A 47-year-old man has revealed how medics missed the early warning signs he had a devastating cancer as they assumed they were just side effects of a weight loss jab.

The unnamed man was taking Mounjaro—dubbed the ‘King Kong’ of weight-loss injections due to its potency—when he began losing too much weight too fast.

He has now been told he has stage 3 colon cancer, an advanced stage of the disease, that kills nearly half of patients within five years of their diagnosis. 

The man claimed medics initially dismissed ‘red flag’ signs of the condition like persistent constipation and nausea as simple side effects of Mounjaro. 

Sharing his story in a Reddit forum for patients taking the drug he urged fellow users not to dismiss worrying changes to their body. 

‘Mounjaro masked the real cause of my rapid weight loss. Looking back, there were red flags. I brought them up with my doctor, but even they chalked it up to side effects,’ he wrote. 

‘Please—listen to your body. Even if the numbers make sense on paper, trust how you feel.’

He made it clear he doesn’t suspect the drug caused his cancer, saying that he simply wanted to share his story as a cautionary tale for other patients. 

The unnamed man was taking Mounjaro—dubbed the ‘King Kong’ of weight-loss injections due to its potency—when he began losing too much weight too fast

The man said he originally started taking Mounjaro about two years ago when he weighed just over 19st (121kg).

While he enjoyed some ‘slow and steady’ success in the first year-and-half—losing 12lbs (5kg) in the first year and then 15lbs (7kg) in six months—he recalled how his weight loss then suddenly accelerated.

He said he lost 30lbs (14kg) in just two months, and this was followed by further rapid weight loss that made him technically underweight, with him now weighing around 9st.

The man said it was at this point, about eight months ago, that medics told him to stop taking Mounjaro.

But he added that despite seemingly suffering side effects and continuing to lose weight in this time, medics said this was still ‘the lingering effects of Mounjaro’.

It was only a recent colonoscopy—where a thin flexible camera is inserted into the rectum—that revealed the real cause of his symptoms, stage 3 colon cancer. 

The man said looking back there were serious signs something was wrong beyond Mounjaro side effects that shouldn’t have been ignored.  

‘The biggest red flag was constipation—except I had none of the usual symptoms. Life was so busy, I didn’t think much of it…until one day I realized I hadn’t had a bowel movement in almost a month. No pain, no discomfort—just nothing,’ he wrote.

Bowel cancer can cause you to have blood in your poo, a change in bowel habit, or a lump inside your bowel which can cause an obstruction. Some people also suffer from weight loss as a result of these symptoms

‘The second red flag was extreme fatigue. I went from going to the gym three times a week to barely having enough energy to walk to the mailbox.

‘The third red flag? Sudden waves of nausea—for no clear reason.

‘The fourth? I completely lost my appetite. No cravings, no hunger—just an emptiness I kept brushing off. Again, I thought it was because of the medication.’

Mounjaro—like other drugs Wegovy and Ozempic—belong to class of medications collectively known as ‘GLP-1′ injections.

For weight-loss they work by mimicking a hormone of the same name which governs our sense of appetite.

While helping patients slim down the drugs commonly cause digestive issues like constipation, nausea and stomach pain in more than one in 10 people, especially when they first start taking the drugs.

Unfortunately, these are also classic warning signs of bowel cancer, of which colon cancer is a type. 

Unexplained weight loss is also considered a serious sign of the disease, but in this case the man said he thought it was simply Mounjaro helping him slim down.

The man said in hindsight he should of questioned why the drug suddenly seemingly became more effective, but added he had been enjoying the weight-loss too much to look into it more deeply. 

He added that he was currently undergoing chemotherapy and that medics have said his prognosis is ‘favourable’. 

Other members of the forum were quick to highlight that medics need to not let the fact a patient was taking Mounjaro overshadow potential signs of cancer. 

One wrote: ‘This is why doctors need to learn so much more about these meds so they spot red flags like this and don’t brush it off.’

Another added: ‘I can see how this med masked the diagnosis for you, but again your doctor should have at least considered something else was going on and evaluated you further.’

Ironically, some studies have suggested GLP-1 medications could actually ward off colon cancer.

One recent piece of research found taking the drugs was linked to a 16 per cent lower risk of colon cancer.

And a 2023 study found that patients who took the jabs had an 44 per cent lower risk of getting colorectal cancer, which includes colon cancer, compared to diabetics treated with insulin.

While part of this effect may be due to helping patients lose weight—with obesity increasing the risk of developing colon cancer—experts have also observed a reduction in cancer risk in non-obese patients. 

Exactly how the drugs may be producing this effect in patients of a healthy weight is still under investigation. 

The case comes amid concerns about rare but potentially deadly side effects of GLP-1 medications, as ministers plan to increase the roll-out of the jabs to combat obesity. 

Last month, the UK medicines regulator launched a probe into the safety of fat jabs after hundreds of users developed pancreatitis, a dangerous inflammation of the pancreas, leaving 10 dead. 

Recent estimates suggest that about 1.5million people in the UK are taking weight loss jabs, many of which are bought privately due to NHS rationing. 

The unnamed man isn’t alone in being hit by a devastating bowel cancer diagnosis as a young adult.

Experts have noted a concerning and mysterious rise in rates of the disease among young adults, defined in cancer terms as those under 50. 

A recent global study found rates of bowel cancer in under 50 year-olds are rising in 27 of 50 nations.

England is averaging a 3.6 per cent rise in younger adults every year, one of the highest increases recorded.

While the disease is known to be linked to obesity, experts have noted that the disease also seems to also be occurring in fit and healthy patients. 

Some experts believe the explanation must lie in environmental factors young people have been exposed to more than previous generations. 

There are around 2,600 new bowel cancer cases in people aged 25-49 in Britain every year, and around 44,100 new cases among all ages.

Around 44,000 cases of bowel cancer are diagnosed every year in the UK, with about 130,000 in the US.

The disease kills almost 17,000 Britons each year, with the death toll rising to about 50,000 in America.

Overall, just over half of bowel cancer patients are expected to be alive 10 years after their diagnosis.

However, survival rates vary vastly on when the disease is diagnosed.

If the cancer is found in its earliest stages 90 per cent of patients will still be alive five years after their diagnosis.

But, if found later, when the cancer has started to show signs of spreading to more distant tissues—stage 3—this drops to just 65 per cent of patients. 

Anyone with symptoms of bowel cancer for three weeks or more, even if minor, should contact their GP for advice.

While most likely not cancer, an early diagnosis—when the disease is typically easier to treat—can vastly improve outcomes for patients.