Mounjaro Study Guide: What the Tirzepatide Trials Show (October 2026)

01 · The core answer

What the Mounjaro studies actually show

The main evidence for Mounjaro comes from large randomised trials, not from news stories or social media. For weight management, the trial most people mean is SURMOUNT-1. The results are averages across everyone in the trial, including people who lost less than the headline figure.

Two further points matter before you act on any number. These results come from people who also followed a reduced-calorie diet and increased physical activity. And in SURMOUNT-1 the figures rose with the dose; the dose steps up gradually, with at least 4 weeks on each dose, depending on how you respond.²

A fuller picture of who lost how much is in the same trial: At 72 weeks, 85% of participants on 5mg, 89% on 10mg and 91% on 15mg had lost at least 5% of their body weight. Half of participants on 10mg and 57% on 15mg lost at least 20% of their body weight.²

If you want to know what these results could mean for you, the next step is a conversation with a prescriber, not a headline. Our weight loss clinic explains how a consultation works.

02 · Is it relevant to me?

Does a Mounjaro study apply to you?

A study applies to you only if you resemble the people in it. The licensed weight-management use is for adults, used alongside a reduced-calorie diet and increased physical activity.¹ Mounjaro is licensed for weight management in adults with a BMI of 30 or above, or a BMI between 27 and 30 with at least one weight-related condition; a prescriber will assess which applies to you.

Trial participants were volunteers who met strict entry criteria and were closely monitored. Real-world results can differ from trial results, so individual results vary. For a related look at how the evidence is moving, see new study on weight loss drugs.

03 · Head-to-head evidence

Tirzepatide versus semaglutide: the direct comparison

SURMOUNT-5 is the first large head-to-head trial of tirzepatide and semaglutide, and comparing numbers from two separate trials is not a fair test. In SURMOUNT-5, 751 adults took tirzepatide or semaglutide 2.4mg for 72 weeks.¹⁵

At least 10% of body weight was lost by 81.6% on tirzepatide vs 60.5% on semaglutide; at least 15% by 64.6% vs 40.1%; at least 20% by 48.4% vs 27.3%; at least 25% by 31.6% vs 16.1%. Average loss was 22.8 kg vs 15.0 kg.¹⁵

Semaglutide has its own landmark study, STEP 1, which is a separate trial against placebo. In STEP 1, 86.4% of adults on semaglutide 2.4mg lost at least 5% of body weight over 68 weeks, compared with 31.5% on placebo.⁹ Those numbers should not be set beside SURMOUNT-1 as though the trials were a contest. For a fuller walkthrough, read our Mounjaro vs Wegovy study guide.

04 · Side effects in trials

What trials say about side effects

Trials record side effects as carefully as weight loss, and stomach and gut symptoms are the usual headline. In SURMOUNT-1, 4.3% of people on 5mg, 7.1% on 10mg and 6.2% on 15mg stopped treatment because of side effects, compared with 2.6% on placebo.²

The full list, with how common each effect is, sits in the patient information leaflet and the SmPC.¹³

Tell your prescriber about any new or lasting symptom, and you can report suspected side effects through the Yellow Card scheme.¹⁹ Online reports are not evidence that a medicine caused something.

05 · Autoimmune and dementia

Autoimmune, dementia and metabolic studies

Mounjaro is licensed in the UK for weight management in adults and for type 2 diabetes. Anything beyond those uses is outside its licence.¹

That does not mean research is not happening. Early findings, observational studies and press releases are not the same as a licensed indication. If you have an autoimmune condition, tell your prescriber at the consultation so they can consider your full history. Our page on Mounjaro and autoimmune goes into this in more detail.

The same caution applies to a metabolic study headline. Tirzepatide helps improve the way the body responds to insulin.¹

06 · Mechanism and clearance

How the studied mechanism works

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 In practice, most people feel fuller sooner and less hungry between meals, which supports a reduced-calorie diet.

This dual action is what distinguishes Mounjaro from single-action GLP-1 medicines such as semaglutide (Wegovy/Ozempic).

Tirzepatide has a half-life of about five days, so after a final dose it takes roughly four to five weeks to clear the system. There is no way to speed this up, and nothing needs doing while it clears.¹ This is relevant to studies about stopping treatment: effects do not vanish the day after the last injection, and what happens next is something to plan with your prescriber.

07 · Pregnancy and breastfeeding

Breastfeeding, pregnancy and PCOS research

Current prescribing information indicates minimal transfer of tirzepatide into breast milk; discuss breastfeeding with your prescriber so the decision reflects your circumstances.¹³

Pregnancy is clearer. 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.¹

Oral contraception also matters. Mounjaro slows stomach emptying, which can reduce how well oral contraceptive pills are absorbed. The SmPC advises switching to a non-oral method, or adding a barrier method (such as condoms), for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase. This applies to the pill only — implants, coils and injections are unaffected.⁷

Mounjaro is not licensed specifically for PCOS, but many women with PCOS meet the licensed weight-management criteria, and weight loss itself is a first-line recommendation that can improve insulin resistance, cycles and symptoms. Mention PCOS and any fertility plans in your consultation.⁴ More in our Mounjaro and PCOS study guide.

08 · Long-term and stopping

Long-term effects and stopping studies

The main weight-loss trials ran for 72 weeks.²

In SURMOUNT-4, people who stopped tirzepatide after 36 weeks regained on average 14% of body weight over the following year, while those who continued treatment lost a further 5.5% on average. Stopping should always be planned with your prescriber.¹⁴ Our guide to the Mounjaro weight regain study covers the topic in detail, and our guide to weight loss injections UK study looks at the wider UK picture.

09 · Serious side effects

Serious side effects: when to get help straight away

Serious side effects are uncommon but need urgent attention. 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/eyes (possible gallbladder problems); or signs of a severe allergic reaction such as swelling of the face or throat and difficulty breathing. If you take insulin or a sulfonylurea alongside Mounjaro, watch for low blood sugar (shakiness, sweating, confusion) — your prescriber may need to adjust those medicines.¹

10 · UK access and case studies

UK access, case studies and pen cases

The practical question is often whether you can get treatment. Mounjaro is available on the NHS, but access is being phased in over several years. NICE recommends it for adults with a BMI of at least 35 plus at least one weight-related condition, and the NHS primary-care rollout began in June 2025 with the highest-need groups first (initially BMI 40+ with four or more qualifying conditions).⁴⁶

Full rollout is expected to take years, which is why many people who meet the licensed criteria choose regulated private treatment rather than waiting.

If you meant a physical case for carrying your pen, our Mounjaro case holder guide covers travel storage.