By Emily Stearn

Slimmers were today warned that weight loss jabs could trigger and unexpected and bizarre side effect: reckless behavior. Game-changing injections such as Ozempic and Wegovy, hailed by the likes of Elon Musk and Jeremy Clarkson, are proven to help people shed up to 20 pounds.

But according to experts, the treatment could cause some users to act 'out of character', engaging in risky sex, compulsive gambling or even making rash major life decisions such as filing for divorce.

They believe changes levels of the brain chemical dopamine, possibly linked to the jabs, may be behind the issue, contributing to the ever-growing list of downsides.

The researchers, from London, say patients should be warned about the potential of these bizarre reactions, known collectively as impulse-control disorder, before starting on the drugs.

Professor Raymond Playford, an expert in molecular medicine at the University of West London and the study's co-author said: 'We have come patients suddenly filing for divorce or splitting up from what seemed to be stable relationships and changing their home situation such as moving house on short notice.

'We have not seen excessive gambling or sexual activity in patients to date but we wouldn't be surprised if this was happening.'

The researchers said they don't know exactly why the drugs may cause this effect.

But Professor Playford added: 'Risky behaviours due to impulsivity is also associated with side effects of Parkinson's disease drugs.'

Weight loss jabs 'share the common mechanism of influencing brain dopamine levels', he added.

'It is well established that hypersexuality and excessive gambling is associated with taking evodopa'.

Dopamine is dubbed the 'happy hormone' for its role in feelings of happiness, pleasure and reward.

Research has shown at higher levels, people become hyperstimulated by everyday activities such as shopping, gambling, eating or sex, and then find themselves essentially 'addicted': having to repeat the behaviours over and over, seeking to replicate that initial thrill.

Currently, impulse control disorder is listed as a potential side effect in patient information leaflets for Parkinson's disease drugs such as levodopa.

No such warnings, however, are included on leaflets for slimming jabs.

Professor Playford said: 'We are advising that doctors also warn patients to look out for urges that are "out of character" or unusual with cost or personal repercussions — for instance divorce — and to "step back" and think whether this is a sensible decision.

'It may also be useful if they share the fact they are starting [drugs such as Ozempic] with a loved one or close friend, so they can give a cautionary note if they see something unusual.

'This warning should allow patients and doctors to reflect and consider whether the decisions some patients have been making are out of character and riskier than one would expect.

'If they are not aware it is a possibility then they don't make the association.'

Semaglutide (sold as Ozempic and Wegovy) and tirzepatide (Mounjaro) have been hailed as a monumental breakthrough in the war on obesity.

The pens, taken once a day, mimics a hormone called GLP-1 and tricks the brain and body into thinking it is full, curbing appetite.

Trials show semaglutide, manufactured by Danish firm Novo Nordisk, helps users lose up to 33lbs (15.3kg) on average in around a year.

Side effects of the jabs including bloating, nausea and acid reflux have long been noted by the government's drugs watchdog National Institute for Health and Care Excellence (NICE).

Increasing numbers of Ozempic users on social media have also complained of being left with gaunt facial features, sagging 'melted candle' skin, 'empty' breasts and hair loss – which are not thought to be a direct side effect of the medication but a consequence of dramatic slimming.

Analysis suggests the cost of tackling knock-on effects of the jabs on the NHS could run into more than £100million per year.

Others, meanwhile, have warned of bad breath and even sexual dysfunction.

Latest NHS data shows 26 per cent of adults in England are obese and a further 38 per cent are overweight but not obese.

Experts have pointed to a lack of exercise, and poor diets high in ultra-processed food, as being key drivers in the UK's obesity epidemic.

Wegovy was approved on the NHS last year, specifically for weight loss.

But eligibility criteria for people wanting the get the drug on the NHS — for the standard prescription rate of £9.90 in England — is strict.

Mounjaro was given the green light by NICE for NHS use in September for patients with type 2 diabetes who do not have the condition under control.

It is not yet used by the health service for obesity.

But in February was made available privately in Britain, with clinics charging around £40 for a week's supply.