Mounjaro (tirzepatide) works by mimicking two natural hormones your body produces after eating: GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide). This dual action is what makes Mounjaro unique among injectable weight-loss and diabetes treatments currently available in the UK.
What Are GLP-1 and GIP?
Both GLP-1 and GIP are incretin hormones — chemical messengers released by your gut when you eat. They play important roles in:
- Stimulating insulin release from the pancreas when blood sugar is elevated
- Suppressing glucagon, a hormone that raises blood sugar
- Slowing gastric emptying, so food stays in your stomach longer and you feel full
- Acting on the brain to reduce appetite and food cravings
In people with type 2 diabetes or obesity, the incretin system often doesn't work as effectively. Tirzepatide essentially tops up these signals.
Why Two Hormones Are Better Than One
Earlier medications like semaglutide (the active ingredient in Ozempic and Wegovy) target only GLP-1. Mounjaro is the world's first dual GIP and GLP-1 receptor agonist. Research from the SURPASS and SURMOUNT clinical trials suggests that targeting both receptors together produces greater reductions in body weight and HbA1c (a long-term measure of blood sugar) compared to GLP-1-only drugs.
How It Affects Appetite
Tirzepatide influences appetite through several pathways:
- Delayed gastric emptying — food moves through your stomach more slowly, keeping you feeling satisfied for longer after meals
- Central appetite suppression — the drug acts on receptors in the hypothalamus, the brain region that controls hunger
- Reduced food reward signalling — many patients report that high-calorie foods simply become less appealing
How It Affects Blood Sugar
For patients with type 2 diabetes, tirzepatide improves glycaemic control in a glucose-dependent manner. This means it primarily boosts insulin secretion when blood sugar is high, which reduces the risk of hypoglycaemia (dangerously low blood sugar) compared to some older diabetes medications.
Clinical Results
In the SURMOUNT-1 trial, participants taking the highest dose of tirzepatide (15 mg) lost an average of 22.5% of their body weight over 72 weeks. In the SURPASS trials for type 2 diabetes, HbA1c reductions of up to 2.4 percentage points were observed.
Summary
Mounjaro's dual-hormone mechanism represents a genuine advance in the treatment of obesity and type 2 diabetes. By targeting both GLP-1 and GIP pathways, it delivers meaningful improvements in weight and blood sugar control for eligible UK patients.