Tirzepatide: 17% weight loss in real care

A prospective observational study of 107 adults with obesity in Palencia, Spain, reports a mean weight reduction of 18.04 kg after six months on tirzepatide, equal to 17.21% of starting body weight. The median dose at the six-month visit was 7.5 mg once weekly^[1]. The paper appeared in Clinical Nutrition ESPEN on 1 August 2026. It is a single-arm cohort with no control group, and that shapes what the headline number can and cannot tell you.

What the study measured

The design was prospective, longitudinal and observational. Participants were followed as they were treated in routine care rather than randomised to anything. Entry required a body mass index of 30 kg/m2 or above, or 27 kg/m2 or above plus at least one weight-related condition, which is the same threshold used in the phase-3 obesity trials. The primary outcomes were change in body weight and body composition at six months. Anthropometric measures, glycaemic and lipid parameters, quality of life and tolerability were recorded alongside.

The authors are based at the endocrinology and nutrition unit of the Complejo Asistencial Universitario de Palencia, with co-authors from the obesity division of the Spanish Society of Endocrinology and Nutrition^[1]. That detail matters when reading the result. These patients were being seen in a specialist obesity service, not collecting pens from a telehealth queue, and a specialist service delivers dietary and behavioural care alongside the injection.

The numbers, and the denominator question

At six months the mean reduction was 18.04 kg, or 17.21% from baseline. A total of 95.35% of participants reached at least 5% weight loss and 60.47% reached at least 15%. Glycaemic and lipid profiles improved, health-related quality of life improved, tolerability was described as favourable, and the dropout rate was 3.7%^[1].

One arithmetic note before anyone compares those responder rates to a trial. 95.35% and 60.47% do not divide cleanly out of 107; they are consistent with a denominator in the mid-eighties. The likeliest reading is that the six-month analysis covers the participants who had reached that visit rather than everyone enrolled, and the abstract does not say how many that was. Anyone weighing this against randomised data should check the full paper for the analysis population, because completer analyses tend to flatter a drug. People who stop early, whether because it is not working or not tolerable, drop out of the average.

The 7.5 mg detail is the interesting part

The median maintenance dose at six months sat at 7.5 mg once weekly. Under the US Zepbound label, the recommended maintenance dosages for weight reduction are 5 mg, 10 mg or 15 mg once weekly, reached by increasing in 2.5 mg increments after at least four weeks on the current dose^[4]. The 7.5 mg pen is a step on the way up rather than one of the labelled maintenance doses. The Mounjaro label lists the same six strengths and caps adults at 15 mg once weekly. In the EU and EEA, tirzepatide is centrally authorised as Mounjaro for both type-2 diabetes and weight management^[5].

The authors read their result as evidence that the effect holds at maintenance doses below the approved maximum. That is a fair description of what they observed. It is not a reason to plan a lower dose. This is where patients happened to land after titration in one clinic over six months, not a dose comparison, and nobody was randomised to 7.5 mg against 15 mg. Our tirzepatide reference page sets out the full titration schedule from the labels. Dose is a conversation with the prescriber who is monitoring you.

What a single-arm cohort cannot settle

There is no placebo group, so the 17.21% carries everything that happened to these patients over six months, not just the drug. For scale: in SURMOUNT-1, the phase-3 randomised trial of 2,539 adults with obesity and without diabetes, the placebo arm still lost 3.1% of body weight by week 72 on lifestyle intervention alone^[2]. A hospital nutrition clinic is doing more than dispensing a peptide, and a single-arm design cannot separate the two contributions.

Three further limits. Selection: people who present to and stay in a hospital obesity service differ from the general population on motivation, comorbidity burden and follow-up. Scale and setting: 107 participants at one Spanish centre, which is a fraction of the SURMOUNT programme enrolment. Duration: six months is short next to the 72-week trials, and the trial weight curves were still falling at that point, so the timepoints are not interchangeable.

Where it sits next to the randomised evidence

SURMOUNT-1 reported mean weight reductions of 15.0% on 5 mg, 19.5% on 10 mg and 20.9% on 15 mg at 72 weeks against 3.1% on placebo^[2]. The Spanish figure of 17.21% at six months on a median 7.5 mg sits inside that range and arrives faster, which is the kind of gap that usually reflects study design rather than a better drug. What real-world cohorts add is the tolerability and persistence picture, and a 3.7% dropout rate in routine care is worth noting given how much of the GLP-1 class discussion is about people quitting within a year.

What no six-month cohort addresses is what happens afterwards. SURMOUNT-4 withdrew tirzepatide from participants who had already lost a mean 20.9% over a 36-week lead-in. Over the following 52 weeks the placebo arm regained 14.0% while those who continued lost a further 5.5%^[3]. Effectiveness at six months and durability at two years are separate questions, and only the first one is on the table here. The same gap applies to real-world reports on semaglutide and to the newer triple agonists such as retatrutide, which have shorter follow-up still.

Tirzepatide is a prescription-only medicine in the United States, the United Kingdom and across the EU and EEA, and the regulatory position does not shift because a 107-person cohort did well. This is educational reporting on one observational study, not medical advice. If you are considering tirzepatide, or already taking it, the dose and the monitoring plan belong with a clinician who knows your history.