Tirzepatide: what SURMOUNT and SURPASS found

Tirzepatide is the most effective licensed obesity drug to date: in SURMOUNT-1, the top 15 mg dose cut body weight by 20.9% over 72 weeks against 3.1% on placebo [1], and in a direct head-to-head trial it beat semaglutide by about six percentage points [3]. For people who stay on it, the effect holds — 89.5% of those who continued treatment in SURMOUNT-4 had kept at least 80% of their initial loss at 88 weeks [4]. It is injected once a week, carries the familiar GLP-1 side-effect profile, and is licensed for obesity, sleep apnoea and type 2 diabetes in the US and England.

What it is

Tirzepatide is a single 39-amino-acid peptide that activates two hormone receptors: GLP-1, the target of semaglutide, and GIP (glucose-dependent insulinotropic polypeptide), a second gut hormone released after eating [8]. Eli Lilly's design includes a 20-carbon fatty acid that binds albumin in the blood [8]. The result has a half-life of roughly five days, so it's injected once a week [8].

Why hitting GIP as well as GLP-1 adds weight loss is still being worked out. The FDA label puts the mechanism cautiously: GLP-1 regulates appetite, and animal studies suggest the GIP component "may further contribute" to food intake regulation [8]. What the trials do show is the result that matters — people eat less and lose more weight than with GLP-1 drugs alone — while the division of labour between the two receptors in humans is the part still being mapped.

It is sold as Mounjaro (type 2 diabetes, and weight management in the UK) and Zepbound (weight management and sleep apnoea in the US).

The human evidence

Lilly ran two large programmes: SURPASS in type 2 diabetes and SURMOUNT in obesity. Both are double-blind or open-label randomised trials with thousands of participants, funded by the manufacturer.

SURMOUNT-1: the main obesity trial

Mean starting weight was 104.8 kg, so the 15 mg group lost around 22 kg on average [1]. Those numbers use a treatment-regimen analysis, which counts people who stopped the drug. Discontinuation due to side effects ranged from 4.3% to 7.1% across doses, against 2.6% on placebo [1].

SURPASS-2 and SURMOUNT-5: against semaglutide

Two trials put tirzepatide directly against semaglutide. In SURPASS-2 (n = 1,879, type 2 diabetes, 40 weeks, open-label), all three tirzepatide doses lowered HbA1c more than semaglutide 1 mg: 2.30 percentage points on 15 mg against 1.86 on semaglutide [2]. Weight fell by an extra 1.9 to 5.5 kg with tirzepatide depending on dose [2]. Serious adverse events were a little more common on tirzepatide (5–7%) than semaglutide (3%) [2]. Semaglutide 1 mg is the diabetes dose, below the 2.4 mg used for obesity, so SURPASS-2 reads mainly as a glycaemic comparison; SURMOUNT-5 is the weight-loss test at each drug's maximum tolerated dose.

SURMOUNT-5 closed that gap. It randomised 751 adults with obesity, no diabetes, to the highest tolerated dose of each drug for 72 weeks [3].

SURMOUNT-5 was open-label, so participants knew which drug they were on, and Lilly paid for it [3]. Even so, a 6.5-point gap in a randomised comparison is hard to explain away.

Stopping the drug

SURMOUNT-4 gave 783 people 36 weeks of open-label tirzepatide, during which they lost 20.9% on average, then randomised 670 of them to keep going or switch to placebo for a further 52 weeks [4]. Those who continued lost another 5.5%; those switched to placebo regained 14.0% [4]. At 88 weeks, 89.5% of continuers had kept at least 80% of their initial loss, against 16.6% on placebo.

Beyond weight

The sleep apnoea result is worth unpacking. SURMOUNT-OSA was two parallel 52-week trials: one in people not using a CPAP machine and one in people who were [5]. Participants started with around 50 breathing interruptions an hour, which is severe. In the first trial the count fell by 25.3 events an hour on tirzepatide against 5.3 on placebo; in the second, by 29.3 against 5.5 [5]. The placebo groups improved a little too, which is why the between-group difference of 20 to 24 events an hour is the fairer number to quote [5]. A starting average near 50 minus about 25 still leaves many people with apnoea that would normally be treated, so the fair reading is a large reduction in breathing interruptions alongside ongoing management. It became the first drug the FDA approved for the condition [11].

SUMMIT also measured how people felt. On the Kansas City Cardiomyopathy Questionnaire, a 0–100 score of heart-failure symptoms and limits, the tirzepatide group improved by 19.5 points against 12.7 on placebo at 52 weeks [6]. Side effects led 6.3% of the tirzepatide group to stop, against 1.4% on placebo [6].

SURPASS-CVOT deserves a closer read, because it is often summarised as a heart-protection win. Tirzepatide was compared against dulaglutide, an older GLP-1 drug with its own proven heart benefit, not against placebo. It met the test for non-inferiority, so tirzepatide protects the heart at least as well as a drug already known to do so; the superiority margin was not significant (p = 0.09) [7]. In SUMMIT, the benefit came from fewer worsening heart failure events, while cardiovascular deaths were few and similar between groups (8 vs 5) [6].

What the animal and lab work suggests

The animal finding that matters most for people is a safety one. In rats, tirzepatide caused thyroid C-cell tumours, so the US label carries a boxed warning and rules the drug out for people with a personal or family history of medullary thyroid cancer [8]. The warning rests on rodent findings, and the label notes that human relevance is unknown [8].

Safety and side effects

Gut. Nausea, diarrhoea, vomiting and constipation are the main complaints. In SURPASS-2, nausea affected 17–22% on tirzepatide against 18% on semaglutide [2], so the two drugs look broadly similar here. Most events happen during dose escalation [1].

Gallbladder. Across the Zepbound trials, gallbladder inflammation was reported in 0.7% on tirzepatide against 0.2% on placebo, and gallstones in 1.1% against 1% [8].

Pancreatitis. In the diabetes trials, 13 people on tirzepatide had adjudicated acute pancreatitis (0.23 per 100 patient-years) against 3 on comparator drugs (0.11 per 100 patient-years) [8]. The label says to stop if pancreatitis is suspected.

Eyes. The label warns about diabetic retinopathy complications in people with type 2 diabetes and says tirzepatide has not been studied in people with more severe existing retinopathy [8].

Lean mass. The FDA label states tirzepatide lowers fat mass more than lean mass [8]. A 2024 review of the GLP-1 class found very inconsistent lean-mass data across studies and warned that DXA "lean mass" includes water and organs as well as muscle [9]. The functional question — how much muscle, and whether it affects strength or falls in older adults — is where the class still has work to do [9].

Legal and regulatory status

NICE advises reviewing treatment if less than 5% of weight has been lost after 6 months on the highest tolerated dose [12]. Tirzepatide sold outside a prescription is not the licensed product.

What is still open

• Whether tirzepatide outperforms older GLP-1 drugs on heart attacks and deaths, rather than simply matching them [7].
• How long people need to stay on it, and whether any tapering strategy prevents regain [4].
• What share of lost lean tissue is muscle, and whether it affects strength or falls in older users [9].
• How much of the extra weight loss comes from GIP itself in humans [8].
• Long-term safety beyond the three to four years covered by the largest trials.

What this means

Our reading is that tirzepatide has settled the question it was asked. For people with obesity it is the strongest weight-loss drug with a licence, and SURMOUNT-5 puts it clearly ahead of semaglutide at each drug's maximum tolerated dose [3]. That is a larger claim than it sounds: the previous generation already worked, and the field had spent years arguing over single-digit percentage points. The honest position is that the ceiling has moved, and treatment decisions should start from there.

Where we think the emphasis belongs is on continuation rather than initiation. SURMOUNT-4 is the most practically useful trial in the programme, because it shows what stopping costs people: 89.5% of continuers held at least 80% of their loss at 88 weeks, against 16.6% of those switched to placebo [4]. A drug that works while you take it is a long-term commitment, not a course, and prescribing conversations should say so from the first appointment.

We also think the cardiovascular story is being told too generously. SURPASS-CVOT compared tirzepatide with dulaglutide, a drug with its own proven benefit, and met non-inferiority while missing superiority [7]. Matching an effective comparator is a real result, and the SUMMIT data in heart failure with preserved ejection fraction are genuinely encouraging [6] — but this is not the placebo-controlled mortality win that many summaries imply.

The open questions above are worth tracking, particularly lean mass and the GIP contribution, but they sit inside a well-replicated core finding rather than casting doubt on it. Read the evidence as a strong, consistent effect paired with a care commitment that lasts as long as the treatment does — and disagree with us if you weight the cardiovascular question more heavily than the trial did.

Educational content only — not medical advice. Many peptides discussed on HPR are not approved for human use. Talk to a qualified clinician before making any decision about your health.