What do long-term studies on Mounjaro actually show?

How much weight did people lose in the longest tirzepatide trials?

SURMOUNT-1 remains the anchor reference point for anyone asking about long-term studies on Mounjaro. Over 72 weeks, adults with obesity or overweight and at least one weight-related condition (but without type 2 diabetes) lost an average of roughly 16% at the 10mg dose and around 20–21% at 15mg. At the highest dose, some analyses put the figure closer to 22.5%, though this depends on how missing data are handled statistically. These aren't small pilot studies: the trial randomised 2,539 participants across multiple sites.

What makes the 72-week timeframe meaningful is that weight loss continued well beyond the early months. Participants were still losing weight at week 36, 48 and beyond, rather than plateauing sharply after the first few months. That trajectory matters for understanding what sustained treatment might look like.

SURMOUNT-5, which ran for the same duration, compared tirzepatide directly with semaglutide 2.4mg in 751 adults. Tirzepatide produced meaningfully greater average weight loss. A fuller breakdown of the SURMOUNT programme sets out each trial arm and what the participant populations looked like.

Does the weight stay off if treatment continues?

This is the question most people are really asking, and the honest answer is: continuing treatment appears to matter a great deal. A sub-study within the SURMOUNT programme looked at what happened when participants switched from active tirzepatide to placebo after the initial treatment period. Weight regain was substantial, confirming a pattern also seen with other GLP-1 medicines.

The data suggest tirzepatide works for as long as it is taken, rather than delivering a one-time correction. That has real implications for how people think about treatment duration, and what the evidence shows about using Mounjaro long term is worth reviewing before making any decisions about stopping or continuing. Whether staying on Mounjaro long-term is appropriate is a clinical decision shaped by individual health profile, tolerability and ongoing benefit — not something to decide alone.

It is also worth knowing that NICE's recommendation (TA1026, updated September 2025) includes a review point: if less than 5% weight loss is achieved after six months at the highest tolerated dose, continuing treatment should be reconsidered. That built-in review is part of responsible prescribing, not a limitation to worry about unnecessarily.

What do long-term studies show about safety over time?

Side effects in the SURMOUNT trials were predominantly gastrointestinal: nausea, diarrhoea, constipation, vomiting and reflux were the most commonly reported. They were most noticeable after starting treatment or after a dose increase, and in the majority of participants they eased over time rather than persisting at the same intensity.

Serious adverse events were low in frequency and broadly consistent with the profile seen in shorter studies. Pancreatitis is a recognised risk with GLP-1 and dual-agonist medicines generally, the MHRA issued a Drug Safety Update in January 2026 specifically highlighting acute pancreatitis as an infrequent but potentially serious concern. Severe or persistent stomach pain that radiates towards the back is a reason to seek medical help promptly, not to wait and see, and anyone wanting to understand the cardiovascular safety picture can find more detail on how tirzepatide relates to major adverse cardiovascular events. The detailed look at Mounjaro's long-term effects covers the full safety picture from the trials.

Mounjaro carries a Black Triangle (▼) designation from the MHRA, meaning it is subject to additional monitoring. Patients and clinicians are encouraged to report any suspected side effects via the MHRA's Yellow Card scheme. Keeping your pen in the fridge door and checking the date each week makes it easier to stay consistent with your prescribed schedule, small habits that support both safety and effectiveness over a longer treatment course.

What are researchers still studying, and what does this mean for treatment decisions today?

The 72-week SURMOUNT-1 timeframe is impressive by pharmaceutical trial standards, but it isn't a lifetime. Longer-term cardiovascular outcome data, the effects of treatment beyond two years, and questions about optimal duration in different patient groups are all areas of active research. The tirzepatide clinical programme is broad (spanning the SURPASS diabetes trials alongside SURMOUNT) and collectively involves well over 10,000 participants, which gives regulators a substantial evidence base to work with.

NICE reviewed all of this when producing its appraisal TA1026 and concluded the benefits justify recommendation for eligible adults. That doesn't mean the evidence is complete; it means the current picture is strong enough to support clinical use with appropriate monitoring.

For anyone weighing up treatment, the evidence base behind tirzepatide is more extensive than for most medicines at a similar stage. Understanding what tirzepatide is and how it works gives useful context before a consultation. If you'd like to explore whether treatment could be appropriate for your own situation, checking your eligibility with our prescribers is the right starting point.