Retatrutide has attracted attention for producing some of the largest weight loss results seen in clinical trials (1-2). But if you're considering treatment when it becomes available, you're probably wondering about the side effects too.
Retatrutide is not yet licensed in the UK, but several large clinical trials have now investigated its safety and effectiveness. While researchers are still collecting long-term data through the ongoing TRIUMPH programme, the most common side effects consistently appear to be digestive symptoms, particularly when starting treatment or increasing the dose (1).
In this guide, we'll look more closely at what trials have found so far and what side effects could mean for future patients.
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In this article
What are the most common Retatrutide side effects?
Exactly how common are they?
Do Retatrutide side effects get better with time?
Why does Retatrutide cause side effects?
Are Retatrutide side effects worse than Wegovy or Mounjaro?
What serious side effects have been reported?
What do we know about long-term safety?
What are the most common Retatrutide side effects?
Across clinical trials completed so far, the most commonly reported Retatrutide side effects have been digestive symptoms, such as (1-2):
Nausea
Diarrhoea
Vomiting
Constipation
Stomach pain
Indigestion
Less commonly, researchers have also reported upper respiratory tract infections, urinary tract infections and dysesthesia (tingling, burning or numbness sensations). These were generally mild to moderate and typically resolved during treatment.
How common are Retatrutide side effects?
The most up-to-date information comes from TRIUMPH-1, a clinical trial involving more than 2,300 adults with obesity or overweight. Researchers found that digestive side effects became more common as the dose increased. At the highest dose studied (12mg), around (1):
42% reported nausea
32% reported diarrhoea
26% reported constipation
25% reported vomiting
At first glance, these numbers may seem high. But it's important to remember that they represent any side effect during the study, not necessarily ongoing or severe symptoms. Most were mild to moderate, meaning unpleasant but bearable, and many occurred when increasing the dose.
Even at the highest dose, only around 11% of people stopped treatment because of side effects, meaning the majority were able to continue treatment (1).
Do Retatrutide side effects go away?
Yes, the evidence so far suggests that they often do. Researchers found that gastrointestinal side effects were most likely to occur (2):
When first starting treatment
When stepping up to the next dose
At higher doses of Retatrutide
As participants remained on a stable maintenance dose, symptoms generally became fewer and easier to tolerate. This pattern is also seen with currently licensed GLP-1 medications such as Wegovy and Mounjaro.
For this reason, treatment is started at a low dose and increased gradually over time, helping the body adapt to the medication and reducing the likelihood of side effects.
Why does Retatrutide cause side effects?
Like other prescription weight management injections, Retatrutide works partly by helping you feel fuller for longer and reducing your appetite. To do this, it affects how quickly food moves through your digestive system.
While this can support weight loss, it is also what’s responsible for digestive side effects such as nausea, vomiting, constipation, diarrhoea and bloating.
What serious side effects have been reported?
Serious side effects appear to be uncommon in Retatrutide clinical trials so far. However, researchers are continuing to monitor for rarer but potentially serious complications that have also been associated with other GLP-1-based medications, including:
Pancreatitis (inflammation of the pancreas)
Gallbladder disease and gallstones
Severe dehydration caused by persistent vomiting or diarrhoea
Severe allergic reactions
If retatrutide is licensed in the future, patients would be advised to seek medical advice if they experience severe abdominal pain or vomiting, or signs of dehydration or an allergic reaction.
What do we know about long-term safety?
So far, no unexpected safety concerns have emerged. The TRIUMPH-1 study followed participants for 80 weeks, with some continuing treatment with Retatrutide for up to 104 weeks in an extension study, and the safety profile appears similar to other medicines in the GLP-1 family (1).
However, Retatrutide is still being studied. Results from TRIUMPH-2 and TRIUMPH-3 are set to be released later this year and will provide more information about long-term safety and how retatrutide performs in different patient groups, while other TRIUMPH trials are just getting underway (2-3).
As more data becomes available, we’ll gain a clearer picture of how Retatrutide performs in real-world use and over longer periods.
What weight loss treatments are currently available?
While Retatrutide is not yet available in the UK, several effective prescription weight loss injections are already licensed, including Wegovy and Mounjaro. Both can be prescribed through Oxford Online Pharmacy, following an online consultation.
Our UK-registered doctors and pharmacists can assess your medical history, weight loss goals and treatment preferences to help determine which option may be suitable for you.
Explore our weight loss injections or complete a short online consultation to get started.
References
Lilly E. Lilly’s triple agonist, retatrutide, delivered powerful weight loss in pivotal Phase 3 obesity trial [Internet]. Eli Lilly and Company. 2025.
Jastreboff AM, Kaplan LM, Frias JP, Wu Q, Du Y, Sirel Gurbuz, et al. Triple–Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial. The New England Journal Of Medicine [Internet]. 2023 Jun 26;389(6).
Lilly E. A Study of Retatrutide (LY3437943) in Participants Without Type 2 Diabetes Who Have Obesity or Overweight (TRIUMPH-9) [Internet]. Lilly Trials. 2026.