Mounjaro is one of the best-known weight loss pens in the UK, and behind the brand name sit a specific device and a specific medicine. This page profiles both in plain English: the KwikPen and single-dose formats, the strengths, the weekly routine, storage, the trial evidence and the UK position — every number drawn from the UK label, the MHRA and the published trials.
The Mounjaro pen delivers tirzepatide, a once-weekly dual GIP and GLP-1 receptor agonist made by Eli Lilly. It comes as a multi-dose KwikPen (four weekly doses per pen) and as single-dose pens, in six strengths — 2.5, 5, 7.5, 10, 12.5 and 15mg — with 15mg once weekly the maximum. It is licensed for weight management in the UK (MHRA, 8 November 2023) but remains prescription-only.
What the Mounjaro pen is
Mounjaro is the brand name for tirzepatide, made by Eli Lilly. Unlike most weight loss injections, which act on a single gut-hormone pathway, tirzepatide is a dual GIP and GLP-1 receptor agonist — activating both receptors at once.[1] It works like the body's own incretin hormones: prompting insulin release when blood sugar rises, reducing appetite and slowing how quickly the stomach empties, so people feel full sooner and for longer.[1]
It is given as a subcutaneous injection — under the skin, not into a vein or muscle — once a week, usually into the abdomen, thigh or upper arm.[1] Tirzepatide is a different molecule from semaglutide, the medicine in the Wegovy pen: both are weekly injections, but semaglutide is a GLP-1 agonist only, so calling Mounjaro "a GLP-1" without the dual qualifier is not accurate.
The KwikPen format and single-dose pens
Mounjaro comes in two device formats, both self-injection pens that sit against the skin with the needle out of sight during use.[1]
- The KwikPen is multi-dose: each holds four weekly doses of 0.6mL, so one pen covers about a month.[1]
- Single-dose pre-filled pens each deliver one weekly dose and are then discarded.[1]
Which format someone receives depends on what their pharmacy supplies; the tirzepatide inside is the same.
Needles, technique and injection sites
Because Mounjaro is injected under the skin, technique matters, but it is straightforward once shown. A prescriber or pharmacist demonstrates how to prepare the pen, rotate the injection site (abdomen, thigh or upper arm) and inject; the leaflet in the box is the definitive step-by-step guide. For a general walk-through, see our using your pen guide, and always follow the instructions supplied with your own device.
Strengths: the 2.5mg to 15mg range
The Mounjaro pen is made in six strengths — the lowest two ease the body into treatment, the higher ones are where most people settle.[1]
| Strength (once weekly) | Role in treatment |
|---|---|
| 2.5mg | Starting dose — first 4 weeks; a step, not maintenance |
| 5mg | Lowest maintenance dose |
| 7.5mg | Titration step |
| 10mg | Maintenance dose |
| 12.5mg | Titration step |
| 15mg | Highest maintenance dose; the maximum |
The three recommended maintenance doses are 5mg, 10mg and 15mg, and 15mg once weekly is the maximum.[1] Not everyone reaches the top; the right dose depends on response and tolerability — the prescriber's judgement.
The once-weekly routine
Mounjaro is injected once a week, and the dose is raised gradually rather than in one jump. Treatment starts at 2.5mg once weekly for four weeks, then moves to 5mg once weekly; after that, the dose can rise in 2.5mg steps at intervals of at least four weeks, up to the 15mg maximum.[1]
This slow climb is not arbitrary: it reduces the gastrointestinal side effects that are most common when a dose first goes up.[1] "At least four weeks" is a minimum — a prescriber can hold a dose for longer, or not raise it at all, if side effects are troublesome. Patients do not adjust the dose themselves.
Storing your Mounjaro pen
Mounjaro is kept refrigerated at 2 to 8 °C. According to the product information, once in use it may be kept unrefrigerated but not above 30 °C for up to 30 days, then discarded; it must not be frozen, or used if it has been frozen.[1] Treat the leaflet inside your own pack as the authority on storage and disposal.
What is inside the pen: the medicine and its headline evidence
The clearest measure of what tirzepatide does is SURMOUNT-1 — the headline trial behind its weight-management use. It ran for 72 weeks in 2,539 adults with obesity but without type 2 diabetes, and the MHRA-cited mean weight reductions rose with the dose.[2]
| Dose (once weekly) | MHRA-cited | NEJM treatment-regimen |
|---|---|---|
| 5mg | −16.0% | ≈ −15.0% |
| 10mg | −21.4% | ≈ −19.5% |
| 15mg | −22.5% | ≈ −20.9% |
| Placebo | −2.4% | — |
Between 89.4% and 96.3% of people on tirzepatide lost at least 5% of their body weight, against 27.9% on placebo.[2] The two columns are not a contradiction: the MHRA quotes an efficacy (on-treatment) estimand, while the NEJM treatment-regimen figures — about 15.0%, 19.5% and 20.9% — reflect an intention-to-treat analysis.[3] Both are valid; honest coverage shows both, not only the bigger number.
Weight loss with this class is generally lower in people who also have type 2 diabetes: in the companion SURMOUNT-2 trial (938 adults with type 2 diabetes), mean reductions were −13.4% at 10mg and −15.7% at 15mg versus −3.3% on placebo.[2]
Side effects and safety
The most common side effects are gastrointestinal. Very common (at least 1 in 10 people) are nausea, diarrhoea, vomiting, constipation and abdominal pain.[1] Common effects (up to 1 in 10) include indigestion, bloating, burping, wind, reflux, gallstones, reduced appetite, dizziness, tiredness, injection-site reactions and a faster heart rate; low blood sugar can occur when tirzepatide is combined with insulin or a sulfonylurea.[1] These are usually mild to moderate and most common while the dose is going up.
The label carries specific warnings: stop the medicine and seek advice if acute pancreatitis (severe, persistent stomach pain) is suspected; watch for dehydration from vomiting or diarrhoea; and note the oral contraceptive warning — the manufacturer advises women on the pill to add a barrier method (such as condoms) around starting Mounjaro and each dose increase, because reduced pill effectiveness cannot be excluded.[1] Mounjaro should not be used in pregnancy, and the only UK-label contraindication is a known allergy to tirzepatide or the pen's other ingredients.[1]
One caution: the US label for tirzepatide carries a boxed thyroid-tumour warning and contraindicates a personal or family history of medullary thyroid carcinoma and MEN 2.[8] The UK/EU label does not — it treats thyroid risk as a precaution — so the US boxed warning is not the UK position.[1] If you experience any side effect, report it through the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk and speak to your GP or pharmacist.[4] Our side effects page covers the full picture.
The UK position: licensed, but prescription-only
Mounjaro's weight-management use was authorised by the MHRA on 8 November 2023.[5] The licensed eligibility is a body mass index (BMI) of 30 kg/m² or above, or 27 to 30 kg/m² with a weight-related health problem such as prediabetes, high blood pressure, abnormal blood fats or cardiovascular disease.[5] Being licensed is not the same as being freely available: it remains prescription-only.
For NHS use in England, NICE published technology appraisal TA1026 on 23 December 2024, recommending tirzepatide for adults with a BMI of 35 or above and at least one weight-related condition, through a phased three-year rollout under NHS England's interim commissioning guidance.[6][7] Thresholds are lowered — usually by 2.5 kg/m² — for people from South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family backgrounds.[6] NHS access is therefore narrower than the licence alone suggests; many people obtain Mounjaro privately, again only on a prescriber's decision.[6]
Mounjaro should only ever be obtained on prescription through a GPhC-registered pharmacy. Buying weight loss injections from an unregulated seller — for example a social-media account or a site with no prescriber — is a real risk, because what arrives may not be a genuine, regulated medicine. The MHRA's FakeMeds campaign explains how to check. We do not sell, supply or link to any provider.
Frequently asked questions
What strengths does the Mounjaro pen come in?
Six: 2.5mg, 5mg, 7.5mg, 10mg, 12.5mg and 15mg, all once weekly. It starts at 2.5mg for four weeks, then 5mg, stepping up by 2.5mg no sooner than every four weeks. The maintenance doses are 5mg, 10mg and 15mg; 15mg once weekly is the maximum.[1]
How often do you inject the Mounjaro pen?
Once a week, as a subcutaneous injection. The multi-dose KwikPen holds four weekly doses of 0.6mL, so one pen lasts about a month; single-dose pens deliver one weekly dose each.[1]
How much weight did people lose with Mounjaro in the SURMOUNT-1 trial?
How do you store a Mounjaro pen?
Refrigerated at 2 to 8 °C. According to the product information, once in use it can be kept unrefrigerated but not above 30 °C for up to 30 days, then discarded. Do not freeze it, or use it if it has been frozen. The leaflet in your pack is the definitive guide.[1]
Can I get the Mounjaro pen in the UK?
It is licensed in the UK for weight management, but prescription-only. On the NHS, NICE recommends it within a phased rollout for adults meeting defined BMI and health criteria; otherwise it is accessed privately. Either way a prescriber must agree it is suitable, and legitimate supply comes only through a GPhC-registered pharmacy. This website does not sell or recommend it.[5][6]
References
- Electronic Medicines Compendium (emc). "Mounjaro KwikPen / pre-filled pen — Summary of Product Characteristics" (product 15481): device format, strengths, dosing and titration, storage, side effects, warnings and contraindications. medicines.org.uk
- MHRA / GOV.UK. "MHRA authorises diabetes drug Mounjaro (tirzepatide) for weight management and weight loss": MHRA-cited SURMOUNT-1 and SURMOUNT-2 results. gov.uk
- Jastreboff AM et al. "Tirzepatide Once Weekly for the Treatment of Obesity" (SURMOUNT-1). New England Journal of Medicine, 2022 — treatment-regimen (intention-to-treat) estimand figures. nejm.org
- MHRA. "Yellow Card scheme — report a side effect." yellowcard.mhra.gov.uk
- MHRA / GOV.UK. "MHRA authorises diabetes drug Mounjaro (tirzepatide) for weight management and weight loss" — UK authorisation date (8 November 2023) and licensed BMI/comorbidity eligibility. gov.uk
- NICE. "Tirzepatide for managing overweight and obesity" (TA1026), published 23 December 2024 — NHS recommendation, BMI ≥35 plus comorbidity, phased rollout and ethnicity-adjusted thresholds. nice.org.uk
- NHS England. "Interim commissioning guidance: NICE TA1026 (tirzepatide)" — phased NHS rollout. england.nhs.uk
- US Food and Drug Administration. "Mounjaro (tirzepatide) US Prescribing Information" (NDA 215866, 2022) — boxed thyroid C-cell tumour warning and US contraindications (medullary thyroid carcinoma, MEN 2), cited for the US-versus-UK contrast only. accessdata.fda.gov