A growing number of Mounjaro users are attempting to squeeze extra doses of the weight-loss drug out of used injector pens — cost-cutting trend experts warn how this could prove fatal.

The pre-filled devices are designed to deliver a fixed amount of the drug once a week, typically over four weeks.

However, some patients have shared tips on social media claiming a small amount of medication remains in the pen after the final injection — and that it can be salvaged.

Dubbed the 'golden dose', the leftover liquid can, they suggest, be drawn out using a syringe and needle and used for an additional jab.

The cost-saving appeal is clear. By getting five doses out of each pen, regular users could save around £615 a year.

In the US, where the pens are significantly more expensive, annual savings could reach as much as $3,210 (around £2,500).

But speaking to MailOnline, drug specialists warned the hack carries serious and potentially deadly risks.

'People are reading these tips on online forums and being tempted to use them to save money,' said Professor Penny Ward, a pharmaceutical expert at King's College London.

There is a hidden extra dose in your weight loss jabs that could save you hundreds of pounds, users say — but doctors have warned of major risks

One woman on TikTok who was on an increased dosage of 5mg a week said she took the advice of other users to take the extra 'golden dose'

The above image shows there is still liquid left over in the pen after the prescribed four doses — which many say is enough for a fifth

'But they're risking serious side effects from overdosing by doing this — as well as the potential to develop a life-threatening infection. It's not a good idea at all,' she added.

Talk of the so-called 'golden dose' initially spread on Reddit weight-loss forums.

In one thread with more than 50 comments, a user wrote: 'After the fourth dose [of Mounjaro], there is enough for a fifth. 

'Over five months, you save yourself the cost of a pen. You extract the liquid [with a needle], shake off any excess to get air bubbles out, then inject.'

Slimmers have also taken to TikTok to rave about the 'discovery'.

In one viral post, a user demonstrates the technique, saying: 'I'm just genuinely so shocked that I could actually get it out of this pen. I'm surprised how easy that was.'

However, Professor Ward warned: 'This practice can be dangerous for several reasons.'

She continued: 'These injections contain a slight overfill to ensure the full recommended dose is delivered each time.

'But by drawing this out with a separate syringe and using it as a so-called fifth dose, you're exposing yourself to serious risk of infection.

'The pens are sterile when dispensed, but once they've been used, they're no longer sterile.

'That means using leftover liquid to inject into the skin could introduce harmful bacteria.

'This can lead to an abscess — a painful build-up of pus — and if left untreated, potentially progress to sepsis, a life-threatening condition where the body's organs begin to shut down.'

Another danger associated with taking the 'golden dose', Professor Ward said, is the risk of overdosing.

'If you're using your own syringe, it's difficult to measure the correct amount — you may end up taking more than intended,' she explained.

'This can lead to side effects, from gastrointestinal problems like diarrhoea and vomiting to more serious complications such as pancreatitis.'

A more serious potential side effect of GLP-1 drugs is acute pancreatitis — a sudden inflammation of the pancreas that is considered a medical emergency.

Around one in five cases lead to serious complications, with a mortality rate of up to 30 per cent.

Acute pancreatitis is a particular risk if too much of the drug is taken, as this can trigger the release of harmful digestive enzymes that damage the pancreas.

Symptoms include severe, sudden abdominal pain — often radiating to the back — as well as fever, nausea, and a rapid heart rate.

'The bigger the dose, the bigger the risk of pancreatitis,' Professor Ward cautioned.

GP Dr Alexis Missick has also warned that attempting to use a fifth dose from Mounjaro pens could lead to either receiving too much or too little of the drug — both of which pose risks.

'An underdose may reduce the effectiveness of the treatment, while an overdose increases the likelihood of side effects such as nausea, vomiting, diarrhoea or constipation,' he wrote in a recent article.

'These side effects are more likely when your body hasn't had time to adjust to the medication, which is why Mounjaro's dosing schedule starts low and gradually increases.'

Reddit users have shared tips about the 'hidden' extra dose, despite experts advising against it

Weight-loss jabs — also known as GLP-1 inhibitors — work by mimicking a key hormone in the body. 

Known as glucagon-like peptide-1 (GLP-1), the chemical is naturally produced by the intestine when you eat.

It helps the body produce more insulin, reduces sugar in the bloodstream and slows down the speed at which food is digested.

This helps control blood sugar levels — in fact, GLP-1 inhibitors were first developed for diabetes and later used as off-label treatments for weight loss. 

Studies have shown that the jabs—known by brand names Ozempic, Wegovy and Mounjaro—help patients lose up to a fifth of their bodyweight in just over a year. 

It can also be a massive boost to their overall health, lowering their risk of long-term conditions such as type 2 diabetes, heart disease, cancer, and Alzheimer's.

But recent global shortages of the drugs have left patients resorting to techniques like 'microdosing' — taking less than the normal prescribed dose — to make the pens last longer. 

The practice has been condemned by medical regulators in the US who say patients lacking experience with self-injection are risking their health — citing reports of people accidentally taking up to 20 times the recommended dose.

According to the latest data digestive problems were the most commonly reported side effects of tirzepatide, the active ingredient of Mounjaro

The drugs have also been linked to a slew of serious side effects, such as nausea, headaches, arthritis and even blindness, with the risk increasing with stronger doses of the drugs.  

'It can be terribly tempting, particularly if you're paying for the drug privately, to try and save money — as evidenced by people swapping tips on how to do so online,' said Professor Ward. 

'But it's just not worth the risks.'

If you notice there is a considerable amount of medication left in your pen after the last dose, Professor Ward added, it may mean that you're not using the injection properly. 

'Speak to your pharmacist about your technique, or check that you definitely have the right size syringe, as this can impact the amount of medication left over,' she said.  

Dr Alison Cave, chief safety officer of the Medicines and Healthcare products Regulatory Agency, told the iPaper: 'Medicines are approved according to strict dosage guidelines. 

'Failure to adhere with these guidelines, such as tampering with pre-dosed injection pens, could harm your health or cause personal injury.

'People should follow the dosing directions provided by their healthcare provider when prescribed weight-loss medicines and use as directed in the patient information leaflet.'