GLP-1 is the hormone most people associate with drugs like semaglutide and tirzepatide, but tirzepatide's name for its mechanism, a "dual GIP/GLP-1 receptor agonist," points to a second incretin hormone that gets far less public attention despite playing a central role in how several major peptide drugs actually work.
What GIP Is
GIP, short for glucose-dependent insulinotropic polypeptide, is an incretin hormone released by K cells in the small intestine after eating. Alongside GLP-1, it's one of the two major incretin hormones responsible for the "incretin effect," the well-documented phenomenon in which oral glucose triggers a much larger insulin release than the same amount of glucose delivered intravenously. GIP's primary natural role is stimulating insulin secretion from the pancreas in response to food intake, a function it shares conceptually with GLP-1 even though the two hormones act on entirely separate receptors.
A Genuinely Confusing Research History
GIP's role in obesity research has an unusually tangled history worth being upfront about. For years, some researchers suspected that blocking, rather than activating, the GIP receptor might support weight loss, based on certain animal and genetic studies. Tirzepatide's development took the opposite approach, pairing GIP receptor agonism with GLP-1 receptor agonism, and it worked: tirzepatide produced greater weight loss in its pivotal trials than GLP-1-only drugs like semaglutide. The leading explanation for why GIP agonism helps rather than hurts in this context is that chronic, sustained receptor activation from a long-acting drug appears to produce different net effects on body weight and appetite than the acute, short-term GIP signaling studied in some earlier animal work, possibly by improving the body's sensitivity to GLP-1 signaling or reducing some of the gastrointestinal side effects that limit how high a GLP-1-only dose patients can tolerate. This remains an active, somewhat unsettled area of pharmacology, not a fully closed question.
Agonism and Antagonism Are Both Being Pursued, and That's Not a Contradiction
Confusingly, the field hasn't settled on one answer, because researchers are simultaneously pursuing GIP receptor agonism and GIP receptor antagonism in different drugs. Tirzepatide, an approved drug, uses GIP agonism. MariTide, Amgen's investigational candidate covered in this site's dedicated guide, instead pairs a GIP receptor antagonist with a GLP-1 receptor agonist, and has also shown substantial weight loss in trials so far. Both approaches appear to work, at least based on the clinical data generated to date, which tells researchers that the relationship between GIP receptor signaling and body weight is more complex than simple "more GIP activity is good" or "less is good" models would suggest, and likely depends heavily on dosing pattern, receptor desensitization over time, and interaction with whatever GLP-1-pathway drug it's paired with.
Why This Matters Beyond Academic Interest
Understanding that two opposite pharmacological strategies, GIP agonism and GIP antagonism, are both showing clinical promise is a useful reminder of a broader pattern in peptide drug research covered throughout this site: a receptor's role in a simplified animal model or an isolated cell system doesn't always predict how a drug targeting that receptor will behave in a full clinical trial with chronic human dosing. It's also a reason researchers remain cautious about over-interpreting mechanism from preclinical data alone, a theme that comes up repeatedly in this site's coverage of animal-versus-human evidence for other peptides.
Where the Evidence Actually Stands
GIP itself is a naturally occurring hormone, not a drug, so it has no FDA approval status of its own. Tirzepatide, which pairs GIP and GLP-1 receptor agonism, is FDA-approved under the brand names Mounjaro and Zepbound. MariTide, which pairs GIP receptor antagonism with GLP-1 agonism, remains an investigational drug still in clinical trials, with no FDA approval to date. Both drugs' current evidence comes from registered human clinical trials rather than animal studies alone, which puts them in a different evidence category than many of the less-studied research-chemical peptides this site covers elsewhere.
The Bottom Line
GIP is the quieter half of the hormone pairing behind some of the most effective obesity drugs on the market and in development, and the fact that both activating and blocking its receptor appear to support weight loss in human trials is a genuinely open and interesting research question, not a sign that one approach is simply wrong. As with any mechanism discussed on this site, understanding why a drug works is informative, but it's the registered clinical trial data, not the mechanism alone, that should guide any medical decision, which is best made with a licensed physician.