How does Mounjaro work? A clear guide to its mechanism

Mounjaro contains the active ingredient tirzepatide.

It stands out among licensed weight and diabetes medicines because it switches on two incretin hormone receptors at the same time.

What are incretins?

Incretins are hormones your gut releases after you eat. They help control blood sugar and appetite. There are two main ones:

• GLP-1 (glucagon-like peptide-1): it triggers insulin release, lowers glucagon, slows how fast your stomach empties and reduces appetite
• GIP (glucose-dependent insulinotropic polypeptide): it boosts insulin release and may affect how the body handles fat and energy

How tirzepatide mimics both:

Tirzepatide is a man-made peptide (a small protein). It binds to and switches on both the GLP-1 and GIP receptors.

This dual action sets Mounjaro apart from single-target medicines such as semaglutide (Ozempic, Wegovy) and liraglutide (Saxenda), which act on the GLP-1 receptor only.

Adding GIP receptor action seems to give extra benefits on top of GLP-1 alone.

In trials, tirzepatide cut body weight and HbA1c (a measure of average blood sugar) more than semaglutide at similar doses.

Clinical significance:

The SURPASS-2 trial compared tirzepatide directly with semaglutide 1 mg. All three tirzepatide doses (5 mg, 10 mg, 15 mg) lowered HbA1c more and led to greater weight loss.

The dual action is thought to be the main reason for this edge.

Mounjaro helps you lose weight in several ways at once. Together, these lower how much you eat and change how your body uses energy.

Central appetite suppression:

GLP-1 and GIP receptors are both found in the hypothalamus, the part of the brain that controls hunger and fullness. Tirzepatide switches on these receptors in the brain.

This gives a steady drop in appetite and food cravings. Patients often feel full sooner and lose interest in high-calorie foods.

Delayed gastric emptying:

Like other GLP-1 medicines, Mounjaro slows how fast your stomach empties into the small intestine. This keeps you feeling full for longer after meals. It also reduces the urge to eat again soon.

Altered food preferences:

Some evidence suggests GLP-1 receptor action changes reward signals in the brain. This can make high-fat and high-sugar foods less appealing.

Many patients on Mounjaro say they start to prefer lighter, lower-calorie food.

Weight loss data:

The SURMOUNT-1 trial in adults with obesity (without diabetes) showed:

• 5 mg dose: 16.0% mean body weight reduction at 72 weeks
• 10 mg dose: 21.4% mean body weight reduction
• 15 mg dose: 22.5% mean body weight reduction
• Placebo: 2.4% reduction

These are among the largest weight reductions seen with any drug treatment so far. More than one third of people on the highest dose lost over 25% of their body weight.

In people with type 2 diabetes, Mounjaro lowers blood sugar through several routes.

Glucose-dependent insulin secretion:

GLP-1 and GIP both prompt insulin release from beta cells in the pancreas. Importantly, this only happens when blood sugar is high. So insulin is released mainly when it is needed.

This greatly lowers the risk of low blood sugar (hypoglycaemia) compared with medicines like sulphonylureas.

Glucagon suppression:

GLP-1 receptor action lowers glucagon release from alpha cells in the pancreas. Glucagon raises blood sugar by making the liver produce more glucose.

By cutting glucagon after meals, Mounjaro helps stop blood sugar spikes.

Improved insulin sensitivity:

Losing weight on its own helps insulin work better. As patients lose belly fat, their cells respond more to insulin. This eases the insulin resistance that drives type 2 diabetes.

Clinical outcomes in diabetes:

Across the SURPASS trials:

• HbA1c fell by up to 2.4 percentage points
• Over 90% of patients on the highest dose reached an HbA1c below 53 mmol/mol (7%)
• About 50% reached an HbA1c below 39 mmol/mol (5.7%), which is in the non-diabetic range

These results beat those of any other single injectable blood sugar medicine. NICE has approved tirzepatide for type 2 diabetes when other treatments have not controlled it well enough.

Knowing how the body handles Mounjaro explains its once-weekly dosing. It also shows how steady levels build up over time.

Administration:

You inject Mounjaro under the skin once a week, into the abdomen, thigh or upper arm. Change the injection site each week.

Absorption:

After the injection, tirzepatide is absorbed slowly from the tissue under the skin. Blood levels peak about 8 to 72 hours after a dose. The middle (median) time to peak is around 24 hours.

Half-life:

Tirzepatide stays in the body a long time. Its half-life (the time for levels to halve) is about 5 days. This is why one dose a week is enough. The medicine keeps working levels going all week.

Steady state:

Levels usually settle into a steady state after 4 weeks of weekly dosing at the same dose.

Metabolism and elimination:

• The body breaks tirzepatide down by cutting it into pieces, much like it does with natural peptides
• The liver and kidneys are not the main route for clearing it. So doses do not normally need to change for mild to moderate liver or kidney problems
• The BNF says no dose change is needed for patients with an eGFR (a kidney function score) above 15 mL/min

Dose titration schedule:

You start at 2.5 mg a week. The dose then rises by 2.5 mg every 4 weeks, up to a maximum of 15 mg a week. This slow build-up lets the body adjust.

It also helps reduce gut side effects.

Mounjaro has a unique place among licensed weight and diabetes treatments. This is because of its dual action.

Mounjaro versus semaglutide (Wegovy/Ozempic):

• Semaglutide acts on the GLP-1 receptor only. Mounjaro acts on both GLP-1 and GIP receptors
• In the SURMOUNT-1 trial, Mounjaro 15 mg gave 22.5% weight loss. Semaglutide 2.4 mg gave about 15% in the STEP-1 trial (an indirect comparison)
• Direct head-to-head data from SURPASS-2 showed tirzepatide beat semaglutide 1 mg for both weight loss and HbA1c

Mounjaro versus liraglutide (Saxenda):

• Liraglutide is a daily GLP-1 injection. It gives about 8% weight loss
• Mounjaro's once-weekly dose and stronger effect make it a more powerful choice

Mounjaro versus orlistat:

• Orlistat works by blocking the fat you absorb from food. It usually gives 3 to 5% weight loss
• Mounjaro works in a completely different way. It acts on appetite and on how the body handles energy

Why dual agonism matters:

The GIP receptor seems to support GLP-1 in controlling appetite and energy use. Switching on both routes at once may give a fuller response than GLP-1 alone. Researchers are still studying this.

Next-generation triple agonists are already being developed.