Researchers spent years trying to block GIP for weight loss. Activating it turned out to work better, and the trial data settled it.
Tirzepatide switches on two gut hormone receptors at once: GLP-1 and GIP. That combination is what separates it from semaglutide, which acts on GLP-1 alone.
Unlike retatrutide, it is an approved medicine. It is sold as Mounjaro for type 2 diabetes. It is also sold as Zepbound, for weight management and for moderate-to-severe obstructive sleep apnoea in adults with obesity. Same molecule, same maker, different labels.
Tirzepatide is a synthetic peptide of 39 amino acids, built on the sequence of natural GIP rather than GLP-1.
Attached to it is a C20 fatty diacid chain. That chain binds to albumin, the most abundant protein in blood, and the molecule travels bound to it. Being carried this way keeps it from being cleared quickly, which stretches the half-life to roughly five days. That is what makes once-weekly dosing possible.
This is the part of the story worth knowing, because it shows how the field corrected itself.
For years the assumption was that blocking GIP would aid weight loss. GIP was associated with fat storage, so suppressing it seemed the obvious move.
It was the wrong way round. Switching GIP on, rather than off, turns out to beat GLP-1 acting by itself. The trial data settled that fairly decisively.
GLP-1 prompts insulin release when blood sugar rises. It slows the stomach's emptying, so food stays put longer. It reduces the liver's glucose output. And it acts on the brain to lower appetite and reduce how rewarding food feels. This is the pathway semaglutide works through, and it is where most of the appetite effect comes from.
GIP supports insulin release alongside GLP-1. It also shapes how nutrients are shared out, meaning how much of what you eat gets taken up and used by tissue rather than left in the blood. Tirzepatide binds this receptor about as tightly as the body's own GIP does.
A receptor can be switched on in more than one way, and which internal pathway gets favoured matters.
The literature calls tirzepatide an imbalanced and biased dual agonist. Inside the cell, its GLP-1 signal leans towards the cAMP route and away from beta-arrestin. That second route drives internalisation, where the receptor is pulled inside the cell and sits unavailable for a while.
Leaning one way rather than the other gives strong insulin-release effects with less internalisation. This is thought to help explain why it is better tolerated than a pure GLP-1 agonist. That link is an inference, though, not a measured endpoint.
SURMOUNT-1 (Jastreboff and colleagues, NEJM, 2022). 2,539 adults with obesity and without diabetes, 72 weeks:
| Weekly dose | Average weight loss |
|---|---|
| 5 mg | 15.0% |
| 10 mg | 19.5% |
| 15 mg | 20.9% |
| Placebo | 3.1% |
At 15 mg, 36% of participants lost a quarter or more of their body weight. Between 4.3% and 7.1% stopped because of side effects. And as with retatrutide, the gains shrink as the dose climbs. Going from 5 to 10 mg added about 4.5 percentage points. Going from 10 to 15 mg added roughly 3 more.
SURMOUNT-5 (Aronne and colleagues, NEJM, 2025) put it directly against semaglutide over 72 weeks. Tirzepatide produced 20.2% average weight loss against 13.7%, with greater waist reduction. Notably, gastrointestinal effects leading to stopping treatment were lower with tirzepatide, 2.7% against 5.6%.
SURMOUNT-4 (Aronne and colleagues, JAMA, 2024) tested what happens on withdrawal. After a 36-week lead-in, participants either continued or switched to placebo. Those continuing reached 25.3% total loss at 88 weeks. Those switched regained 14% of body weight. That contrast is the most important single result here. It describes a treatment for an ongoing condition, not a course that finishes.
SURPASS-2 (Frias and colleagues, NEJM, 2021) set it against semaglutide 1 mg in type 2 diabetes. HbA1c fell further at every dose. Note the comparator, though: 1 mg, not the 2.4 mg used for weight management.
SURPASS-CVOT (2025) followed 13,165 patients with type 2 diabetes and existing cardiovascular disease for a median of four years, against dulaglutide. Cardiovascular death, heart attack or stroke occurred in 12.2% on tirzepatide against 13.1%. That met non-inferiority but not superiority. The comparator was an active drug, not placebo. So the finding is that tirzepatide is not worse than another GLP-1 agonist. It is not a finding that it beats no treatment.
SYNERGY-NASH (Loomba and colleagues, NEJM, 2024) studied 190 adults with biopsy-confirmed MASH. Resolution without worsening fibrosis reached 44%, 56% and 62% across the three doses, against 10% on placebo.
SURMOUNT-OSA (Malhotra and colleagues, NEJM, 2024) reduced sleep apnoea severity by up to 62.8%, with around half of participants reaching resolution.
A DEXA substudy of SURMOUNT-1 measured body composition directly. Roughly 75% of the weight lost was fat and 25% lean tissue. That ratio sat close to the placebo group's, despite far greater total loss.
That proportion is reassuring, but the absolute number deserves attention. A quarter of a 50-pound loss is still around 12 pounds of lean tissue. The ratio holding steady is not the same as lean mass being unaffected.
Gastrointestinal effects dominate: nausea in roughly a quarter to a third of participants depending on dose, along with diarrhoea, vomiting and constipation. They cluster around dose increases and generally ease with time at a given level.
Gallbladder problems were reported in about 0.6%, against none on placebo, in the weight-loss trials. That fits the known link between rapid weight loss and gallstones, whatever drives the loss.
Pancreatitis rates were similar to comparators in the obesity trials and slightly higher in the diabetes trials.
In rodents, tirzepatide produced dose-related increases in thyroid C-cell tumours. Whether that carries over to humans is unknown, and no cases of medullary thyroid carcinoma appeared in human trials. It is the basis for the contraindications below.
It is not for use by anyone with a personal or family history of medullary thyroid carcinoma, or with multiple endocrine neoplasia type 2. Nor in pregnancy, nor while breastfeeding.
Caution applies with a history of pancreatitis, gallbladder disease, type 1 diabetes, diabetic retinopathy, gastroparesis, or kidney disease.
The oral contraceptive interaction is the one most often overlooked. By slowing gastric emptying, tirzepatide substantially reduces absorption of oral hormonal contraceptives. A pharmacokinetic study found peak ethinyl estradiol exposure cut by 59% and overall exposure by 20%. The manufacturer advises a non-oral method, or an added barrier method, for four weeks after starting and after each dose increase. This is specific to tirzepatide and does not apply to GLP-1-only drugs — which is exactly why it gets missed.
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