Retatrutide study: an experimental weight loss drug tops Mounjaro?

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In a new study, an experimental weight loss drug outperformed Mounjaro. Learn what the results really mean.

Here at SemaPen, we specialise in obesity treatment programmes using the injectable weight loss medications Wegovy and Mounjaro.

We use these medications for a reason. They're part of a class of drugs called 'GLP-1 agonists', which are known to be effective and tolerable in most cases and for most people.

Wegovy and Mounjaro are the best of the bunch. Mounjaro is clinically proven to produce an average of 21% weight loss when taken as part of a treatment programme with diet and exercise.¹ Wegovy, meanwhile, produces an average of 15% weight loss² – not as much as Mounjaro, but still far ahead of many of its peers.

But medicine is a fast-moving field. Novel, experimental weight loss drugs are being developed all the time.

One of these experimental drugs, retatrutide, was found to be more effective than Wegovy or Mounjaro in a new study.

Does this mean we should all stop taking Wegovy or Mounjaro and move to retatrutide? Not quite.

Let's look at the study in a little more detail.

What did the study find out?

The retatrutide study was a systematic review. This is a research method that looks at other studies in a structured and controlled way.

The researchers wanted to look at several new GLP-1 drugs to learn more about their safety and effectiveness. To do this, they gathered data from 26 clinical trials covering 15,491 participants. These trials looked at nine experimental drugs plus the commercially available medications tirzepatide (Mounjaro), semaglutide (Wegovy) and liraglutide (Saxenda).

Some of the results were to be expected. The Mounjaro trials tended to report better results than the Wegovy trials. Mild side effects like nausea were fairly common, but serious side effects were rare.³ This all aligns with what we already know.

But the surprise contender was retatrutide. In trials, it caused up to 22.1% weight loss.³ That was more than Wegovy and Mounjaro. And it was more than all the other experimental weight loss drugs included in the review.

The researchers made another interesting point. As noted by Medscape, "The review shows that it is likely that GLP-1s would have to be used chronically to have the greatest effect".

In other words, these types of weight loss drugs are most effective as part of a long-term treatment plan. This chimes with similar studies and our own observations of SemaPen patients.

What do the results really mean?

The review provides compelling evidence for retatrutide's effectiveness as a weight loss treatment.

However, it's just that – a piece of evidence. It's not a smoking gun that conclusively proves retatrutide outperforms Wegovy or Mounjaro.

To prove that, we'd need to conduct a trial that compares the three medications under identical circumstances. That's similar to what happened recently with a trial that proved Mounjaro is better than Wegovy.

While the researchers in the retatrutide study did their best to compare similar studies, they couldn't account for all differences. And in medical research, even the smallest difference can change the results and make like-for-like comparisons impossible.

The researchers looked at studies of different lengths, of different sizes and with different types of participants.

That's why it's best to see the study not as conclusive evidence but as a useful signpost for future research. At this stage, it's nothing more than that.

When can I get retatrutide in the UK?

Retatrutide has a long way to go before it's available to patients in the UK – and that's if it even makes it to market.

That's because new medications must go through extensive clinical trials to make sure they're safe and effective. After that, they need to be signed off by the agencies in charge of approving medications. Each country has its own such agency – the MHRA in the UK, for example, and the FDA in the States.

Mounjaro and Wegovy have already been through this process. They're clinically proven and have been licensed for use in the UK.

Right now, they're the best medications on the market in terms of safety and effectiveness. That's why we'll continue offering them to our subscribers – at least until there's conclusive evidence for a better option.

Will retatrutide take their place? Only time – and trials – will tell.

SemaPen delivers expert-led obesity treatments using clinically proven non-surgical options for weight loss. Why not explore our weight loss programmes in more detail?

Sources

1. Jastreboff, A.M. et al. (2022) "Tirzepatide Once Weekly for the Treatment of Obesity" New England Journal of Medicine, 387(3) https://doi.org/10.1056/NEJMoa2206038

2. Wilding, J.P.H. et al. (2021) "Once-weekly semaglutide in adults with overweight or obesity" New England Journal of Medicine, 384(11) https://doi.org/10.1056/NEJMoa2032183

3. Moiz, A. et al. (2025) "Efficacy and Safety of Glucagon-Like Peptide-1 Receptor Agonists for Weight Loss Among Adults Without Diabetes: A Systematic Review of Randomized Controlled Trials" Annals of Internal Medicine [Epub 7 January 2025]

https://doi.org/10.7326/ANNALS-24-01590

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