that population and those doses. It isn't proof that "two hormones always beat one," and individual responses vary widely. If you're on tirzepatide and progress feels slow, our piece on Mounjaro and not losing weight looks at what research says.

Is every tirzepatide product approved for the same thing?

No, and the brand name matters. In the US:

• Mounjaro is approved to improve blood sugar control in adults with type 2 diabetes and, since August 2026, to reduce the risk of major cardiovascular events in adults with type 2 diabetes at high cardiovascular risk.
• Zepbound is approved for chronic weight management and for moderate-to-severe obstructive sleep apnea in adults with obesity.

Approvals and brand names differ in other countries. For a side-by-side of tirzepatide and semaglutide brands, see Mounjaro vs Ozempic.

Common questions

What is GLP-1 and GIP, in plain words?

Both are gut hormones your body releases when you eat, and both tell the pancreas to release insulin when blood sugar is rising. GLP-1 comes mostly from L cells further down the gut, lowers glucagon, slows stomach emptying and affects appetite and fullness. GIP comes from K cells in the upper small intestine and is also involved in how the body handles nutrients and fat.

What is a GIP/GLP-1 agonist?

It's a medicine built to activate the receptors for both hormones instead of just GLP-1. Tirzepatide, sold as Mounjaro and Zepbound, is the one on the market. Semaglutide (Ozempic) activates only the GLP-1 receptor.

Is a GLP-1 plus GIP medicine better than GLP-1 alone?

In the SURMOUNT-5 trial, adults with obesity but without type 2 diabetes lost an average of 20.2% of their weight on tirzepatide versus 13.7% on semaglutide over 72 weeks. That's a meaningful result for that population and those doses, but it isn't proof that two hormones always beat one, and individual responses vary widely.

Why was GIP a less obvious drug target than GLP-1?

In type 2 diabetes, the pancreas responds much less to natural GIP than it does in people without diabetes. Researchers are still working out exactly how much the GIP part of tirzepatide contributes to its effects.

The takeaway

Knowing what GIP is makes drug news easier to follow, especially as newer medicines add even more targets. But biology isn't a shortcut to a personal decision. The useful questions are what the trials showed, which approved product fits your situation where you live, and how your clinician will monitor your response and side effects.