If you take birth control pills and are starting Mounjaro or Zepbound (tirzepatide), the prescribing information says the pill may work less reliably for a while. It advises switching to a non-oral method, or adding a barrier method such as condoms, for 4 weeks after you start and for 4 weeks after each dose increase. The reason is digestive: tirzepatide slows stomach emptying, which can delay and blunt how much of the pill’s hormones reach your blood. Hormonal methods that are not swallowed are not expected to be affected, according to the label.
What the label says, and why
Tirzepatide is a modified peptide built on the sequence of GIP, a gut hormone, and it also acts on GLP-1 receptors. The UniProt record for GIP describes the natural hormone as a potent stimulator of insulin release, and the UniProt record for proglucagon notes that GLP-1 inhibits gastric emptying in humans.
The Mounjaro prescribing information and the Zepbound prescribing information both say tirzepatide may reduce the effectiveness of oral hormonal contraceptives because of delayed gastric emptying, and that this delay is largest after the first dose and lessens over time. That is why the backup window is tied to the start of treatment and to each dose increase, not to the whole time you are on the drug.
The labels also report the study behind the warning. When a combined pill containing ethinyl estradiol and norgestimate was taken with one tirzepatide injection, peak blood levels of the hormones fell by roughly 55% to 66%, overall exposure fell by about 20% to 23%, and the peak arrived 2.5 to 4.5 hours later. That was a drug-level study, not a count of pregnancies, but it is enough for the manufacturer to advise backup protection.
What counts as “non-oral,” and a wording difference to know about
The labels say hormonal contraceptives that are not taken by mouth should not be affected. By that wording, the warning would not cover options such as the patch, ring, injection, implant and hormonal IUD, as well as a copper IUD, which has no hormones. The consumer page from MedlinePlus is broader: it says tirzepatide may decrease the effectiveness of hormonal contraceptives including pills, patches, rings and injections, and that your doctor may suggest another type of birth control for 4 weeks after starting and after each dose change. Because the two sources differ, ask your prescriber which applies to your method rather than assuming.
Is this only a tirzepatide issue?
The specific 4-week backup advice appears in the tirzepatide labels. The Ozempic (semaglutide) prescribing information gives a more general caution that slowed gastric emptying may affect absorption of medicines taken by mouth. A 2026 review in the European Journal of Contraception and Reproductive Health Care adds that stomach side effects such as vomiting and diarrhea, which the tirzepatide labels list among the most common reactions, could in theory also compromise pill absorption, and suggests that non-oral and long-acting methods may be more reliable during treatment.
The same review notes that these medicines can improve fertility in some women with obesity and polycystic ovary syndrome, largely through weight loss and better insulin sensitivity. In other words, starting one of these drugs is not a time to count on being less fertile.
If pregnancy is possible or planned
The Mounjaro label says available human data are insufficient to judge the risk in pregnancy and that animal studies suggest possible harm to a developing fetus; MedlinePlus says to call your doctor right away if you become pregnant while taking it. If you are planning a pregnancy, talk with your prescriber ahead of time about when to stop. Useful questions for your next visit:
- Does my birth control method need a backup while I start or increase this medicine, and for how long?
- Would a non-oral or long-acting method suit me better while I am on it?
- What should I do about my pill if I have vomiting or diarrhea?
- If I want to get pregnant, when should I stop this medicine?
By The Everyday Brief · AI-assisted editorial. Drafted with AI research assistance; claims checked against the linked sources (fetched 2026-10-05 PT). Not medical advice.
Sources
- DailyMed: Mounjaro (tirzepatide) prescribing information
- DailyMed: Zepbound (tirzepatide) prescribing information
- MedlinePlus (U.S. National Library of Medicine): Tirzepatide injection
- DailyMed: Ozempic (semaglutide) prescribing information
- Dilbaz B, Ateş Ç. The effects of glucagon-like peptide-1 receptor agonists on fertility, contraception, and pregnancy: clinical perspectives. European Journal of Contraception and Reproductive Health Care, 2026.
- UniProt: Gastric inhibitory polypeptide (GIP) (P09681)
- UniProt: Pro-glucagon, including glucagon-like peptide 1 (P01275)