What Is Mounjaro? Tirzepatide for Weight Loss Explained
Summary: Mounjaro (tirzepatide) is a once-weekly injectable prescription medicine approved by the MHRA in November 2023 for chronic weight management. It is the first dual GIP and GLP-1 receptor agonist available in the UK and produced an average weight loss of 20.9% in the landmark SURMOUNT-1 trial — more than any previously approved weight-loss medicine.
20.9%
Mean body weight lost at 15 mg (SURMOUNT-1, 72 weeks)
91%
Of participants lost ≥5% of body weight at 15 mg
57%
Of participants lost ≥20% of body weight at 15 mg
Nov 2023
MHRA approval date for weight management in the UK
Overview: what is Mounjaro?
Mounjaro is the brand name for tirzepatide, a once-weekly subcutaneous injection manufactured by Eli Lilly. It is classified as a prescription-only medicine (POM) in the UK and is licensed by the Medicines and Healthcare products Regulatory Agency (MHRA) for chronic weight management in adults with an initial body mass index (BMI) of 30 kg/m² or greater, or a BMI of 27–29.9 kg/m² in the presence of at least one weight-related comorbidity.
Tirzepatide was first approved in the United States in 2022 for type 2 diabetes under the brand name Zepbound, and subsequently for obesity. In the UK, the MHRA granted approval for weight management in November 2023, making Mounjaro the most effective pharmacological weight-loss treatment available through a licensed UK pharmacy at that time.
Mounjaro belongs to a new class of medicines: dual glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) receptor agonists. Unlike earlier injectable weight-loss medicines such as liraglutide (Saxenda) which target only one receptor, tirzepatide simultaneously activates both the GIP and GLP-1 receptors, producing a markedly stronger effect on appetite suppression, satiety, and metabolic function.
It is intended for use alongside a calorie-reduced diet and increased physical activity — it is a medical aid to lifestyle change, not a substitute for it.
How does Mounjaro work?
Mounjaro works by mimicking two natural gut hormones — GIP and GLP-1 — that are released after eating. These hormones signal to the brain that the body is full, slow the rate at which food leaves the stomach, stimulate insulin release, and suppress glucagon (a hormone that raises blood sugar). By activating both receptors simultaneously, tirzepatide produces a synergistic effect that is significantly greater than activating either receptor alone.
The practical result is a marked reduction in appetite and food intake, prolonged feelings of fullness after meals, reduced preoccupation with food (sometimes called “food noise”), and improved metabolic markers including blood glucose, blood pressure, and blood lipids.
For a full scientific explanation of the dual mechanism, see the companion guide: How Does Mounjaro Work? The Science Behind Tirzepatide.
SURMOUNT trial results
The clinical evidence for Mounjaro comes primarily from the SURMOUNT programme, a series of phase 3 randomised controlled trials. The key findings are as follows.
SURMOUNT-1 (adults without type 2 diabetes)
SURMOUNT-1 enrolled 2,539 adults with a BMI ≥30 kg/m² or ≥27 kg/m² with a weight-related comorbidity (excluding type 2 diabetes). Participants were randomised to tirzepatide 5 mg, 10 mg, or 15 mg, or placebo, for 72 weeks. All groups received lifestyle intervention support.
−20.9%
Mean weight change at 15 mg (placebo: −3.1%)
−15.0%
Mean weight change at 10 mg
−16.0%
Mean weight change at 5 mg
At the 15 mg dose, 91% of participants achieved ≥5% weight loss, 57% achieved ≥20% weight loss, and 38% achieved ≥25% weight loss. These figures represent the greatest weight-loss efficacy of any pharmacological agent recorded in a phase 3 clinical trial at the time of publication.
SURMOUNT-2 (adults with type 2 diabetes)
SURMOUNT-2 demonstrated a mean weight loss of 15.7% at the 15 mg dose over 72 weeks in adults with type 2 diabetes — a population in whom weight loss is typically more difficult to achieve. Participants also showed substantial improvements in HbA1c (glycated haemoglobin), a marker of long-term blood glucose control.
SURMOUNT-4 (weight maintenance)
SURMOUNT-4 examined what happens when Mounjaro is stopped. Participants who discontinued tirzepatide after achieving weight loss regained approximately two-thirds of their lost weight within 52 weeks. This underlines that Mounjaro is a long-term treatment for a chronic condition, not a short course to be stopped once a target weight is reached.
Who is eligible for Mounjaro?
Mounjaro is a prescription-only medicine and eligibility is assessed by a qualified prescriber. The MHRA-licensed indication in the UK is:
- BMI ≥30 kg/m² (obesity), regardless of comorbidities
- BMI 27–29.9 kg/m² (overweight) with at least one weight-related comorbidity, such as type 2 diabetes, hypertension, dyslipidaemia, obstructive sleep apnoea (OSA), or established cardiovascular disease
- Used alongside a reduced-calorie diet and increased physical activity
- Treatment reviewed regularly by the prescriber
Who should not take Mounjaro?
Mounjaro is contraindicated in the following circumstances:
- Pregnancy or breastfeeding
- Personal or family history of medullary thyroid carcinoma (MTC)
- Multiple Endocrine Neoplasia syndrome type 2 (MEN2)
- Personal history of pancreatitis
- Concurrent use of another GLP-1 or GIP receptor agonist
- Known hypersensitivity to tirzepatide or any excipient
Caution is required in patients with diabetic retinopathy, a history of gallbladder disease, or those taking insulin or sulphonylureas (risk of hypoglycaemia). A full medical assessment is conducted before any prescription is issued.
Dosing schedule
Mounjaro is initiated at a low dose and escalated gradually over several months. This step-up approach is designed to minimise gastrointestinal side effects while the body adjusts to the medicine. Injections are given once weekly at any time of day, with or without food.
1
Weeks 1–4: 2.5 mg
Starting dose. Not intended to produce significant weight loss at this stage — the purpose is tolerability. Inject subcutaneously into the abdomen, thigh, or upper arm, rotating sites each week.
2
Weeks 5–8: 5 mg
First dose increase. Most patients begin to experience noticeable appetite reduction at this stage. If tolerability is poor, the prescriber may advise remaining at 2.5 mg for a further 4 weeks.
3
Weeks 9–12: 7.5 mg
Continued escalation. Weight loss typically becomes more consistent. Store unused pens in the refrigerator at 2–8°C; do not freeze.
4
Weeks 13–16: 10 mg
Mid-range therapeutic dose. Many patients achieve clinically significant weight loss (≥5%) by this point. A pen removed from the fridge may be kept at room temperature (below 30°C) for up to 21 days.
5
Weeks 17–20: 12.5 mg
Penultimate escalation step. Side effects, if present, are usually well established by now and should be improving.
6
Week 21 onwards: 15 mg
Maximum maintenance dose. This is the dose at which SURMOUNT-1 demonstrated 20.9% average weight loss. Not all patients will need or tolerate 15 mg; some achieve their treatment goals at a lower dose.
Missed dose: If a dose is missed and the next scheduled dose is more than 4 days away, inject as soon as possible and then return to the usual schedule. If the next dose is within 4 days, skip the missed dose and continue as normal. Do not take two doses in the same week.
Mounjaro vs Saxenda: key differences
Saxenda (liraglutide) is a once-daily injectable GLP-1 receptor agonist that has been available in the UK since 2015. Mounjaro represents a significant advance in efficacy. The table below summarises the main differences. For a detailed comparison, see the guide: Mounjaro vs Saxenda: Which Is More Effective?
| Feature | Mounjaro (tirzepatide) | Saxenda (liraglutide) |
|---|---|---|
| Mechanism | Dual GIP + GLP-1 agonist | GLP-1 agonist only |
| Injection frequency | Once weekly | Once daily |
| Average weight loss (trial) | 20.9% at 15 mg (72 weeks) | ~8% at 3 mg (56 weeks) |
| ≥5% weight loss achieved | 91% of participants | ~63% of participants |
| MHRA approval (weight) | November 2023 | January 2015 |
| Maximum dose | 15 mg weekly | 3 mg daily |
| Storage | 2–8°C (up to 21 days at room temp) | 2–8°C (up to 30 days at room temp) |
How to get Mounjaro in the UK
Mounjaro is a prescription-only medicine. It cannot be purchased over the counter. A prescription may be obtained through an NHS specialist weight management service, a GP (where commissioned), or a regulated online pharmacy following a remote clinical consultation.
Access Doctor is a GPhC-registered online pharmacy (registration number 9011198) offering a full online consultation with a GMC-registered prescriber. If you meet the eligibility criteria, a prescription can be issued and medication dispatched directly to your address.
Start your Mounjaro assessment
Complete a free online consultation with one of our GMC-registered GPs. If suitable, your prescription is issued and dispatched from our GPhC-registered pharmacy (#9011198) — no GP appointment needed.
View weight loss treatmentsFrequently asked questions
Is Mounjaro available on the NHS?
NICE published guidance in 2024 recommending tirzepatide (Mounjaro) for use within NHS specialist weight management services for eligible patients. Availability through GP practices varies by integrated care board (ICB). Many patients currently access it through private prescription from regulated online pharmacies such as Access Doctor.
How long does it take to see results with Mounjaro?
Most patients begin to notice a reduction in appetite within the first 4–8 weeks. Measurable weight loss typically becomes consistent from months 2–3 onwards as the dose escalates. The greatest weight loss is generally observed between months 6–18 of treatment. Results vary by individual and are enhanced by dietary changes and increased physical activity.
Does Mounjaro need to be kept in the fridge?
Yes. Unused Mounjaro pens must be stored in the refrigerator at 2–8°C and must not be frozen. A pen that has been removed from the refrigerator can be kept at room temperature (below 30°C) for up to 21 days. Discard any pen that has been frozen or exposed to temperatures above 30°C.
Can I take Mounjaro if I have type 2 diabetes?
Yes. Tirzepatide is licensed both for glycaemic control in type 2 diabetes (as Mounjaro) and for weight management. In the SURMOUNT-2 trial, participants with type 2 diabetes achieved an average weight loss of 15.7% at the 15 mg dose. If you are taking insulin or a sulphonylurea alongside Mounjaro, your prescriber will monitor for hypoglycaemia and may adjust your diabetes medication.
What are the most common side effects of Mounjaro?
The most common side effects are gastrointestinal: nausea, diarrhoea, vomiting, constipation, and dyspepsia. These are most pronounced during dose escalation and typically improve by weeks 6–10. In the SURMOUNT-1 trial, only 4.3% of participants at the 15 mg dose discontinued due to gastrointestinal side effects — meaning more than 95% continued treatment. For a full side-effect guide, see Mounjaro Side Effects: What to Expect & How to Manage Them.
Can I drink alcohol while taking Mounjaro?
There is no absolute contraindication to alcohol, but patients are advised to moderate intake. Alcohol is calorie-dense and may undermine weight-loss progress. Heavy alcohol use can also increase the risk of pancreatitis, a rare but serious side effect of tirzepatide. Speak to your prescriber if you have concerns about alcohol use.
References
- Jastreboff AM, et al. Tirzepatide once weekly for the treatment of obesity. N Engl J Med. 2022;387(3):205–216. doi:10.1056/NEJMoa2206038
- Garvey WT, et al. Tirzepatide once weekly for the treatment of obesity in people with type 2 diabetes (SURMOUNT-2). Lancet. 2023;402(10402):613–626.
- Medicines and Healthcare products Regulatory Agency. Mounjaro 2.5 mg solution for injection: summary of product characteristics. November 2023. www.medicines.org.uk
- National Institute for Health and Care Excellence. Tirzepatide for managing overweight and obesity (TA1026). London: NICE; 2024. www.nice.org.uk/guidance/ta1026
- Eli Lilly. SURMOUNT-4: tirzepatide vs placebo following tirzepatide-induced weight loss. JAMA. 2023;330(22):2151–2161.
- NHS. Weight loss surgery and medicines. www.nhs.uk
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Mounjaro is a prescription-only medicine (POM); it must be prescribed by a qualified healthcare professional following an individual clinical assessment. Do not start, stop, or alter any prescribed medication without consulting your prescriber. If you experience any side effects, contact your prescriber or call NHS 111. In an emergency, call 999.