Mounjaro injection, explained

What is the Mounjaro injection?

Mounjaro is the brand name for the active substance tirzepatide, developed and made by Eli Lilly and Company^[2], and given as a once-weekly injection under the skin^[5]. Its mechanism is what makes it unusual: at its first approval, tirzepatide was the first and only medicine to act on two gut-hormone receptors at once — the glucose-dependent insulinotropic polypeptide (GIP) receptor and the glucagon-like peptide-1 (GLP-1) receptor^[2]. That dual action is why it is nicknamed a "twincretin"; older medicines such as semaglutide act on the GLP-1 receptor alone.

In the UK the single brand "Mounjaro" covers both of its licensed uses — type 2 diabetes and weight management^[1]. There is no "Zepbound" brand in the UK: here, patients receive Mounjaro for weight loss, so any American "Mounjaro versus Zepbound" comparisons you may come across do not apply in this country^[1].

Is Mounjaro licensed in the UK?

Yes. Mounjaro was first licensed in Great Britain for type 2 diabetes, and the MHRA then authorised it for weight loss and weight management in adults aged 18 and over on 8 November 2023^[1]. That makes it a legal, licensed medicine here — not a grey-market product. But "licensed" is not the same as "available to anyone who wants it": Mounjaro is prescription-only, and its supply is controlled, as the sections below explain.

How Mounjaro works

Tirzepatide is a single peptide molecule that switches on both incretin receptors, GIP and GLP-1^[2][5]. Between them they do three things: help the pancreas release insulin when blood sugar is high, slow the rate at which the stomach empties, and reduce appetite and food intake^[3]. The appetite effect is the one most people notice — smaller portions feel like enough sooner — which is why, used alongside diet and activity, the medicine leads to weight loss^[3]. Because it is a peptide, it has to be injected rather than swallowed: a tablet would be broken down in the stomach before it could work^[5].

Who is Mounjaro for? The UK label

Under the UK weight-management licence, Mounjaro is for adults with a body-mass index (BMI) of 30 kg/m² or above (obesity), or 27 to 30 kg/m² (overweight) who also have at least one weight-related health problem — for example prediabetes, high blood pressure, abnormal blood fats or cardiovascular disease^[1]. It is licensed for use alongside a reduced-calorie diet and more physical activity, not as a replacement for them^[1]. Whether you fit that label, and whether it is safe given your history, is a judgement for a prescriber.

Who should not take Mounjaro

Some people should not use it at all. The UK Summary of Product Characteristics rules out Mounjaro for anyone with a known serious allergy (hypersensitivity) to tirzepatide or any of its ingredients^[5]. The US prescribing information goes further and contraindicates tirzepatide for people with a personal or family history of medullary thyroid carcinoma (a type of thyroid cancer) or Multiple Endocrine Neoplasia syndrome type 2 (MEN 2), because in a two-year animal study tirzepatide caused thyroid C-cell tumours in rats — whether that risk applies to people is not known^[6]. Warnings and precautions include acute pancreatitis, gallbladder disease, and low blood sugar if it is combined with insulin or a sulfonylurea^[6]. The UK product information also reports that tirzepatide can make the oral contraceptive pill less reliable, so patients on the pill are advised to add a barrier method for a time after starting and after each dose increase^[5]. All of this is why the medical-history check belongs with a prescriber, not a website.

Mounjaro doses

Mounjaro comes in six fixed strengths — 2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg and 15 mg — each a 0.6 mL injection^[5]. Nobody starts at the top. Treatment begins at 2.5 mg once weekly for four weeks — a starter dose meant to settle the stomach, not to drive weight loss — then moves up to 5 mg^[1]. After that the dose can rise in 2.5 mg steps, no more often than every four weeks, up to a maximum of 15 mg once weekly, as tolerated and as the prescriber directs^[1][5]. You can inject on any day, with or without food, rotating between the abdomen, thigh or upper arm^[5]. Our Mounjaro dosage page walks through the full ladder, and the how-to-inject guide covers technique.

In the UK, Mounjaro is supplied as the KwikPen, a disposable pre-filled pen that holds four fixed weekly doses of one strength; it is kept refrigerated at 2°C to 8°C before first use^[5]. Our how-to-inject guide covers storage and technique in full.

How well does it work? The SURMOUNT-1 trial

The headline weight-management evidence for Mounjaro is SURMOUNT-1, a 72-week, placebo-controlled trial in 2,539 adults with obesity (or overweight plus a weight-related condition) who did not have type 2 diabetes^[3]. Average weight loss was about 16.0% at 5 mg, 21.4% at 10 mg and 22.5% at 15 mg, versus 2.4% on placebo^[3]. At 15 mg that was roughly 24 kg lost, against about 2 kg on placebo^[4]. Put another way, 96% of people on 15 mg lost at least 5% of their body weight, and 39.7% lost a quarter or more of it — against 0.3% on placebo^[4]. The weight lost was mostly fat: the 15 mg group saw roughly three times as much reduction in fat mass as in lean mass^[4].

Read those numbers honestly. They are the "on-treatment" (efficacy estimand) figures — the effect seen while people stayed on the medicine; the New England Journal of Medicine also reports a stricter analysis that counts everyone originally randomised, whether or not they kept taking it, which gives somewhat lower averages^[3]. And they are trial averages: individual results vary, and weight tends to return if the medicine is stopped without other changes in place.

Is Mounjaro safe? Side effects

No medicine is risk-free. Mounjaro's side effects are predominantly gastrointestinal — nausea, diarrhoea, constipation and vomiting — mostly mild to moderate and dose-related, so they become more common as the dose goes up^[3]. Nausea is the most common: in SURMOUNT-1 it affected about 31% of people on the 15 mg dose, against roughly 10% on placebo^[3]. Even so, most people kept taking the medicine — stopping because of side effects was uncommon, at about 6% on 15 mg versus 3% on placebo^[3]. The slow, four-weekly increases exist to keep these effects manageable, and rarer, more serious risks are part of why Mounjaro is prescription-only. Our Mounjaro side effects page has the fuller frequency table and label warnings.

Can I take Mounjaro, and how do you get it in the UK?

Mounjaro is prescription-only, so you cannot simply buy it: a qualified prescriber decides whether it is appropriate after a consultation^[1], and it must then be dispensed by a GPhC-registered UK pharmacy. There are two lawful routes.

• The NHS. NICE recommended tirzepatide for managing overweight and obesity in appraisal TA1026, with final guidance on 23 December 2024^[7]. Because so many people are eligible, NHS England is rolling it out in phases through primary care, starting on 23 June 2025 and prioritising the highest clinical need first — an eligible group of about 220,000 people over the first three years^[8]. NHS access is real but narrow and gradual, not open to everyone straight away^[8].
• Regulated private prescribers and pharmacies. Where the NHS route is not yet open, a consultation with a regulated private prescriber is the other lawful path. We do not recommend, rank or link to any provider, and we never discuss price. What matters is that the pharmacy is registered with the GPhC and that a genuine prescriber assesses you first.

Either way, the decision is clinical, not commercial. If a service offers to sell you Mounjaro without any assessment, treat that as a warning sign, not a convenience. More questions are answered on our Mounjaro FAQ page.

Frequently asked questions