Women starting the weight loss drug tirzepatide, known as Mounjaro, are advised to use additional contraceptive methods for at least the first four weeks, and for four weeks after any dose increase, according to official guidance.
The medicine slows stomach emptying, which can reduce the absorption of oral contraceptives, potentially making them less effective. The guidance recommends barrier methods, such as condoms, in addition to continuing the oral pill, or switching to a non-oral contraceptive for the entire time Mounjaro is used. Women changing their weight-loss medication or dose may also need to take similar precautions for four weeks after the switch.
Side Effects and Contraceptive Effectiveness
There is currently no evidence that other weight loss jabs, such as semaglutide (the main ingredient in Wegovy), interfere with oral contraceptives in the same way. However, a common side effect of almost all weight loss medications could still impact effectiveness.
Faculty of Sexual and Reproductive Healthcare guidance states: “Diarrhoea and vomiting are common side effects of the GLP-1 agonists and can reduce the effectiveness of the pill.” If vomiting occurs within three hours of taking birth control, the pill may not have been absorbed properly and should be treated as a ‘missed pill’. Similarly, severe diarrhoea lasting more than 24 hours should also lead to treating the last oral contraceptive as a ‘missed pill’.
Managing Missed Pills and Alternatives
Missing just one contraceptive pill can reduce its effectiveness, but the procedure to follow depends on the type of pill and where you are in the pack cycle. The information leaflet with the contraceptive pills provides instructions on missed pills, or a pharmacist can offer guidance.
Non-oral contraceptives, such as the coil, implant, injection, or patches, do not require extra precautions when used with weight loss medications. As the official guidance notes: “There is no reason to believe that GLP-1 agonists affect methods of contraception that are not taken by mouth, so it is okay to use any of these options.”
