A UK patient has died after taking an unapproved drug dubbed the 'Godzilla' of weight-loss jabs - and dozens more have suffered serious side effects, the Daily Mail can reveal.
Figures show that a man in his 30s died after reporting complications from taking the illicit appetite-suppressing medication retatrutide, also known as 'Reta'.
The drug, which has been shown to help people lose up to a third of their body weight in less than a year, is seen as a potential successor to blockbuster fat jab treatments such as Mounjaro and Wegovy.
But unlike those jabs, retatrutide - which is manufactured by Eli Lilly - remains in clinical trials and has not yet been approved by health officials anywhere in the world, meaning it cannot legally be used.
Experts are concerned that soaring demand for weight-loss jabs - now used by an estimated 2.5 million Britons - is fuelling a booming black market where patients may be obtaining fake or contaminated versions of the drug from unlicensed sellers.
Now, the Daily Mail can reveal that the UK medical safety regulator, the Medicines and Healthcare products Regulatory Agency (MHRA), has recorded 77 reports of suspected retatrutide side effects since 2025.
The MHRA stresses that reports do not prove that a medicine caused the suspected adverse reaction, only that the patient was taking the drug at the time.
Figures show that a man in his 30s died after reporting serious side effects from use of the illegal appetite-suppressing medication retatrutide, also known as 'reta'
Experts also warn that products sold as retatrutide on the black market may not actually contain the drug at all - meaning some of the reported reactions could instead have been caused by other, potentially harmful, substances.
Naveed Sattar, professor of cardiometabolic medicine at the University of Glasgow, told the Daily Mail: 'No one in the UK should be on a drug that is not licensed or has not been approved by MHRA.
'People need to be very wary of something that's been sold to them called retatrutide because it is likely not retatrutide.
'They could be injecting into their body any substance that could potentially cause them harm.
'I would urge people not to buy things that are not licensed and not from proper manufacturers. Otherwise, you could be putting yourself into harm's way.'
The MHRA runs a database called the Yellow Card scheme, which allows patients and doctors to log concerning side effects.
In 2025, the MHRA received 14 reports of suspected side effects linked to retatrutide, including 12 classified as serious and two as non-serious.
That total included the report involving the death of the man in his 30s. No other details about this patient were recorded.
So far this year, the number of reports has risen by 350 per cent, with 48 serious and 15 non-serious suspected adverse reactions recorded. No deaths have yet been reported in 2026.
The reports includes a wide range of side effects, most commonly gastrointestinal symptoms, which made up 46 of the 77 cases.
Such symptoms are typically associated with weight-loss drugs and include stomach pain, nausea, diarrhoea, vomiting, constipation and bloating.
However, there were also eight suspected hepatobiliary reactions - complications typically affecting the liver, gallbladder, pancreas and bile ducts - following use of retatrutide. This includes the man who died in 2025.
The data does not specify the exact diagnosis in each case, meaning it is not possible to determine the exact cause of the man's death.
But it comes just weeks after health officials in Australia revealed six people had been hospitalised with severe liver damage after taking unlicensed doses of the drug.
One user, Megan Hancocks, 32, was rushed to hospital with acute liver failure within a week of using off-market retatrutide that she had bought for cash. Her symptoms included severe jaundice - which brought on a yellow colour to her eyes - as well nausea and vomiting, extreme fatigue, stomach swelling and debilitating constipation.
Doctors treated her for a month and even considered a liver transplant but her condition stabilised before the procedure was required.
Megan Hancocks (pictured) said she used off-market retatrutide late last year
The MHRA has also received one report of an eye disorder linked to the drug. It is believed some people starting approved weight-loss jabs can experience temporary changes in vision as blood sugar levels adjust.
Women accounted for the majority of reports, with 52 submitted by female patients, compared with 23 submitted by men. Two reports did not specify the patient's sex.
In cases where an age was recorded, patients in their 30s accounted for the largest group, with 16 reports. This was followed by those in their 40s (10), 20s (9) and 10 to 19-year-olds (3). The ages of patients were not recorded in 29 reports.
Professor Sattar said the figures suggest people have become so desperate to lose weight that they are willing to buy products marketed as weight-loss drugs, even though products sold as retatrutide may not actually contain the drug.
He added: 'A lot of people just want to lose weight. Hopefully, over the next few years, more drugs that have got really good evidence from trials are licensed and these issues become less of an issue in five to 10 years time.'
An MHRA spokesperson told the Daily Mail: 'Retatrutide has not been approved for medicinal use in the UK. It is currently undergoing clinical trials and cannot be legally sold or supplied outside of those settings.
'Buying medicines from unregulated sources significantly increases the risk of receiving products that are unsafe or ineffective.
'Their use may result in serious and unpredictable side effects due to unknown composition and a lack of quality controls. We urge the public to only source medicines from registered healthcare professionals and legitimate pharmacies.
'Where illegal activity is identified in the UK, the MHRA's Criminal Enforcement Unit will not hesitate to use the full extent of its powers to protect public health, including, where appropriate, through criminal prosecution.'
Drug manufacturer Eli Lilly added: 'Retatrutide is an investigational, once-weekly, triple hormone receptor agonist, which activates the body's receptors for glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide-1 (GLP-1), and glucagon.
'It is an investigational molecule that is legally available only to participants in Lilly's clinical trials. It has not been reviewed or approved by any regulatory agency in the world.
'No one should consider taking anything claiming to be retatrutide outside of a Lilly-sponsored clinical trial. The company continues to warn the public about the potential dangers of fake medicines.'
