Mounjaro vs Wegovy Ingredients: What Each Medicine Actually Contains

The active ingredients: what each medicine is made of

Tirzepatide, the active ingredient in Mounjaro, is a synthetic peptide that binds to both the GIP receptor and the GLP-1 receptor simultaneously. That dual action sets it apart from every other weight-loss medicine currently licensed in the UK. It was designed to mimic two gut hormones at once (glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1)) slowing gastric emptying and reducing appetite through two complementary pathways. Comparing Wegovy and Mounjaro as a whole reveals how much of that distinction traces back to this single molecular difference.

Semaglutide, the active ingredient in Wegovy, is also a synthetic peptide, but it targets the GLP-1 receptor alone. It was derived from human GLP-1 with modifications that extend its half-life so a once-weekly injection delivers a sustained effect. Structurally it shares roughly 94% sequence homology with the body's own GLP-1. The Wegovy formulation includes semaglutide at a 2.4mg weekly maintenance dose, a higher concentration than the same compound used in diabetes treatment. The dosing schedules for Wegovy and Mounjaro reflect these different molecules and their titration requirements.

Both medicines are delivered subcutaneously via pre-filled pen devices and carry a Black Triangle (▼) designation from the MHRA, meaning they are subject to additional monitoring. The excipients (the inactive carrier components) differ between the two pens, but the clinical conversation centres on those two active compounds.

What the trial evidence shows about each ingredient's effect

The clearest evidence on tirzepatide's weight-loss effect comes from SURMOUNT-1, published in the New England Journal of Medicine, which enrolled 2,539 adults with obesity and no diabetes over 72 weeks. At the 15mg dose, average body-weight reduction was around 20 to 21%. For semaglutide 2.4mg, the STEP 1 trial (also published in the NEJM) reported around 15% average weight loss over 68 weeks in a similar population.

A question our prescribers hear most weeks is whether the ingredient difference translates into a meaningful real-world outcome. SURMOUNT-5, a head-to-head open-label trial published in 2025, compared tirzepatide directly with semaglutide 2.4mg in 751 adults with obesity over 72 weeks. Tirzepatide produced greater average weight reduction. NICE's technology appraisal of tirzepatide (TA1026) notes that indirect comparisons also favour tirzepatide, though it acknowledges the limitations of cross-trial analysis. The fuller picture of which ingredient suits a given person is explored on the clinical comparison page.

It is worth being precise about what these figures represent: averages across trial populations, under controlled conditions, alongside lifestyle support. Individual results vary. Neither medicine is assessed by the MHRA for cosmetic weight loss, and a prescriber will look at your full clinical picture rather than a league table of percentages.

Comparing the two ingredients side by side

The oral contraceptive note is a practical example of how two ingredients with a similar mechanism can still differ in their clinical management. Tirzepatide's dual-receptor activity slows gastric emptying more markedly in some patients, which can reduce absorption of an oral pill. NHS guidance advises adding a barrier method for the first four weeks of tirzepatide treatment and for four weeks after any dose increase. There is no equivalent recommendation for semaglutide. The broader question of which suits your situation involves factors beyond the ingredients alone.

What the ingredient difference means for eligibility and prescribing

Both active ingredients carry the same broad licensed eligibility threshold in the UK: adults with a BMI of 30 or above, or 27 or above with at least one weight-related condition such as high blood pressure, type 2 diabetes, or obstructive sleep apnoea. Lower thresholds apply for some ethnic backgrounds under UK guidance. But NICE's criteria for NHS prescribing differ between the two molecules, TA1026 for tirzepatide and TA875 for semaglutide set different conditions around specialist services and treatment duration. Reading the NICE appraisal for tirzepatide alongside TA875 makes the contrasts concrete.

Private prescribing follows the licensed indications rather than the NHS commissioning criteria, so the eligibility conversation is somewhat different through a service like nume. BMI alone is never the whole story, a prescriber weighs your medical history, current medicines, and the specific properties of each ingredient before recommending one over the other. The cost context for each treatment is a separate consideration worth understanding, and a fuller picture of what to ask is on the weight-loss treatment overview.

Ozempic, which also contains semaglutide, is licensed in the UK for type 2 diabetes, not weight management, a distinction that matters when patients ask about it alongside Mounjaro. That separation is explained in more detail on the Mounjaro vs Ozempic page. Which ingredient is right for you is a clinical decision our prescribers make with you, based on your full picture rather than the trial averages alone. Speak to our prescribers through a free consultation to explore which option fits.