But today marks the first official agency warning about the possibility of these drugs—specifically Mounjaro—decreasing the effectiveness of oral contraceptives.

“Women taking popular medicines for weight loss and diabetes, sometimes referred to as ‘skinny jabs,’ are being reminded to use effective contraception while taking these medicines and, in some cases, for up to two months between stopping the medicine and trying to get pregnant,” warns the U.K.’s Medicines and Healthcare products Regulatory Agency (MHRA)—akin to the U.S. Food and Drug Administration. 

It goes on to specify that “Mounjaro may reduce the effectiveness of oral contraceptives in those who are overweight. Therefore, those taking Mounjaro who are overweight and are using an oral form of contraception are advised to also use a non-oral form of contraception. This only applies to those taking Mounjaro and is especially important for the four weeks after starting Mounjaro and after any dose increase.”

The MHRA characterized its announcement as a “reminder,” as the decreased effectiveness of birth control pills is already one of several known risks included in Mounjaro’s package inserts, which includes the following warning with its product: 

“If you take birth control pills by mouth, talk to your healthcare provider before you use Mounjaro. Birth control pills may not work as well while using Mounjaro. Your healthcare provider may recommend another type of birth control for 4 weeks after you start Mounjaro and for 4 weeks after each increase in your dose of Mounjaro.”

A spokesperson for Eli Lilly, the maker of Mounjaro, emailed the following statement to Fortune: “Patient safety is Lilly’s top priority, and we actively engage in monitoring, evaluating, and reporting safetyinformation for all our medicines. The Summary of Product Characteristics for Mounjaro (tirzepatide) explains that tirzepatide ‘has the potential to impact the rate of absorption of concomitantly administered oral medicinal products’ and that the impact ‘is most pronounced at the time of tirzepatide treatment initiation.’ The tirzepatide Patient Information Leaflet advises that if you are pregnant, think you may be pregnant or are planning to have a baby, ask your doctor for advice before using this medicine.”

While Ozempic and Wegovy contain semaglutide and work by mimicking a hormone called GLP-1 that triggers an increase in the production of insulin, Mounjaro contains the active ingredient tirzepatide, and also acts on a second hormone involved in appetite and blood sugar control.

The MHRA warning calls out Mounjaro, specifically, regarding the possible disruption of birth control, but then goes on to remind women that none of the drugs should be taken during pregnancy, as the effects on a fetus are unknown. 

“These medicines must not be taken during pregnancy, while trying to get pregnant, or during breastfeeding,” reads the warning. “Anyone who gets pregnant while using them should speak to their healthcare professional and stop the medicine as soon as possible. This is because there is not enough safety data to know whether taking the medicine could cause harm to the baby.” 

Today’s warning was notable, said ABC News chief medical correspondent Dr. Tara Narula on Good Morning America, because it marks “the first time we’re really seeing a regulatory agency speaking out about this.” But, she added, “we’ve heard talk about this particularly on social media and online in this country women dubbing them ‘Ozempic babies.’”

Narula explained theories that the decreased effectiveness of birth control while taking any of these drugs is due to two factors: One, the fact that being obese or overweight, or having syndromes like PCOS can decrease your fertility, and that, with these drugs inducing weight loss, “you’re essentially changing your menstrual cycle, your ovulation, and increasing your chances of getting pregnant.”

The second issue, she explains, is that oral contraceptives work by getting absorbed into our body—but that GLP-1s work “by changing our gut motility,” which is how food and waste moves through the body, and can also sometimes cause nausea, vomiting, and diarrhea. “So you can put those two together and see how the GLP drugs changing your gut motility and potentially affecting absorption of oral contraceptive pills could cause you to also see an increase in fertility,” Narula said.

Regarding the other part of the warning, that the drugs should not be taken during pregnancy and should be stopped two months before attempting to conceive, she said, “We don’t study pregnant women in trials, it’s not considered safe. But what we do know from animal studies is that there can be some birth defects, growth restriction, even miscarriage.”

More on GLP-1 drugs:

• Most Americans who want to lose weight don’t want to take GLP-1 drugs like Wegovy and Zepbound, new survey says
• Exclusive: The COO of Hims & Hers was the third person to try their new weight loss drugs—and she’s ready to share her story
• Largest-ever Ozempic and GLP-1 drug study finds they lower risk of 42 health conditions, including heart attacks and Alzheimer’s disease