How does Mounjaro work? A clear guide to its mechanism

Mounjaro contains the active ingredient tirzepatide, which is unique among licensed weight management and diabetes medicines because it activates two incretin hormone receptors simultaneously.

What are incretins?

Incretins are hormones released by the gut after eating. They play a central role in blood sugar regulation and appetite control. The two main incretins are:

• GLP-1 (glucagon-like peptide-1): stimulates insulin release, suppresses glucagon, slows gastric emptying and reduces appetite
• GIP (glucose-dependent insulinotropic polypeptide): enhances insulin secretion and may influence fat metabolism and energy balance

How tirzepatide mimics both:

Tirzepatide is a synthetic peptide that binds to and activates both the GLP-1 and GIP receptors.

This dual agonism is what distinguishes Mounjaro from single-agonist medicines such as semaglutide (Ozempic, Wegovy) and liraglutide (Saxenda), which target only the GLP-1 receptor.

The addition of GIP receptor activation appears to provide additive benefits beyond GLP-1 agonism alone.

In clinical trials, tirzepatide produced greater reductions in body weight and HbA1c than semaglutide at comparable doses.

Clinical significance:

The SURPASS-2 trial directly compared tirzepatide with semaglutide 1 mg. Tirzepatide at all three doses (5 mg, 10 mg, 15 mg) produced superior HbA1c reductions and greater weight loss.

The dual mechanism is thought to be the key reason for this advantage.

Weight loss with Mounjaro results from several overlapping mechanisms that collectively reduce calorie intake and alter energy balance.

Central appetite suppression:

Both GLP-1 and GIP receptors are expressed in the hypothalamus, the brain region that controls hunger and satiety.

Tirzepatide activates these receptors centrally, producing a sustained reduction in appetite and food cravings. Patients commonly report feeling full sooner and losing interest in high-calorie foods.

Delayed gastric emptying:

Like other GLP-1 agonists, Mounjaro slows the rate at which the stomach empties its contents into the small intestine.

This prolongs the sensation of fullness after meals and reduces the urge to eat again quickly.

Altered food preferences:

Some evidence suggests that GLP-1 receptor activation modifies reward signalling in the brain, making high-fat and high-sugar foods less appealing.

Many patients on Mounjaro describe a shift in food preferences towards lighter, less calorie-dense options.

Weight loss data:

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

• 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 figures represent some of the largest weight reductions achieved with any pharmacological treatment to date.

Over one third of participants on the highest dose lost more than 25% of their body weight.

For patients with type 2 diabetes, Mounjaro's mechanism of action provides robust glycaemic control through multiple pathways.

Glucose-dependent insulin secretion:

Both GLP-1 and GIP stimulate insulin release from pancreatic beta cells, but crucially, this effect is glucose-dependent.

This means insulin is released primarily when blood sugar is elevated, significantly reducing the risk of hypoglycaemia compared with medicines like sulphonylureas.

Glucagon suppression:

GLP-1 receptor activation suppresses glucagon release from pancreatic alpha cells. Glucagon raises blood sugar by promoting glucose production in the liver.

By reducing glucagon levels after meals, Mounjaro helps prevent post-meal blood sugar spikes.

Improved insulin sensitivity:

Weight loss itself improves how the body responds to insulin. As patients lose visceral fat, cells become more responsive to insulin, reducing the insulin resistance that underpins type 2 diabetes.

Clinical outcomes in diabetes:

Across the SURPASS trial programme:

• HbA1c reductions of up to 2.4 percentage points were observed
• Over 90% of patients on the highest dose achieved an HbA1c below 53 mmol/mol (7%)
• Approximately 50% achieved an HbA1c below 39 mmol/mol (5.7%), which is within the non-diabetic range

These results exceed those seen with any other single injectable glucose-lowering medicine. NICE has approved tirzepatide for type 2 diabetes when other treatments have not achieved adequate control.

Understanding the pharmacokinetics of Mounjaro helps explain its once-weekly dosing schedule and how steady-state levels are achieved.

Administration:

Mounjaro is given as a subcutaneous injection once weekly, into the abdomen, thigh or upper arm. The injection site should be rotated each week.

Absorption:

After injection, tirzepatide is absorbed slowly from the subcutaneous tissue. Peak blood levels are reached approximately 8 to 72 hours after dosing, with a median time to peak of around 24 hours.

Half-life:

Tirzepatide has a long elimination half-life of approximately 5 days. This supports the once-weekly dosing regimen and means the medicine maintains therapeutic levels throughout the week.

Steady state:

Steady-state concentrations are typically reached after 4 weeks of once-weekly dosing at the same dose level.

Metabolism and elimination:

• Tirzepatide is broken down by proteolytic cleavage, similar to natural peptides
• It is not primarily cleared by the liver or kidneys, which means dose adjustments are not routinely required for mild to moderate hepatic or renal impairment
• The BNF notes that no dose adjustment is needed for patients with an eGFR above 15 mL/min

Dose titration schedule:

The recommended titration begins at 2.5 mg weekly, increasing by 2.5 mg every 4 weeks to a maximum of 15 mg weekly.

This gradual increase allows the body to adapt and helps minimise gastrointestinal side effects.

Mounjaro occupies a distinct position among licensed weight management and diabetes treatments due to its dual mechanism.

Mounjaro versus semaglutide (Wegovy/Ozempic):

• Semaglutide is a GLP-1 receptor agonist only; Mounjaro activates both GLP-1 and GIP receptors
• In the SURMOUNT-1 trial, Mounjaro 15 mg produced 22.5% weight loss versus approximately 15% with semaglutide 2.4 mg in the STEP-1 trial (indirect comparison)
• Direct head-to-head data from SURPASS-2 showed tirzepatide was superior to semaglutide 1 mg for both weight loss and HbA1c reduction

Mounjaro versus liraglutide (Saxenda):

• Liraglutide is a daily GLP-1 agonist injection producing approximately 8% weight loss
• Mounjaro's once-weekly dosing and greater efficacy make it a more potent option

Mounjaro versus orlistat:

• Orlistat works by blocking dietary fat absorption and typically produces 3 to 5% weight loss
• Mounjaro's mechanism is entirely different, acting on appetite regulation and metabolic pathways

Why dual agonism matters:

The GIP receptor appears to play a complementary role to GLP-1 in appetite regulation and energy balance.

Activating both pathways simultaneously may produce a more complete metabolic response than targeting GLP-1 alone.

This is an active area of research, and next-generation triple agonists are already in development.