When you've been a GP for over 20 years, you start to feel like you've seen it all. But a few months ago, I had an experience with a patient that left me shaken – and makes me worried for the health of countless others.

It started, as it so often does, with vomiting. The patient, let's call her Jane, a 35-year-old mother of two, called up to complain that she could not keep food down. 

She explained that she was violently vomiting every hour but – at first – she claimed she did not know what could have caused the symptoms. 

We told her to drink plenty of water and get back in touch in a few days if she wasn't feeling any better.

Three days later, Jane called, in even more of a panic, to say that she was still vomiting. She was also intensely nauseous and had been constipated for several days. 

It was only at this point, when we questioned her more closely, that she admitted the cause: she had taken a black-market weight-loss drug called Reta.

Reta, in case you haven't heard of it, is short for retatrutide. It's an experimental drug made by Eli Lilly – the same firm behind the blockbuster weight-loss drug Mounjaro. 

Trial data has shown that Reta patients can lose up to a third of their body weight, making it the most powerful weight-loss drug to date. It's no wonder it's gained the nickname, the 'Godzilla shot'.

But what makes the story of Reta so strange is that it is not even for sale yet. 

It appears that black-market drug makers have been able to cook up their own version of Reta in laboratories and are now selling it illicitly – either online or via street drug dealers. 

At least, that's what people claim. It's impossible to know what is in the vials that these shady dealers are selling. It could be Reta, but it could just be Mounjaro – or it could be something else completely.

Jane certainly didn't know what she was taking. I was shocked when she explained that she had got the Reta from a friend who bought it online – she had injected it three times over three weeks.

It's easy to judge Jane for this reckless decision. However, I do understand the desperation that led her to try Reta. Jane was overweight – and had been for more than a decade. 

She had tried exercise and dieting but nothing seemed to stick. She had wanted to try weight-loss jabs for some time, but the £200-a-month price had been prohibitively expensive. 

While she watched friends and celebrities on social media shedding a previously unimaginable amount of weight, Jane felt she was stuck. So when her friend offered to let her try the jabs for free, she jumped at the opportunity.

Unfortunately, it turned out to be a disastrous decision. Three months on, Jane is still seriously unwell. She is still vomiting every day. Her nausea hasn't abated. Even her periods have stopped. 

Worst of all, she can't eat – still unable to keep even the lightest, blandest food down. Jane took this drug to improve her relationship with food. Instead, she has further complicated it, and potentially done permanent damage to her body.

And it's not just Jane I'm worried about. Countless Britons – the majority of whom are women – are buying Reta online. 

A new survey, published today, found that a quarter of GPs have treated severely unwell patients who took black-market weight-loss drugs. 

It's shockingly easy to access. To test this, I went on social media and searched for Reta – it took me less than two minutes to find multiple people selling it via post. 

Every day, there are people injecting themselves with this black-market drug – and we don't even know what is in it.

What I find so strange about this is that, in recent years, I have seen a massive increase in scepticism about pharmaceutical drugs. 

More mothers than ever are refusing to have their children vaccinated; older people tell me they don't want to take statins due to dangers they've read about online. 

Yet thousands of us are willing to try an untested drug sent in the post from a dodgy social media account. It really demonstrates that people will go to any length to lose weight.

And it is another reminder that it is a scandal that licensed weight-loss drugs are still so hard to get on the NHS. More than two million Britons now pay for them privately – but fewer than 220,000 are getting them on the NHS. 

If the Government doesn't fix this widening inequality, we will end up with many more patients like Jane – driven by a desire to be healthier, and ending up deathly unwell.