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Retatrutide is an experimental weight loss injection from Eli Lilly, the company behind Mounjaro. Known as the “triple G” obesity drug, it’s a triple agonist that targets three hormone pathways in your body, while existing weight loss medications target one or two.
Mounjaro and Wegovy have already changed how obesity is treated, and retatrutide could push results further still. In phase 3 trials, people on the highest dose lost an average of 28.3% of their body weight. It isn’t licensed anywhere yet, so you can’t get it on prescription in the UK.
Retatrutide is called a ‘triple G’ drug because it targets three different hormone receptors at the same time: GLP-1, GIP, and glucagon. This triple action is what sets it apart from other weight loss medications currently available:
Retatrutide works as a GLP-1 receptor agonist in the same way as Ozempic and Wegovy. GLP-1 helps you feel full after eating and slows down how quickly food leaves your stomach. It also helps regulate blood sugar levels.
GIP is a hormone your gut releases after you eat. Like GLP-1, it helps with feeling full and blood sugar control, but it works in a slightly different way and is particularly good at helping your body respond to insulin. Mounjaro targets GIP alongside GLP-1.
This is the part that sets retatrutide apart. Glucagon normally tells your liver to release stored sugar. By activating the glucagon receptor in a controlled way, alongside GLP-1 and GIP, retatrutide is thought to increase how much energy your body burns and encourage it to break down more fat, particularly in the liver.
Retatrutide has now been tested in three large phase 3 clinical trials, known as the TRIUMPH programme. Lilly has shared the headline results, and the full data are due to be published in peer-reviewed journals.
TRIUMPH-1 followed 2,339 adults with obesity, or overweight with at least one weight-related health condition, for 80 weeks. Average weight loss by dose was:
On the 12mg dose, 45.3% of people lost at least 30% of their total body weight, and 65.3% finished the trial with a BMI under 30. People with a starting BMI of 35 or above who carried on into a 104-week extension lost an average of 30.3%, which is a promising sign for long-term weight loss.
To put this in perspective, if someone weighed 100kg (about 15.7 stone) at the start, an average loss of 28.3% would take off around 28kg (about 4.5 stone).
TRIUMPH-2 looked at adults with type 2 diabetes and obesity or overweight. They lost an average of 12.7%, 19.1% and 20.8% on the 4mg, 9mg and 12mg doses, with HbA1c (a measure of long-term blood sugar) falling by up to 1.6%. People with type 2 diabetes tend to lose less weight on these medicines, which is why these figures are lower than in TRIUMPH-1.
TRIUMPH-3 included adults with a BMI of 35 or above and established cardiovascular disease, who lost up to 22.6% on average. The highest dose also cut triglycerides by 37.0%, non-HDL cholesterol by 16.5% and systolic blood pressure by 9.3 mmHg.
In the earlier phase 2 trial, people on the highest dose lost an average of 24% of their starting body weight over 48 weeks. The phase 3 results show that weight loss carried on well beyond this point.
The reported side effects in the phase 3 trials were similar to those of other weight loss injections, but they were more common on the higher doses. Most were mild to moderate.
Digestive side effects were the most common in TRIUMPH-1. On the 12mg dose:
Other digestive effects reported in trials include stomach pain or discomfort and loss of appetite.
Dysaesthesia isn’t a common side effect of Mounjaro or Wegovy, so it’s one to watch as more data on the safety of retatrutide comes out. In TRIUMPH-1, most cases were mild to moderate and cleared up during treatment.
Most side effects are worse when starting the medication or increasing the dose, and often improve as your body adjusts. Some people did stop treatment because of them: 11.3% on 12mg, 6.9% on 9mg and 4.1% on 4mg, compared with 4.9% on placebo. How you respond will depend on you, and the lower 4mg dose looked much easier to tolerate.
Retatrutide isn’t approved for use outside clinical trials yet, so we can only look at who has taken part in studies so far. Across the TRIUMPH programme, participants included:
People generally couldn’t take part if they had a history of certain conditions, such as pancreatitis or severe kidney disease, or if they were pregnant or breastfeeding.
In clinical trials, retatrutide has been given as a once-weekly injection under the skin (subcutaneous). If it’s approved, it’s likely to come as pre-filled retatrutide pens, like other weight loss injections.
In TRIUMPH-1, everyone started on 2mg a week and stepped up every four weeks until they reached their target dose of 4mg, 9mg or 12mg. The 4mg dose needed just one increase, while 12mg took four steps over 16 weeks.
Increasing the dose gradually helps reduce side effects, particularly digestive ones, when starting treatment.
There haven’t been any head-to-head trials yet, so the best we can do is compare the results of separate trials.
In TRIUMPH-1, people on the top dose of retatrutide lost an average of 28.3% of their body weight over 80 weeks. That compares with around 15-17% with Wegovy (semaglutide) over 68 weeks and around 21% with Mounjaro (tirzepatide) over 72 weeks. These trials ran for different lengths of time and included different groups of people, so treat the comparison as a rough guide.
Because retatrutide targets three hormone pathways instead of one or two, digestive side effects were more common at the higher doses, and more people stopped treatment at 12 mg than on placebo.
When it comes to convenience, retatrutide would be similar to Wegovy and Mounjaro: a once-weekly injection you can do at home, with the dose increased gradually to help minimise side effects.
Retatrutide isn’t the only new medicine using glucagon to boost weight loss. Survodutide, made by Boehringer Ingelheim, targets GLP-1 and glucagon but not GIP.
Not yet. Retatrutide isn’t available in the UK because it hasn’t been approved by the MHRA (Medicines and Healthcare Products Regulatory Agency) or any other regulator.
With all three TRIUMPH trials reported, Lilly says it now has the data to support global submissions for obesity, knee osteoarthritis pain and obstructive sleep apnoea. It plans to submit to the US FDA in early 2027, and it can’t be approved by the FDA until that review is complete. UK approval would need a separate MHRA review, and there’s no confirmed UK timeline yet.
No. Lilly states that retatrutide is legally available only to people taking part in its clinical trials. Anything sold online as retatrutide, including products labelled as “research retatrutide peptides”, hasn’t been through any quality checks. It could be the wrong strength, be contaminated or not contain retatrutide at all, and there’s no prescriber checking it’s safe for you.
If you’re looking for weight management treatment now, Wegovy, Mounjaro and Saxenda injections are all licensed in the UK. So are Wegovy tablets and Foundayo, a daily tablet from Lilly that contains orforglipron. At Click2Pharmacy, we can help you access these treatments through our online weight loss clinic, where you can have an online consultation with qualified healthcare professionals.
