Weight loss drugs, including Wegovy (semaglutide) and Mounjaro (tirzepatide), are becoming increasingly popular among those trying to lose weight. But a rise in so-called "Ozempic babies" has led the UK's medicines regulator to issue guidance on their use by women of reproductive age.
Author
- Simon Cork
Senior Lecturer in Physiology, Anglia Ruskin University
The guidance comes after the agency received 40 reports of unintended pregnancies by women who had been using a weight loss drug. Of particular note is the effect that these drugs may have on the effectiveness of oral contraceptives .
Weight loss jabs (including both semaglutide and tirzepatide) act by mimicking the naturally occurring hormone GLP-1, which is released from the gut after we eat. One of the things this hormone helps to do is suppress appetite. Tirzepatide also acts on another naturally occurring hormone system called GIP, also known to suppress appetite .
The mechanism through which these drugs impact appetite is multifaceted. First, they inhibit regions of the brain associated with hunger. This suppresses the increase in appetite that occurs when people lose weight. GLP-1 drugs also slow how quickly food leaves the stomach.
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There's currently very little published literature which has investigated the interactions between GLP-1 drugs and oral contraceptives. However, the effect these drugs have on stomach emptying appears to at least partly explain why the oral contraceptive pill may not work as well as expected.
One 2024 study demonstrated that tirzepatide reduced the amount of ethinylestradiol (a synthetic form of oestrogen, which is a component of the combined oral contraceptive pill) in the bloodstream by 20%. It also increased the amount of time it took the ethinylestradiol to be fully absorbed into the bloodstream by two to four hours.
This reduced absorbency hampers the drug's ability to suppress the action of the reproductive system in women. This will affect its contraceptive effects. Notably, the effects of semaglutide on ethinylestradiol absorption were less pronounced.
The increased length of time it took the contraceptive to be fully absorbed is probably a consequence of reduced gastric emptying since ethinylestradiol is primarily absorbed in the small intestine . The reasons why these effects were more pronounced in tirzepatide compared with semaglutide remain unclear. However, one study showed that while both of these drugs affect gastric emptying to a similar degree, these effects are much longer lasting with tirzepatide.
Other possible factors
Two commonly observed side-effects of GLP-1 drugs include vomiting and diarrhoea - affecting 12% and 23% of patients taking tirzepatide respectively. Vomiting and diarrhoea have the potential to interfere with the absorption of all types of oral medications - including contraceptives.
This is because the drugs may be expelled from the body before they have an opportunity to be absorbed into the blood stream. People taking the contraceptive pill are advised to use a back-up contraceptive for this reason if they vomit or have diarrhoea to avoid unintended pregnancy.
Another factor that could explain the link between GLP-1 drug use and unintended pregnancy could be the effect that weight loss in general has on fertility.
Obesity has long been associated with reduced fertility . Obesity can also exacerbate other conditions which affect fertility - such as polycystic ovary syndrome , a hormonal disorder that affects how the ovaries work.
It is likely weight loss associated with taking GLP-1 drugs leads to an increase in fertility . This in turn could make women more likely to become pregnant - independent of whether they're using oral contraceptives or not.
So far, it doesn't appear that other forms of contraceptives are affected by GLP-1 weight loss drugs. Non-oral contraceptives, such as intrauterine devices (IUDs), transdermal patches and implants, are unlikely to be affected as their active ingredients are absorbed into the blood stream independently of the gastrointestinal tract. Likewise, physical barriers such as condoms and copper IUDs are also unaffected.
But women who use an oral contraceptive are advised to use an additional, non-oral form of contraception (such as condoms) for four weeks after starting semaglutide or tirzepatide. This is when side-effects are typically at their highest .
Because of a lack of evidence around the safety of these medications during pregnancy, women who do become pregnant while using a weight loss drug are advised to speak to their doctor to find alternative medications .
Simon Cork does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
