The active ingredients in weight loss injections licensed in the UK

Step 1: understand which ingredient is inside your pen

Before anything else, it helps to know what is actually in the injection. The two licensed active ingredients differ structurally and pharmacologically. Tirzepatide, the active ingredient in Mounjaro, is a synthetic peptide that binds to both the GIP (glucose-dependent insulinotropic polypeptide) receptor and the GLP-1 (glucagon-like peptide-1) receptor. Semaglutide, the active ingredient in Wegovy, binds to the GLP-1 receptor alone. That single-versus-dual distinction shapes the clinical evidence and the way each medicine is prescribed.

Both ingredients mimic hormones that the gut releases naturally after eating. They slow gastric emptying, signal to the brain that you are full, and reduce appetite. The effect builds over weeks rather than days, which is partly why the dose is increased gradually. A good overview of how tirzepatide works at a patient level is published on the NHS tirzepatide medicines page, and the corresponding semaglutide information appears on the NHS semaglutide medicines page.

One practical point worth knowing: because the injection slows the stomach, oral medicines (including the contraceptive pill) may be absorbed differently, particularly with tirzepatide. Your prescriber should factor this in during assessment. If you want to dig deeper into what each active ingredient actually contains and how it works, our guide to weight loss injections ingredients covers both in detail. You can read more about how the two licensed injections compare in terms of results on the effectiveness of weight loss injections page.

Step 2: follow how the dose of each ingredient is titrated

Neither tirzepatide nor semaglutide is started at its full therapeutic dose. The titration schedule exists to give your body time to adjust, and reducing early nausea is the main reason the first pen's dose is set conservatively.

For tirzepatide, treatment starts at 2.5 mg once weekly. The prescriber reviews progress and tolerability before increasing, typically in 4-week steps, through 5 mg, 7.5 mg, 10 mg, 12.5 mg and up to 15 mg. For semaglutide (Wegovy), the starting point is 0.25 mg weekly, escalating through 0.5 mg, 1.0 mg and 1.7 mg to a 2.4 mg maintenance dose, again at roughly 4-weekly intervals. A higher 7.2 mg maintenance dose was approved by the MHRA in January 2026 and a dedicated single-dose 7.2 mg pen received approval in April 2026.

The prescriber (not the patient) decides when a dose increase is appropriate, based on weight loss progress, tolerability and any relevant health changes. At nume, a named GPhC-registered prescriber reads every consultation; dose increases go through clinical re-review before a new pen is dispatched. If you are curious about what different strengths mean for your treatment, the weight loss injections overview covers the practicalities clearly.

Step 3: weigh the clinical evidence for each ingredient

The trial evidence for both active ingredients is substantial. In the SURMOUNT-1 trial, participants using tirzepatide at 15 mg achieved an average body-weight reduction of around 20 to 21 percent over 72 weeks. The STEP 1 trial, published in the New England Journal of Medicine, reported an average reduction of around 15 percent over 68 weeks with semaglutide 2.4 mg. More recently, the SURMOUNT-5 head-to-head study (published in the New England Journal of Medicine in 2025) compared the two active ingredients directly in adults with obesity and found tirzepatide produced greater average weight loss than semaglutide 2.4 mg over 72 weeks. Some patients ask whether a triple action weight loss injection could offer even broader benefits, and our dedicated page explains what that approach involves.

NICE recommends tirzepatide under technology appraisal TA1026 for adults with a BMI of 35 or above and at least one weight-related comorbidity, with lower BMI thresholds applying for some ethnic backgrounds. Results in any individual trial participant varied considerably, and no prescriber can guarantee what a given person will experience. The right question at consultation is not which number sounds biggest, but which active ingredient suits your health profile. Our guide to the most effective weight loss injection explores this in more detail.

A useful frame from our prescribers: trial percentages are group averages. Your own response to either active ingredient depends on starting weight, dose reached, lifestyle changes made alongside treatment and how long you stay on it.

Step 4: know the shared safety profile and when to seek help

The side-effect profile of tirzepatide and semaglutide overlaps considerably because both act on gut-hormone receptors. Nausea is the most commonly reported effect, especially in the first week or two after starting or after a dose increase; constipation, diarrhoea, indigestion and fatigue are also frequently noted. These tend to settle as the body adjusts.

There are more serious effects that require prompt medical attention. The MHRA's January 2026 Drug Safety Update highlighted acute pancreatitis as a known, infrequent but potentially serious risk: if you develop severe stomach pain that spreads to your back, with or without vomiting, get medical help the same day rather than waiting. Both ingredients carry a Black Triangle (▼) designation, meaning they are under additional monitoring by the MHRA and patients are encouraged to report suspected side effects via the Yellow Card scheme.

Pregnancy, breastfeeding and trying to conceive are situations where neither active ingredient is recommended. The same applies to anyone under 18. Certain conditions (including a history of medullary thyroid carcinoma or pancreatitis) require particular scrutiny at the prescribing stage. Storing your pen correctly matters too: both active ingredients require refrigeration between 2 and 8 degrees Celsius; always check the Patient Information Leaflet for the exact room-temperature window if you need to carry it with you. A sharps container is needed for safe disposal of used pens. For anything touching drug interactions (for example, taking weight loss injections alongside methotrexate) speak with your prescriber before starting.

If you would like a prescriber to assess which active ingredient may suit your circumstances, you can start your free consultation with nume today.