The UK's medical safety regulator has issued an urgent health warning after a patient died and 76 others reported serious side effects from taking an unapproved weight-loss drug, retatrutide, also known by the nickname 'Reta'.
The drug, manufactured by Eli Lilly, has shown potential for significant weight loss, with studies indicating users can lose up to a third of their body weight in less than a year. However, it remains in clinical trials and has not received approval from any global health authorities, rendering its use illegal outside of controlled studies.
Experts are concerned that the growing demand for weight-loss medications is fueling a black market where individuals may obtain counterfeit or contaminated versions of unapproved drugs. The Medicines and Healthcare products Regulatory Agency (MHRA) has logged 77 reports of suspected adverse reactions to retatrutide since 2025.
The MHRA emphasizes that these reports indicate a patient was taking the drug at the time of the suspected reaction but do not definitively prove causation. Concerns are also raised that black market products sold as retatrutide might not contain the actual drug, potentially leading to reactions from other harmful substances.
"No one in the UK should be on a drug that is not licensed or has not been approved by MHRA," said Naveed Sattar, professor of cardiometabolic medicine at the University of Glasgow. "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."
Reported Side Effects and Incidents
In 2025, the MHRA's Yellow Card scheme, which collects reports of suspected side effects, received 14 reports related to retatrutide, including 12 classified as serious. One of these reports involved the death of a man in his 30s. So far in 2026, the number of reports has surged by 350%, with 48 serious and 15 non-serious suspected adverse reactions recorded, though no further deaths have been reported this year.
Gastrointestinal symptoms, such as stomach pain, nausea, diarrhea, vomiting, constipation, and bloating, were the most common complaints, accounting for 46 of the 77 cases. Additionally, there were eight suspected hepatobiliary reactions, affecting the liver, gallbladder, pancreas, and bile ducts. The death in 2025 was associated with such a reaction.
These reports follow similar incidents in Australia, where six people were hospitalized with severe liver damage after using unlicensed doses of the drug. Megan Hancocks, 32, was hospitalized with acute liver failure within a week of using off-market retatrutide purchased illicitly. After a month of treatment, her condition stabilized, averting the need for a liver transplant.
One report also detailed an eye disorder, which can sometimes be a temporary side effect of approved weight-loss drugs as blood sugar levels adjust.
Women submitted the majority of the reports (52) compared to men (23). Among patients whose age was recorded, those in their 30s represented the largest group with 16 reports.
Regulatory Warnings and Expert Advice
Professor Sattar reiterated that desperation for weight loss may lead individuals to purchase unregulated products that may not even contain the advertised drug. "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," he added.
A spokesperson for the MHRA stated, "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."
The agency warned that acquiring medicines from unregulated sources significantly increases the risk of receiving unsafe or ineffective products, potentially leading to serious and unpredictable side effects due to unknown composition and lack of quality control. The MHRA urges the public to obtain medicines only from registered healthcare professionals and legitimate pharmacies. The MHRA's Criminal Enforcement Unit is prepared to prosecute illegal activities related to medicine sales to protect public health.