The single most important pharmacological fact about Mounjaro is also the most easily missed. Tirzepatide does something that no previously approved weight management medication has done. It activates two hormone receptors at once. GLP-1, the receptor that Wegovy and Ozempic act on, and GIP, a receptor that earlier generations of weight management medication ignored entirely.
This is not a marginal pharmacological detail. The dual action is the reason Mounjaro produces stronger average weight reduction than semaglutide in head to head trials. It is the reason the SURMOUNT trial programme produced 20 percent average weight reduction at the maximum 15 mg dose, against semaglutide's 15 percent at 2.4 mg. And it explains why the side effect profile is broadly similar to single-receptor GLP-1 agonists despite the stronger effect.
What GIP Does
GIP, or glucose-dependent insulinotropic polypeptide, was discovered in the 1970s. It is a gut hormone released by special cells in the upper small intestine, particularly when fats and carbohydrates arrive after a meal. Its primary documented function is signalling the pancreas to release insulin in response to food, but research over the last two decades has shown its effects extend further:
- It promotes insulin release from the pancreas, particularly when blood glucose is elevated
- It acts on fat tissue, affecting how energy is stored and released
- It has effects in the brain on appetite signalling, though more subtly than GLP-1
- It influences bone metabolism, lipid handling, and some aspects of inflammation
For decades, GIP was viewed as a less interesting hormone than GLP-1. Animal studies in the 2000s even suggested that blocking GIP might reduce obesity, which seemed to mean that activating GIP would be the wrong direction for a weight management drug. The story was more complicated than that.
Why Earlier Work Missed The Combination
The reason earlier pharmaceutical research overlooked GIP is partly historical and partly methodological. Most studies looked at GIP alone, in animals with otherwise normal metabolic states. Those studies produced mixed results. Some showed GIP promoting fat storage. Others showed neutral or beneficial effects depending on the specific experimental setup.
What changed was the realisation that GIP behaves differently in the context of simultaneous GLP-1 activation. The two hormones interact. Activating both at once produces effects that neither produces alone. Eli Lilly's research programme around what would become tirzepatide tested this hypothesis directly, designing molecules that bound to both receptors and measuring outcomes. The results were strong enough that tirzepatide moved through development quickly.
The Mechanistic Story
The current best understanding of why the dual action works:
Appetite suppression is amplified
GLP-1 acts on appetite centres in the hypothalamus. GIP appears to act on overlapping but distinct neural circuits. Activating both simultaneously produces stronger appetite suppression than activating GLP-1 alone, particularly for highly palatable food.
Energy expenditure may be modestly increased
Some preclinical evidence suggests dual receptor activation increases energy expenditure modestly. Whether this translates clinically into a meaningful effect is still being studied. If real, it would mean tirzepatide does not work by appetite reduction alone.
Fat tissue handles energy differently
GIP receptors are present on fat cells. Activation in the context of GLP-1 signalling may shift how the body uses and stores energy, separately from the calorie reduction.
Insulin sensitivity improves more
The diabetes data from SURPASS showed stronger HbA1c reduction than semaglutide at comparable dose stages. Both pathways contribute to glucose handling, and their combination outperforms either alone.
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Start ConsultationWhat This Means For Trial Outcomes
The clearest evidence for the dual action's value comes from head to head comparison. SURPASS-2 compared tirzepatide to semaglutide in people with type 2 diabetes. SURMOUNT-5 compared tirzepatide to semaglutide in adults with obesity. Both showed superiority for the dual agonist, with effect sizes consistent with the proposed mechanism.
| Outcome | Tirzepatide (max dose) | Semaglutide (max dose) |
|---|---|---|
| SURMOUNT-5 weight reduction at 72 weeks | ~20% | ~14% |
| SURPASS-2 HbA1c reduction at 40 weeks | ~2.3% | ~1.9% |
| Proportion achieving 20% weight reduction | ~50% | ~25% |
| Side effect profile (GI dominant) | Similar magnitude | Similar magnitude |
The pattern is consistent across trials. Tirzepatide produces stronger average effects without proportionally worse side effects. This is the practical signature of the dual mechanism.
What This Does Not Mean
It does not mean tirzepatide is automatically the right choice for every patient. Several considerations push the other direction:
- It is more expensive than semaglutide in SA
- It is not yet SAHPRA approved for cardiovascular risk reduction (semaglutide is)
- It interacts with oral contraceptives, which Wegovy does not
- Individual variation in response is wide. Some people respond better to semaglutide than to tirzepatide, despite the averages
- It is not approved for adolescents, unlike Wegovy
The dual mechanism is what tirzepatide brings to the table. Whether it is the right table for any given patient depends on more than the mechanism. A fuller comparison lives here.
What Comes Next
Tirzepatide is the first dual agonist. It will not be the last. Several pharmaceutical companies have triple agonist molecules in trials, adding activation of a third receptor (glucagon) to the GIP and GLP-1 combination. Retatrutide, Eli Lilly's own follow on programme, has shown larger average weight reductions in early phase trials, potentially exceeding 25 percent.
None of these are available in South Africa yet. Tirzepatide is, and the dual mechanism it pioneered is what makes it work the way it does.
Frequently Asked
Glucose-dependent insulinotropic polypeptide. It is a gut hormone released when you eat, particularly in response to carbohydrate and fat intake.
Early research suggested GIP might worsen obesity by promoting fat storage. Later work showed that pharmacological GIP receptor activation in the context of GLP-1 co-activation produces different effects than GIP alone. The combination, not GIP by itself, is what tirzepatide exploits.
The side effect profile of tirzepatide is broadly similar to GLP-1 medications. Most side effects appear to come from the GLP-1 pathway. There are no distinct GIP-specific side effects that have emerged in clinical use.
Yes. Several pharmaceutical companies have triple agonist molecules in clinical trials, adding glucagon receptor activation to the GIP and GLP-1 combination. Retatrutide from Eli Lilly is the most advanced. These are years away from SA registration.