What Is Tirzepatide — and How Does It Work for Weight Loss?

Why two receptors matter: the mechanism NICE cited in its recommendation

Most GLP-1 medicines target a single gut hormone receptor. Tirzepatide targets two: GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide). Both receptors are involved in regulating appetite, gastric emptying and blood-glucose control, but they work through partly distinct pathways. Activating both at once appears to produce a stronger combined signal than activating either alone, and that difference shows up clearly in the trial numbers.

In SURMOUNT-1, a 72-week randomised trial involving 2,539 adults with obesity and no diabetes, participants taking the 15 mg dose of tirzepatide lost an average of around 20–21% of their body weight. That figure, published in the New England Journal of Medicine, was substantially larger than the reductions seen with the GLP-1-only medicines that preceded it. SURMOUNT-5, a later head-to-head trial published in the NEJM in 2025, compared tirzepatide directly with semaglutide 2.4 mg and found tirzepatide produced greater average weight loss over 72 weeks. Regulators and NICE considered this body of evidence when reaching their assessments.

The mechanism also slows how quickly the stomach empties, which extends the sensation of fullness after eating. This is why most people find they feel satisfied on smaller portions, appetite genuinely falls rather than requiring constant willpower to override.

What the licence actually covers, and who meets the criteria

In the UK, tirzepatide holds two licensed indications: weight management in adults, and blood-glucose control in type 2 diabetes. For weight management, the licence covers adults with a BMI of 30 or above, or a BMI of 27 or above alongside at least one weight-related health condition such as high blood pressure, high cholesterol, obstructive sleep apnoea, prediabetes or cardiovascular disease. The medicine is not licensed for anyone under 18, and it is not recommended during pregnancy, while breastfeeding, or when actively trying to conceive.

NICE's recommendation in TA1026 set a higher bar for NHS access, a BMI of 35 or above with at least one qualifying comorbidity, with thresholds 2.5 kg/m² lower for people of South Asian, Chinese, Middle Eastern, Black African or African-Caribbean backgrounds. Private prescribing follows the licensed indication rather than the narrower NICE threshold, which is one reason people who do not yet qualify for NHS access explore regulated private treatment options.

A BMI alone is never the only thing a prescriber looks at. Other medicines, existing conditions, and the whole clinical picture all factor in. The prescriber's judgement, not a number on a calculator, is the deciding step.

Side effects: what the evidence shows and when to seek help

The side-effect profile is led by the gut. Nausea is the most commonly reported effect, followed by loose stools, constipation, indigestion, burping and fatigue. These tend to be most noticeable in the days after starting treatment or after each dose increase, and for many people they settle considerably within one to two weeks as the body adjusts. Starting at 2.5 mg is specifically designed to reduce this early discomfort before the dose is raised.

A smaller number of people experience more significant effects. The MHRA issued a drug safety update in January 2026 specifically about GLP-1 medicines and acute pancreatitis: if you develop severe stomach pain that spreads to your back, with or without vomiting, stop the medicine and get urgent medical help. Gallbladder problems have also been reported. Injection-site reactions (redness, mild bruising) are generally minor and rotate with the injection site (abdomen, thigh or upper arm).

Patients and carers can report any suspected side effect through the MHRA's Yellow Card scheme, and doing so actively contributes to the ongoing safety monitoring that the Black Triangle status requires. If you have questions about how a side effect is affecting you, our support team is available seven days a week.

Tirzepatide in practice: the pen, the schedule, and a note on timing

Mounjaro is supplied as a pre-filled KwikPen, each pen contains four weekly doses, so one pen lasts a month. The injection is taken on the same day each week, into the skin of the abdomen, thigh or upper arm, and the site is rotated each time. Pens are stored in the fridge between 2 and 8°C; the exact room-temperature window for short-term storage is set out in the Patient Information Leaflet rather than here, because it is a prescriber and leaflet matter rather than general guidance.

People often ask about what happens if their usual injection day falls on a bank holiday or over a long break away. The short answer is: check with your prescriber, because the leaflet allows some flexibility in timing, but the specific window and what to do with a missed dose are decisions for your clinical team, not a general guide. It is worth thinking ahead, if payday or a holiday trip means your repeat order might arrive late, ordering a few days early avoids the gap. Our pharmacy dispatches orders the same day for consultations approved before 12pm on working days, so the planning is fairly straightforward.

For a fuller picture of Mounjaro as a product (including how it compares with other options currently available) the Mounjaro overview page covers the detail in one place. Questions about whether tirzepatide and Mounjaro are technically the same thing are answered on this dedicated page, which is one of the questions our prescribers hear regularly. If you want to understand what tirzepatide actually is at the formulation level, including how it works as a molecule, that page covers the science in accessible detail, and if you are looking for a broader introduction, this guide explains what tirzepatides are and how the drug class fits into modern weight management. If you are curious about which products and formulations actually contain tirzepatide, that page sets out exactly what is available and in what form.

If you think tirzepatide might be right for you, the place to start is a free clinical consultation. A GPhC-registered prescriber, not a screening algorithm, reads every submission and gives you a clear answer the same day. Check your eligibility here.