Slimmers who come off weight-loss injections such as Mounjaro or Wegovy can regain their lost pounds four times faster than those who stop conventional dieting and exercising, new research suggests.

Data published in the British Medical Journal suggests overweight people shed large amounts when using jabs – about a fifth of their body weight – but once they quit they regain 0.8 kg every month, on average.

That means they return to their pre-treatment weight in a year and a half.

“People buying these need to be aware of the risk of fast weight regain when the treatment ends,” warns investigator Dr Susan Jebb, from Oxford University.

She stressed that the findings come from medical trials rather than real life and more studies of the longer-term effects of new slimming jabs would be helpful.

The researchers looked at 37 studies with over 9,000 patients to compare the blockbuster weight-loss jabs with conventional dieting or other pills.

Only eight of the studies assessed treatment with the newer GLP-1 drugs, such as Wegovy and Mounjaro, and the maximum follow-up period in these studies was a year after medication stopped, so the figures are an estimate.

People who diet instead can expect to lose less weight than with the jabs, but afterwards weight creeps back on more slowly – perhaps around 0.1 kg a month – say the researchers, although it does vary.

The NHS recommends the jabs for people who are overweight with obesity-related health risks – not people who just want to get a bit slimmer.

And doctors should also prescribe lifestyle changes that include eating healthily and getting enough exercise to help people keep the weight off.

Many say the treatment should be considered life-long, given the risk of relapse.

People who have tried coming off the jabs describe it as “a switch that goes on and you’re instantly starving”.

One woman said: “It was like something opened up in my mind and said: ‘Eat everything, go on, you deserve it because you haven’t eaten anything for so long.'”

Dr Adam Collins, an expert in nutrition at the University of Surrey, says the way the jabs work in the brain and body might explain why weight regain is amplified once you stop taking them.

They mimic a natural hormone called GLP-1, which regulates hunger.

“Artificially providing GLP-1 levels several times higher than normal over a long period may cause you to produce less of your own natural GLP-1, and may also make you less sensitive to its effects.

“That’s not a problem when taking the drugs, but as soon as you withdraw this GLP-1 ‘fix’, appetite is no longer kept in check and overeating is far more likely.”

Going cold turkey is a real challenge, he says.

“This is further exacerbated if the individual in question has relied solely on GLP-1 to do the heavy lifting… artificially suppressing their appetite without them establishing any dietary or behavioural changes that would help them in the long run.”

According to latest best estimates, around 1.6 million UK adults have used these injections in the past year – mostly bought through private prescriptions rather than on the NHS.

An additional 3.3 million people say they would be interested in using “skinny jabs” over the next year, meaning one in 10 adults has either tried them or would like to, says Cancer Research UK, based on nationally representative surveys in the first quarter of 2025.

Use was twice as common among women compared to men, and more common among people in their 40s and 50s.

Prof Naveed Sattar from Glasgow University said the jabs could provide added health benefits by working fast to get weight down.

“It’s plausible that being lighter for even two to three years due to short-term use of the jabs could help slow damage to joints or the heart and kidneys. Larger and longer outcome trials will be needed to answer that question.

“Importantly, continued use of these medicines over three to four years enables people to maintain significantly lower weight than they would otherwise – a benefit not typically seen with lifestyle-induced weight loss, where many regain weight over time.”

GPs and specialist weight-management services cannot automatically prescribe Mounjaro and Wegovy, even if patients have already been prescribed it privately.

The drugs can be offered to those in greatest clinical need who meet certain criteria, such as having weight-related health problems.

Currently, there is no specified time limit for Mounjaro prescriptions on the NHS, while Wegovy can only be prescribed for a maximum of two years.

A spokeswoman for Eli Lilly, a pharmaceutical company that makes Mounjaro, said use of weight-loss drugs needed to be accompanied by healthy eating, physical activity and medical follow-up.

“When treatment is stopped, weight can return, which reflects the biology of the condition rather than a lack of effort.”

Pharmaceutical company Novo Nordisk, which makes Wegovy, said: “These findings highlight the chronic nature of obesity and suggest that ongoing treatment is necessary to maintain improvements in weight and overall health for patients, similar to the management of other chronic conditions such as diabetes or hypertension.”