Mounjaro Function: What Tirzepatide Does in the Body (October 2026)

01 · The core answer

What is the function of Mounjaro?

Mounjaro is the brand name for tirzepatide, a once-weekly injectable prescription medicine made by Eli Lilly, licensed in the UK for weight management in adults and for type 2 diabetes.¹ Its function is hormonal: it mimics two natural gut hormones that signal fullness and help regulate blood sugar.

Tirzepatide is the first treatment of its kind to activate two natural gut-hormone receptors at once: GIP and GLP-1. Together these help regulate appetite, slow how quickly the stomach empties, and improve the way the body responds to insulin.¹ This dual action distinguishes it from single-action medicines such as semaglutide.

02 · Your next step

Is this relevant to you?

If you are an adult looking at medically supervised weight management, your prescriber will weigh how Mounjaro works against your history. Treatment works alongside a reduced-calorie diet and increased physical activity, not instead of them. Individual results vary.

If you want to explore treatment, our weight loss clinic explains the process. Every consultation is reviewed by a GPhC-registered prescriber.

03 · Thyroid function

Mounjaro and thyroid function

Mounjaro is not known to change how a healthy thyroid works, but there is a specific precaution. In rodent studies tirzepatide caused thyroid C-cell tumours; it is not known whether this applies to humans, and no increase has been shown in human trials.¹

As a precaution, tell your prescriber if you or a close relative have had medullary thyroid cancer or the inherited condition MEN 2. Routine thyroid monitoring is not required for most people.¹ Our pages on Mounjaro and the thyroid and thyroid tumour concerns go further.

04 · Kidney and liver function

Kidney and liver function

Mounjaro is not known to damage kidneys or liver directly, and no dose adjustment is needed for mild to moderate kidney or liver impairment.¹ The practical risk is indirect.

Severe vomiting or diarrhoea can cause dehydration, which can strain the kidneys, so keep fluids up and get medical help straight away if you cannot keep fluids down. People with severe impairment need individual prescriber assessment.¹ Read more on tirzepatide and kidney function, whether it helps the kidneys, liver function and liver effects.

05 · Mood, cognition and sexual function

Mood, cognitive and sexual function

Regulators have looked specifically at mood. The EMA's safety committee reviewed the evidence and concluded that it does not support a causal link between GLP-1 medicines and low mood or suicidal thoughts, and the MHRA keeps this under continuous review.¹⁷ Big changes in weight, appetite and routine can still affect how you feel. Tell your prescriber about any history of depression, and seek help promptly if your mood drops or you have distressing thoughts during treatment.

If you have thoughts of harming yourself or ending your life, get help straight away. Call 999 or go to A&E if you are in immediate danger. In England you can call NHS 111 and choose the mental health option, any time. You can also call Samaritans free on 116 123, day or night. Tell your prescriber about any change in your mood while taking Mounjaro.

If you notice a new symptom, discuss it with your prescriber and report it through the Yellow Card scheme.¹⁹

06 · Wider body effects

Effects beyond weight

In trials, weight loss on tirzepatide came with average improvements in waist size, blood pressure, cholesterol and blood-sugar measures, and most participants with prediabetes returned to normal blood-sugar levels.² These are trial-average secondary outcomes, not promises.

On weight, SURMOUNT-1 people taking tirzepatide alongside diet and exercise lost on average 15% of their body weight on 5mg, 19.5% on 10mg and 20.9% on 15mg over 72 weeks, compared with 3.1% with lifestyle changes alone.² Individual results vary.

07 · Safety and clearance

Safety and who should avoid it

You should not take Mounjaro if you are allergic to tirzepatide or any of its ingredients, and it should not be used during pregnancy or while trying to conceive. Mounjaro must not be used during pregnancy, and it should be stopped at least one month before trying to conceive. If you become pregnant during treatment, stop the medicine and tell your prescriber promptly. Your prescriber will take extra care if you have a history of pancreatitis, severe gut or digestive disease, diabetic retinopathy, or gallbladder problems.¹

Seek immediate medical help for severe, persistent stomach pain that may spread to your back, with or without vomiting (possible pancreatitis); pain in the upper right abdomen, fever or yellowing of skin or eyes (possible gallbladder problems); or signs of a severe allergic reaction such as swelling of the face or throat and difficulty breathing.¹ After a final dose, tirzepatide takes roughly four to five weeks to clear the system; nothing needs doing while it clears.¹