Wegovy and Mounjaro: why weight loss drugs may affect oral contraceptives

Weight loss drugs, including Wegovy (semaglutide) and Mounjaro (tirzepatide), are growing in popularity among people seeking to lose weight. However, an increase in reports of so-called “Ozempic babies” has prompted the UK’s medicines regulator to publish advice for women of reproductive age using them.

The guidance follows 40 reports to the agency of unintended pregnancies in women taking a weight loss drug. A key concern is the potential effect of these medicines on how well oral contraceptives work.

Weight loss injections, including semaglutide and tirzepatide, mimic GLP-1, a hormone naturally released by the gut after eating. Among its roles is reducing appetite. Tirzepatide also targets GIP, another naturally occurring hormone system that is likewise linked with appetite suppression.

These drugs curb appetite through several mechanisms. They first inhibit areas of the brain involved in hunger, counteracting the appetite increase that commonly accompanies weight loss. GLP-1 drugs also delay the rate at which food leaves the stomach.

There is currently little published research examining interactions between GLP-1 drugs and oral contraceptives. Yet their effect on stomach emptying may partly account for why the oral contraceptive pill may be less effective than anticipated.

Weight loss drugs and oral contraceptive absorption

A 2024 study found that tirzepatide lowered bloodstream levels of ethinylestradiol - a synthetic form of oestrogen used in the combined oral contraceptive pill - by 20%. It also extended the time needed for ethinylestradiol to be completely absorbed into the bloodstream by two to four hours.

Lower absorption can reduce the drug’s ability to suppress activity in the female reproductive system, potentially affecting its contraceptive effect. The impact of semaglutide on ethinylestradiol absorption was notably less marked.

The longer time to complete absorption is likely to result from slower gastric emptying, as ethinylestradiol is mainly absorbed in the small intestine. Why these effects are stronger with tirzepatide than semaglutide is still uncertain.

One study did, however, show that although both drugs affect gastric emptying to a similar extent, the effect persists for far longer with tirzepatide.

Other possible factors

Vomiting and diarrhoea are two frequently reported side-effects of GLP-1 drugs, affecting 12% and 23% of people taking tirzepatide respectively. Both can disrupt the absorption of oral medicines of all kinds, including contraceptives.

This may happen because a medicine can leave the body before it has had the chance to enter the bloodstream. To prevent unintended pregnancy, people using the contraceptive pill are advised to use a back-up contraceptive if they vomit or have diarrhoea.

Weight loss itself may be another explanation for the association between GLP-1 drug use and unintended pregnancy, because it can affect fertility.

Obesity has long been linked to lower fertility and may worsen other fertility-related conditions, including polycystic ovary syndrome, a hormonal disorder that affects ovarian function.

Weight loss resulting from GLP-1 drugs is likely to increase fertility. This could make pregnancy more likely, regardless of whether women are using oral contraceptives.

Contraception advice for semaglutide and tirzepatide users

At present, other contraceptive methods do not appear to be affected by GLP-1 weight loss drugs. Non-oral options, including intrauterine devices (IUDs), transdermal patches and implants, are unlikely to be influenced because their active ingredients enter the bloodstream without relying on the gastrointestinal tract. Physical barrier methods, such as condoms, and copper IUDs are also unaffected.

Women who use an oral contraceptive are nevertheless advised to use an additional non-oral contraceptive method, such as condoms, for four weeks after beginning semaglutide or tirzepatide. This is the period when side-effects are usually most severe.

As evidence on the safety of these medicines in pregnancy is lacking, women who become pregnant while taking a weight loss drug are advised to speak with their doctor about alternative medication.

Simon Cork, Senior Lecturer in Physiology, Anglia Ruskin University

This article is republished from The Conversation under a Creative Commons licence. Read the original article.