A weight loss drug dubbed the 'Godzilla' of fat jabs may be linked to a swathe of troubling side effects, a study suggests.
Researchers found patients claiming to use retatrutide reported 129 symptoms from burning skin to rapid heart rate, panic attacks and depression.
The drug – commonly known as 'reta' – is still undergoing clinical trials and is not licensed for use anywhere in the world.
However, patients in the UK and across the world have been buying the jab illegally and injecting themselves with it.
Reta earned its 'Godzilla' nickname after trials found it to be even more powerful than Mounjaro, helping slimmers shed a quarter of their body weight in as little as 18 months.
But experts say using the drug before it is licensed – and not subject to safety checks – is dangerous, because patients could be injecting substances that don't, in fact, contain the medication.
The new study, published in the journal JAMA Network Open, saw researchers examine 148,640 social media posts related to retatrutide on the website Reddit.
They whittled it down to 13,589 patients who claimed to be using the drug, and analysed the side effects reported by them.
Researchers say patients using retatrutide have reported 129 symptoms from burning skin to rapid heart rate, panic attacks and depression
In total, the researchers – from the University of Pennsylvania – found 7,823 users reported at least one side effect.
That's more than half of those who claimed to be using the jab. Many users reported multiple changes.
The study separated the side effects into 14 different categories, ranging from digestive issues to mental disorders, heart problems and skin changes.
The most commonly reported change was, surprisingly, increased appetite, affecting 1,336 people – almost one in five.
Weight loss drugs generally reduce appetite by mimicking a hormone in the gut named GLP-1 that is released after eating and curbs hunger.
Retatrutide also targets the GLP-1 hormone, alongside two other receptors – earning it the name 'triple agonist'.
Following increased hunger was fatigue, reported by 1,188 patients – around 15 per cent of people.
Meanwhile, nausea topped the list of digestive complaints, affecting fewer than one in 10, ahead of diarrhoea and vomiting - common side effects of weight loss jabs. Food cravings (571) and insomnia (475) followed this.
Crucially, the findings should be treated with caution because the authors could not verify the claims made by those posting to the site.
But even so, experts say the study raises concerns about illegal use of retatrutide.
'I would urge people to avoid buying unlicensed weight-loss products online,' Naveed Sattar, professor of cardiometabolic medicine at the University of Glasgow, told the Daily Mail.
'There is no certainty about what these products contain, and without proper regulatory oversight there is a risk of adverse effects.
'People may inadvertently do themselves more harm than good by taking substances that have not been rigorously tested for safety and efficacy.'
Some of the most concerning complaints flagged involved changes to the heart and mental health.
A total of 374 users said their heart rate had increased, which is around 5 per cent. Meanwhile, 475 reported insomnia and 301 flagged anxiety.
Another 206 reported anhedonia – the term for no longer finding joy or pleasure in things you used to enjoy – while 68 reported depression and 64 said they had suffered panic attacks.
Elsewhere, small numbers of patients (55) reported a burning sensation in their skin while 58 said they had developed an intolerance to alcohol.
Other effects impacting smaller numbers included binge eating, decreased libido, obsessive thoughts, nicotine dependence and emotional distress.
There were, however, some more positive changes.
A total of 175 of the 13,589 users said they could exercise for longer, 164 reported an increased libido and 149 said they felt euphoric.
The authors stressed that the study cannot prove the drugs cause these problems.
'The social media users studied here may not represent all retatrutide users, and adverse events may be preferentially reported,' the scientists said.
They added that information such as the product's contents, dose and the patient's medical history could also not be verified.
While retatrutide targets three receptors, Mounjaro, on the other hand, targets two. Both drugs are made by the same manufacturer, Eli Lilly.
There is an expectation that it could be available to buy in Britain, and even be prescribed by the NHS, within three years.
Eli Lilly was approached for comment.