Blockbuster weight loss jabs could make contraception less effective, and prove harmful to unborn babies, according to an urgent warning from UK drugs chiefs.

In a new alert, the Medicines and Healthcare products Regulatory Agency (MHRA) urged women using any slimming injection including Ozempic and Wegovy to use birth control, due to potential risks to both the pregnant woman and the foetus. 

Meanwhile, those using the ‘King-Kong’ of weight loss jabs, Mounjaro, were told to ‘double-up’ their methods of contraception if using The Pill.

‘Mounjaro may reduce the effectiveness of oral contraceptives in those who are overweight,’ the alert read, adding that safe sex is ‘especially important for the four weeks after starting Mounjaro and after any dose increase.’   

Officials added that women who become pregnant while using the medications must cease taking them immediately.

Those who are breastfeeding should the injections stop too. 

‘There is not enough safety data to know whether taking the medicine could cause harm to the baby,’ the MHRA said.

The regulator added that, to date, it had received more than 40 reports relating to pregnancy among women on the drugs.

The warning comes after numerous reports of women having unplanned pregnancies while on jabs like Ozempic, Wegovy and Mounjaro, despite using contraception

Deb Oliviara revealed how she became pregnant while taking Ozempic. She said she stopped taking it as soon as she realised

Of these, 26 were related to Mounjaro, one of which was for an unintended pregnancy.

Eight related to semaglutide, the active ingredient in both Ozempic and Wegovy. 

The reports related to pregnancy, recorded by medics and patients, include problems such as birth defects, miscarriages, and unplanned pregnancies.

The regulator added in some cases women should continue to use contraception for up to two months after stopping the medications before trying to get pregnant.

Mounjaro, which boosts levels of appetite-supressing hormones, is known to make The Pill less effective due to disruption it causes to the digestive system, flushing out the contraceptive.

Patients taking weight-loss injections are already advised to use contraception, and to stop taking the drugs if they get pregnant, in leaflets that come with the drugs. 

Some 35,000 British women of child-bearing age were prescribed the jabs on the NHS in England in 2024, official figures suggest.

However, this is likely to be an underestimate, given that many acquire the drug privately. 

The MHRA alert also warned against buying the medicines from unregulated and non-medical sellers such as beauty salons or via social media.

Dr Alison Cave, MHRA chief safety officer, said: ‘Skinny jabs are medicines licensed to treat specific medical conditions and should not be used as aesthetic or cosmetic treatments.

‘They are not a quick fix to lose weight and have not been assessed to be safe when used in this way.

‘Our guidance offers patients a ‘one-stop shop’ for our up-to-date advice on how to use these powerful medicines safely.

‘This guidance should not be used as a substitute to reading the patient information leaflet or having a conversation with a healthcare professional as part of the prescribing process.’

Experts said it was suspected how weight-loss jabs work to help patients shed pounds, was also thought to be diminishing the effectiveness of contraceptives. 

Dr Channa Jayasena, an expert in reproductive health at Imperial College London, explained: ‘We think that the absorption of oral contraceptive pills may be reduced with GLP-1 drugs which slow down emptying of the stomach, though more research is needed to confirm this.’ 

He added, that the MHRA’s alert seemed ‘sensible’. 

‘We don’t know how harmful GLP-1 drugs are during pregnancy; however, we know that other forms of weight loss like weight loss surgery can increase chances of a miscarriage,’ he said.

‘So, women are advised to do all they can to prevent pregnancy while taking GLP-1 drugs.’

Professor Rebecca Reynolds, an expert in metabolic medicine at the University of Edinburgh, added that some studies had indicated the drugs could increase the risk of birth defects.  

‘There is hardly any available data from human studies to be able to advise if these weight loss drugs are safe in pregnancy. 

‘The data from animal studies suggests the potential for harm with low birthweight and skeletal abnormalities, though more evidence is needed to assess if there are risks of taking these drugs in pregnant humans.’

Reacting to the MHRA’s announcement Jasmine Shah, medication safety officer at the National Pharmacy Association, urged women using the drugs to take note.

‘Community pharmacies have been experiencing unprecedented levels of interest for weight loss injections,’ she said.

‘It is therefore important that regulations and guidance keep pace with this demand and that patient safety is put at the heart of everything we do.

‘Medicines are not like ordinary goods for sale; they must be handled with great care because they have the power to harm as well as to heal.’

There are dozens of reports of women experiencing unexpected pregnancies while taking weight loss injections. 

Both Wegovy and Mounjaro work by regulating patients’ appetite, resulting in people feeling full quicker, but Mounjaro is said to have an additional effect as it appears to change the body’s metabolism

Some of these are linked to the drugs helping people lose weight which in turn, boosts their fertility. 

However, there have been a smaller number whereby it is believed weight loss jabs have made contraceptive less effective.

One US woman, Deb Oliviara, who is based in the US, detailed how she became pregnant while using semaglutide.

‘It lessens the effectiveness of birth control and it also heightens your fertility,’ she said in a clip viewed nearly a million times.

She added she stopped using the drug immediately upon learning she was expecting. 

Another woman, who posted on a Reddit forum, revealed she had become pregnant while using Mounjaro, despite taking contraception. 

She shared an image of her positive pregnancy test and wrote that weight loss jabs ‘ make it hard for oral medications (like birth control) to be effective’.

Weight-loss injections belong to class of drugs known as GLP- agonists, which help encourage fullness by mimicking a natural hormone released after eating.

Some, like Mounjaro, also act on a second hormone involved in appetite and blood sugar control.

The alert comes as findings suggest taking Ozempic and similar drugs may raise the risk of a ‘silent cancer’. 

Reports of suspected side effects for drugs in the UK are logged under the MHRA’s ‘Yellow Card’ scheme. 

Officials use the same database, set-up in the wake of the 1960s thalidomide scandal, to track the safety of Covid vaccines.

Although impossible to prove, it allows doctors, pharmacists and patients to report adverse reactions believed to be caused by drugs used in Britain.

This can lead to them being reviewed, having warnings added to the labels or being taken off the market completely.