What the phase 3 trials tell us about tirzepatide — and what they don't

SURMOUNT-1: what the core phase 3 dataset found

SURMOUNT-1 is the trial most often cited when tirzepatide's weight-loss results come up, and for good reason. Published in the New England Journal of Medicine, the 72-week randomised trial enrolled 2,539 adults with obesity or with overweight plus at least one weight-related condition, but specifically excluded people with type 2 diabetes. Participants were randomised to tirzepatide at 5 mg, 10 mg or 15 mg weekly, or placebo, all alongside lifestyle counselling.

At the 15 mg dose, average weight reduction from baseline was around 20–21%, with some analyses putting the figure slightly higher among those who stayed on treatment throughout. The 10 mg group averaged roughly 19%, and the 5 mg group around 16%. All three doses substantially outperformed placebo, where average reduction was about 3%. These are mean figures from a clinical trial population; individual results in real life vary, and no outcome is predictable before a clinical assessment takes place.

One detail worth knowing: the titration schedule starts at 2.5 mg for the first four weeks. That starter dose isn't therapeutic, it exists to give the body time to adjust and reduce early side effects, particularly nausea. The prescriber then steps the dose upward based on tolerability and response. The SURMOUNT-1 results reflect the full escalation pathway, not just the maintenance dose.

Beyond SURMOUNT-1: the wider phase 3 landscape

SURMOUNT-1 wasn't the whole story. The SURMOUNT programme was designed to test tirzepatide across overlapping clinical populations, and the phase 3 data now covers several distinct trials.

SURMOUNT-2 enrolled adults with type 2 diabetes alongside obesity, where weight loss is complicated by medications that can blunt appetite reduction. Even in that population, tirzepatide produced clinically meaningful reductions (typically around 14–16% at the highest dose) alongside improvements in blood glucose markers. The Lancet published parts of this dataset.

SURMOUNT-5 addressed a question regulators and clinicians had been waiting for: how does tirzepatide compare to semaglutide 2.4 mg directly? Over 72 weeks in 751 adults with obesity but without diabetes, tirzepatide produced greater average weight loss than semaglutide 2.4 mg. NICE's appraisal of tirzepatide (TA1026) notes that indirect comparisons also favour tirzepatide, though direct comparisons of this kind are rare in obesity medicine. The results from tirzepatide's earlier phase 2 obesity work had pointed in this direction, but SURMOUNT-5 confirmed it in a head-to-head setting.

There is also a separate SURMOUNT trial examining tirzepatide in non-alcoholic steatohepatitis, liver disease linked to metabolic factors. That evidence set is covered in detail on our page about tirzepatide's phase 3 NASH trial.

What NICE concluded, and what that means in practice

When NICE published its recommendation for tirzepatide in December 2024, updated in September 2025, it made tirzepatide the first dual-agonist medicine recommended for weight management in England. The criteria are specific: adults with a BMI of 35 or above plus at least one weight-related comorbidity, with lower BMI thresholds applying for some ethnic backgrounds under UK guidance. If less than 5% of body weight is lost after six months at the highest tolerated dose, continuing is reviewed.

NHS access is being phased in gradually by eligibility cohort. For people who don't meet those criteria yet, or who'd rather not wait, a private prescription route exists through a regulated online pharmacy, it's worth checking any pharmacy you consider against the GPhC register, which takes under a minute. The full Mounjaro overview explains what private treatment involves and what NICE's recommendation means for eligibility today.

From a clinical standpoint, the phase 3 data established that tirzepatide's dual mechanism (activating both GIP and GLP-1 receptors) translates into meaningfully greater average weight loss than GLP-1 single-agonists. That's the finding regulators and NICE acted on. What the trials couldn't settle is which individual will respond best; that's still a clinical judgement, made prescription by prescription. More background on how the treatment works in practice is on our tirzepatide information page.

Side effects in the phase 3 trials, what the data actually reported

The SURMOUNT trials documented side effects systematically, and the pattern is consistent across the programme. Gastrointestinal effects were the most common: nausea, diarrhoea, constipation and vomiting occurred in a meaningful proportion of participants, particularly after starting treatment or stepping up the dose. Most were rated mild to moderate in severity and tended to ease within a few days to two weeks.

Discontinuation due to side effects was higher in the tirzepatide arms than placebo, but completion rates were still high, the majority of participants on active treatment finished the trials. Injection-site reactions, fatigue and headache were also recorded. Questions about tirzepatide's pH and formulation sometimes come up in this context, since the medicine's chemical properties are part of why injection-site reactions can occasionally occur. The NHS patient-level information on tirzepatide from the NHS summarises the side-effect profile in accessible language and is worth reading before starting treatment.

One point the trials reinforced: starting at the lower dose and titrating slowly is not optional preamble, it's the mechanism by which most people tolerate the medicine at higher doses. Skipping that schedule isn't supported by the trial design that generated these results. The detail of how doses are managed, what to do if a dose is missed, and when to contact a prescriber is covered on our page about tirzepatide's licensed indications and prescribing detail, and in the Patient Information Leaflet that comes with every pen; a prescriber at our treatment page can also advise at consultation.