How Mounjaro Works for Weight Loss

Last reviewed: [January 2026]

How Mounjaro Works for Weight Loss: Benefits, Side Effects and UK Eligibility

Obesity is a significant health issue in the UK. A large proportion of adults are now overweight or living with obesity. In population-level studies, higher body weight is associated with increased risks of type 2 diabetes, cardiovascular disease, obstructive sleep apnoea, osteoarthritis and several cancers. [1–8]

Weight management is rarely a simple short-term effort. Many people can lose weight for a period of time, but sustaining that loss can be difficult. Lifestyle measures remain the foundation of treatment. A reduced-calorie diet, regular physical activity and behavioural support are central. However, for some individuals, medication can make these changes more achievable and sustainable. [9]

Mounjaro (tirzepatide) is a once-weekly injectable medication licensed in the UK for type 2 diabetes and for weight management in adults with obesity, or overweight with weight-related medical conditions. [30] It works by influencing hormone pathways involved in appetite regulation and metabolic control. [10–12]

What Is Mounjaro?

Mounjaro is the brand name for tirzepatide. It is administered as a subcutaneous injection (under the skin), once per week. [30]

Tirzepatide activates two receptors in the body:

• GLP-1 (glucagon-like peptide-1) receptor
• GIP (glucose-dependent insulinotropic polypeptide) receptor [10–12]

These receptors normally respond to hormones released from the gut after eating. Other injectable weight-loss medicines, such as semaglutide (Wegovy) and liraglutide (Saxenda), activate the GLP-1 receptor alone. Tirzepatide activates both GLP-1 and GIP receptors. [10–12]

In the UK, tirzepatide is licensed for use alongside a reduced-calorie diet and increased physical activity. [30]

How Mounjaro Works

Appetite and Fullness

After we eat, the gut releases hormones that signal to the brain that food has arrived. These signals contribute to the feeling of fullness during and after a meal.

Tirzepatide mimics two of these hormones. In mechanistic studies, tirzepatide reduced appetite ratings and reduced energy intake during test meals. [13–15] In practical terms, a portion that previously felt moderate may feel sufficient sooner. Hunger between meals may become less persistent, and snacking may become less frequent. [13–15]

Slowing Gastric Emptying

Gastric emptying refers to the speed at which food leaves the stomach and moves into the small intestine.

Tirzepatide delays gastric emptying most noticeably early in treatment, with the effect reducing over time. [16,17] When food remains in the stomach for longer, the sensation of fullness can last longer. [16,17]

Effects on Blood Sugar

Tirzepatide was originally developed for type 2 diabetes.

In people with type 2 diabetes, tirzepatide improves glucose control and is associated with weight loss in clinical trials. [18–20]

Food Motivation and Cravings

Eating is influenced not only by physical hunger but also by reward systems in the brain. Food can trigger anticipation and desire even when energy needs have been met.

Clinical studies of GLP-1–based treatments show reduced activation in brain regions involved in responding to food cues. [21,22] In trials of tirzepatide, participants reported lower appetite scores and reduced energy intake. [13–15]

How This Leads to Fat Loss

Weight loss occurs when energy intake is consistently lower than energy expenditure.

Mounjaro does not directly “burn fat” independently of food intake. Instead, by reducing appetite and increasing fullness, it can make sustained calorie reduction more achievable. When calorie intake falls below the body’s energy needs, stored fat is used to supply energy, leading to weight loss over time. [13–15]

Effectiveness and Clinical Results

Tirzepatide has been studied in several large clinical trial programmes.

SURMOUNT-1 (Adults Without Diabetes)

The SURMOUNT-1 trial followed adults with obesity, or overweight without diabetes, for 72 weeks. [23]

Average weight loss was approximately:

• 15% of body weight at 5 mg weekly
• 19–20% at 10 mg weekly
• 21–22% at 15 mg weekly [23]

A substantial proportion of participants at higher doses lost 20% or more of their starting weight. These results were achieved alongside lifestyle advice. [23]

To understand what these levels of weight loss mean for your health, see our full guide

SURPASS Trials (Adults With Type 2 Diabetes)

In people with type 2 diabetes, weight loss was generally more modest, though still clinically meaningful. Across major SURPASS trials, weight loss was dose-dependent and greater than comparator arms. [18–20]

Average weight loss in these studies ranged from around 5% to the low teens, depending on the population, dose and comparator. [18–20]

In people with type 2 diabetes, average weight loss is typically lower than in trials in people without diabetes. [18–20,23]

Comparison With Semaglutide

In a head-to-head clinical trial in adults with type 2 diabetes, tirzepatide produced greater average weight loss than semaglutide at the doses studied. [20]

Both medications are effective treatments. Individual results vary, and the choice between them depends on medical history, tolerability and treatment goals.

How Quickly Does It Work?

Appetite reduction can occur early in treatment, and weight loss typically builds gradually over time with continued use. [13,23]

Potential Side Effects and Safety

As with all prescription medicines, side effects can occur. [30]

Common Side Effects

The most common side effects affect the gastrointestinal system and are usually mild to moderate. These include nausea, diarrhoea, constipation, vomiting and abdominal discomfort. [30] In trials, gastrointestinal symptoms were common, especially during dose escalation. [18,23,24,30]

“For a full timeline of how long Mounjaro’s side effects typically last, see our dedicated guide.”

Important Safety Considerations

Mounjaro is not suitable for everyone. A full medical assessment is required before starting treatment to determine suitability and discuss risks. [30]

Who Can Benefit From Mounjaro?

Tirzepatide is licensed for weight management in adults with obesity, or overweight with weight-related medical conditions. [30]

For some ethnic groups at higher metabolic risk, lower BMI thresholds may apply. [25]

Suitability depends on overall health, previous attempts at weight loss, current medications and tolerance of potential side effects.

How to Use Mounjaro

Mounjaro is injected once weekly under the skin. Common injection sites include the abdomen, thigh or upper arm. [30]

Treatment starts at a low dose. The dose is increased at set intervals, provided it is tolerated, until a maintenance dose is reached. [30] This gradual approach helps reduce gastrointestinal side effects. [23,30]

Medication should be combined with dietary structure and physical activity. For example, maintaining regular meal patterns, ensuring adequate protein intake and incorporating consistent activity can support results.

We would not normally prescribe Mounjaro alongside other weight-loss medicines.

Regular clinical review allows monitoring of weight progress and any side effects.

Comparing Mounjaro With Other Weight-Loss Medications

Semaglutide (Wegovy) is a weekly GLP-1 receptor agonist. In STEP trials, average weight loss with semaglutide 2.4 mg was in the mid-teens percent range, alongside structured lifestyle intervention. [26,27]

Liraglutide (Saxenda) is a daily GLP-1 receptor agonist. In a large trial, average weight loss was about 8% at 56 weeks with liraglutide 3.0 mg alongside lifestyle intervention. [28]

Tirzepatide (Mounjaro), which activates both GLP-1 and GIP receptors, has produced average weight loss of up to around 21–22% in adults without diabetes in clinical trials. [23]

Long-Term Maintenance

Weight regain can occur after stopping weight-loss medication. In withdrawal studies, stopping treatment has been associated with weight regain, while continuing treatment maintained more of the weight loss. [24,27,29]

Long-term weight management may involve continued medication, structured eating patterns, physical activity and behavioural strategies. The appropriate duration of treatment should be discussed with a clinician.

Alternatives to Mounjaro

Mounjaro is one option within a broader range of weight-management treatments.

Other GLP-1–based medications include semaglutide (Wegovy) and liraglutide (Saxenda). These act on the GLP-1 receptor but do not activate the GIP receptor. [26–28]

There are also non-GLP-1 medications available in the UK, such as:

• Orlistat, which reduces fat absorption from the gut
• Naltrexone/bupropion combination therapy, which acts on appetite pathways in the brain [9]

In addition, non-medication approaches remain important. These include structured dietary programmes, behavioural interventions, increased physical activity and, in appropriate cases, bariatric surgery. [9]

The most appropriate approach depends on individual health status, risk profile and treatment goals.

“If you are wondering about whether Mounjaro is suitable as a long-term treatment, we explore the clinical evidence in a separate article.”

Frequently Asked Questions

These answers provide a general overview. For detailed explanations, evidence summaries, and treatment comparisons, see our in-depth guides in the Knowledge Hub.