The main ingredient in Mounjaro, and why it matters

Step 1: understanding the two receptors tirzepatide activates

Most GLP-1 medicines target a single gut-hormone receptor. Tirzepatide does something different — it is engineered to bind to both the GLP-1 receptor (glucagon-like peptide 1) and the GIP receptor (glucose-dependent insulinotropic polypeptide). Both receptors are found in the brain, the gut and the pancreas, and both play a role in telling you when you have had enough to eat.

GLP-1 receptors slow the rate at which food leaves the stomach and signal fullness to the hypothalamus. GIP receptors work through a slightly different pathway, modulating fat storage and also contributing to satiety signals. Activating the two together appears to produce a stronger combined effect than activating either alone, which is part of why tirzepatide's clinical results caught researchers' attention. The full tirzepatide profile on our site goes into the pharmacology in more detail if you want to explore further.

The practical upshot is that people on tirzepatide often find their appetite drops more noticeably than they expected, and that feeling full arrives sooner during meals. This is a pharmacological effect, not willpower, and it is the mechanism that underpins the medicine's licensed use for weight management.

Step 2: how tirzepatide became a licensed UK medicine

Tirzepatide was developed by Eli Lilly. In the UK it is licensed for weight management in adults with a body mass index of 30 or above, or from 27 upwards where at least one weight-related condition (such as high blood pressure, type 2 diabetes or obstructive sleep apnoea) is present. It is also separately licensed for type 2 diabetes, though the formulation and pen used for weight management is specifically the Mounjaro brand.

The weight-management licence followed the SURMOUNT clinical programme, which involved thousands of adults across multiple international trials. SURMOUNT-1 alone randomised 2,539 participants and ran for 72 weeks; at the 15 mg dose, average body-weight reductions reached around 20–21%, as published in the New England Journal of Medicine. NICE subsequently recommended tirzepatide for NHS use in December 2024, under specific eligibility criteria set out in technology appraisal TA1026.

If you want to read the full patient-level information (including every listed ingredient, not just the active one) the Mounjaro patient information page is a good place to start, and the official Patient Information Leaflet inside each pack covers it comprehensively.

Step 3: what the excipients (inactive ingredients) do

Tirzepatide is the active ingredient, the part of the medicine doing the pharmacological work. The KwikPen solution also contains excipients: inactive substances that keep the medicine stable, control its pH and make it suitable for injection. These include sodium phosphate, sodium chloride, polysorbate 80 and water for injections, among others.

For most people these excipients are irrelevant, but they matter if you have a known allergy to any component. One quick habit worth building: before your first pen, read the ingredients section of the Patient Information Leaflet, which takes under a minute and lists every component in plain language. If anything gives you pause, mention it to your prescriber before you inject. Our Mounjaro ingredients page covers the full inactive-ingredient list in more detail, and the complete ingredient breakdown answers common allergen questions too.

The cost of Mounjaro as a private prescription is also worth understanding before you start, since treatment continues across several dose steps over many months.

Step 4: what tirzepatide does not do, and who assesses suitability

Tirzepatide is not a stimulant, a fat-blocker or a diuretic. It does not burn calories directly. Its effect is rooted in appetite regulation and slowed digestion, which makes it more sustainable for many people than extreme dietary restriction, but it still works best alongside a reduced-calorie diet and regular physical activity, as stated in its UK licence.

It is not suitable for everyone. People who are pregnant, breastfeeding or trying to conceive should not take it. Those under 18 are outside the licensed population. Certain conditions (including a personal or family history of medullary thyroid carcinoma or MEN2 syndrome) require particular care, and a prescriber will screen for these before approving a prescription. The Mounjaro active ingredient overview touches on these clinical considerations in more depth.

A prescriber assesses the whole picture, not just the BMI figure. At nume, a GPhC-registered Independent Prescriber reads every consultation personally, the same day it is submitted. If you have been considering treatment and want a clinical view, you can speak to our prescribers through a free consultation, there is no obligation and no algorithm making the call on your behalf.