Mounjaro reviews: what the trials show and what patients report

Opinions about Mounjaro are fairly close to what the trials measured, for better and for worse. In SURMOUNT-1, the main obesity trial, average weight loss at 72 weeks was 15.0% to 20.9% depending on the dose, compared with 3.1% on placebo. The most common complaints are also well founded: nausea and diarrhoea as the dose goes up, a cost the public health system does not cover, and weight that tends to come back if you stop.

Mounjaro is the brand name of tirzepatide, a once-weekly injectable medicine that acts on two hormone receptors involved in appetite and blood sugar control at the same time: GIP and GLP-1. If you want to understand that mechanism properly, we explain it in what GLP-1 medicines are and how they work. It is authorised in Spain, as across the European Union, and is only dispensed with a medical prescription. Its summary of product characteristics covers two uses: type 2 diabetes and weight management in adults with a BMI of 30 or more, or 27 or more with at least one weight-related condition.

One fact weighs on many reviews: it is not funded by the Spanish National Health System. The Ministry of Health's Nomenclátor has listed it as not funded since 1 July 2024, so anyone using it pays for the whole treatment.

Why opinions about Mounjaro contradict each other so much

On forums and social media, stories of «I've lost twenty kilos» sit alongside «it did nothing for me» or «I had to stop after a few weeks». Both can be true. Everyone starts from a different point, with a different dose, a different length of treatment and different conditions, and the people who write reviews tend to be those who had a very good or a very bad experience. A single review does not tell you how many people responded the same way; a placebo-controlled trial does.

There is a second, less well-known reason. In September 2025, the European Medicines Agency and the heads of the national agencies, including Spain's AEMPS, warned of a sharp rise in illegal products sold online and on social media as semaglutide, liraglutide or tirzepatide. According to the EMA, they may not contain the active substance at all or may contain harmful amounts of other substances. Some of the bad experiences in circulation may come from products that were not the authorised medicine.

What the trials measured: how much weight is lost

SURMOUNT-1, published in The New England Journal of Medicine in 2022, followed 2,539 adults with obesity, or with overweight and a related complication, without diabetes, for 72 weeks. The first 20 weeks were a gradual dose increase and the average starting weight was 104.8 kg. Average weight loss was 15.0% with 5 mg, 19.5% with 10 mg and 20.9% with 15 mg, compared with 3.1% on placebo. Between 85% and 91% of those on tirzepatide lost at least 5% of their weight, and 50% and 57% of the 10 mg and 15 mg groups lost more than 20%.

Those figures come with two caveats. They are averages: within each group some people lost much more and some considerably less. And all participants were advised to follow a reduced-calorie diet and increase physical activity, with close follow-up that does not always happen outside a trial. The study was funded by Eli Lilly, the manufacturer.

In the head-to-head comparison with semaglutide, the active substance in Ozempic and Wegovy, the SURMOUNT-5 trial (NEJM, 2025) included 751 people with obesity without diabetes. Average weight loss at 72 weeks was 20.2% with tirzepatide and 13.7% with semaglutide. It was an open-label trial, meaning participants and doctors knew which medicine each person was receiving, and it was also funded by Eli Lilly.

There are longer-term data in one specific group. In the 1,032 SURMOUNT-1 participants who also had prediabetes, treatment continued for 176 weeks, just over three years: average weight loss was 12.3% to 19.7% depending on the dose, compared with 1.3% on placebo, and 1.3% developed type 2 diabetes compared with 13.3%. If that is your situation, you may want to read what prediabetes is and how it is managed.

The most common complaints and what lies behind them

Nausea, diarrhoea and other digestive symptoms

This is the number one complaint, and the summary of product characteristics approved in Spain puts numbers on it. In the weight-management trial in people without diabetes, digestive disorders affected 55.6%, 60.8% and 59.2% with 5, 10 and 15 mg, compared with 30.3% on placebo. Nausea occurred in 24.6%, 33.3% and 31.0% (9.5% on placebo) and diarrhoea in 18.7%, 21.2% and 23.0% (7.3% on placebo).

What the summary of product characteristics adds explains why many opinions change over the weeks: these symptoms were mostly mild (60.8%) or moderate (34.6%), more frequent while the dose was being increased, and became less common over time. Even so, between 1.9% and 4.4% stopped treatment for this reason, compared with 0.5% on placebo. We look at how they are managed in our guide to Ozempic and Mounjaro side effects and how to manage them.

Less frequent effects that also appear in people's stories

The summary of product characteristics lists hair loss as an adverse reaction, mainly in people with overweight or obesity. It also lists injection-site reactions, which in the weight-management trials occurred in 8.0% compared with 1.8% on placebo, and gallbladder problems such as cholecystitis, in 0.6% compared with 0.2%. It further warns that vomiting and diarrhoea can lead to dehydration and harm the kidneys, and that cases of acute pancreatitis have been reported, so it is used with caution in people who have had it before.

The cost and the weight that comes back

Many negative reviews are not about the medicine but about the bill: because it is not funded, it is an expense that continues month after month. And that meets the most uncomfortable finding of all, from SURMOUNT-4 (JAMA, 2024). After 36 weeks of treatment, with an average loss of 20.9%, 670 participants were randomly assigned to continue or switch to placebo. Over the following year, those who continued lost a further 5.5% and those who stopped regained 14.0%. 89.5% of those who continued kept at least 80% of the weight they had lost, compared with 16.6% of those who switched to placebo.

This does not mean everything is lost when you stop, but it does mean obesity behaves like a chronic disease and that stopping treatment needs a plan. We go into this in why weight comes back after stopping treatment and how it is managed.

What happens outside the trials

Real-world studies, based on the medical records of thousands of patients, are closer to what reviews describe. One published in JAMA Network Open in 2025, covering 125,474 adults in the United States who started liraglutide, semaglutide or tirzepatide, found that during the first year 64.8% of those without diabetes and 46.5% of those with diabetes stopped treatment. Moderate or severe digestive symptoms were associated with more people stopping; greater weight loss, with fewer.

Another, from Cleveland Clinic, published in Obesity in 2025 with 7,881 patients without diabetes treated with semaglutide or tirzepatide, found an average weight loss of 8.7% at one year. Those who did not interrupt treatment lost 11.9%; those who stopped in the first three months, 3.6%. In addition, 80.8% were on low maintenance doses. The authors attribute the gap with the trials to those two factors: people stopping and low doses.

The other side comes from an academic obesity clinic at Vanderbilt (Diabetes, Obesity and Metabolism, 2025): among those who stayed on semaglutide or tirzepatide for at least 12 months, median weight loss was 14.4%, close to what was seen in the trials. Much of the distance between an enthusiastic review and a disappointed one lies in how long, and at what dose, the treatment was followed.

What people notice beyond the scales

SURMOUNT-1 also asked participants about their quality of life, using validated questionnaires on physical function, psychosocial wellbeing and general health. According to the analysis published in Diabetes, Obesity and Metabolism in 2025, all tirzepatide groups improved more than placebo, the improvement grew from 5% weight loss upwards and was greater in those who started with more physical or psychosocial limitations. In questionnaire language, it is what many positive reviews describe in their own words.

Before you trust a review: what changes depending on your case

There are situations where someone else's experience is of little use. The summary of product characteristics recommends contraception for women of childbearing potential during treatment, does not recommend it in pregnancy and asks for it to be stopped at least one month before trying to conceive, because of how long it stays in the body. If you are going to have a general anaesthetic or deep sedation, you need to tell the team, because cases of pulmonary aspiration have been reported with this type of medicine as it slows stomach emptying. And in people with diabetes who use insulin or sulfonylureas, the risk of hypoglycaemia increases.

That is why the useful question is not whether it went well for others, but whether your profile matches what was studied: your BMI, related conditions, the medication you already take and your plans. That is assessed by a doctor with your history in front of them.

What to take away

Reviews of Mounjaro have a basis in both directions: in the trials people lose considerably more weight than on placebo, and in return there are digestive symptoms at the start, a cost paid in full and the need for a plan so as not to regain what was lost. What does exist today in Spain for anyone who wants to lose weight with medical support is medical weight-loss treatment with a prior assessment, a prescription when appropriate from a registered doctor and ongoing follow-up. You can check in a few minutes whether you are eligible.