The pill, tirzepatide and unplanned pregnancies

Two effects that add up

The first is pharmacokinetic. On the tirzepatide label, a single 5 mg dose reduced the Cmax of ethinyl estradiol, norgestimate and norelgestromin by 59%, 66% and 55%, and AUC by 20%, 21% and 23%, with Tmax delayed by 2.5 to 4.5 hours¹. According to the label, the mechanism is delayed gastric emptying, and the effect is largest after the first dose and diminishes over time¹. With subcutaneous semaglutide, by contrast, a study in 43 postmenopausal women found no reduction in the bioavailability of the combined ethinyl estradiol/levonorgestrel contraceptive¹¹. Pharmacokinetic reviews agree that most GLP-1 drugs do not meaningfully alter oral contraceptives and single out tirzepatide as the exception¹²¹³.

The second is reproductive: GLP-1s may improve ovulatory function and menstrual regularity, particularly in women with obesity or polycystic ovary syndrome⁸. In a randomized pilot trial in infertile women with obesity and polycystic ovary syndrome (28 participants), the pregnancy rate per embryo transfer was 85.7% with liraglutide plus metformin vs 28.6% with metformin alone, despite similar weight loss in both groups⁷. The pooled evidence is more cautious: a 2026 Swedish meta-analysis of 11 trials in polycystic ovary syndrome judged it insufficient to draw conclusions about menstrual regularity¹⁵. A 2026 review discusses concerns about unintended pregnancies due to improved fertility⁸.

What labels and regulators say

The Zepbound label advises oral-contraceptive users to "switch to a non-oral contraceptive method, or add a barrier method of contraception for 4 weeks after initiation ... and for 4 weeks after each dose escalation"¹. In June 2025 the UK's MHRA advised that overweight Mounjaro users taking oral contraception should also use a non-oral method². The Wegovy label says to stop it at least 2 months before a planned pregnancy³. A 2026 clinical review proposes an 8-week washout for both semaglutide and tirzepatide, warns that vomiting and diarrhea could compromise oral contraceptive absorption, and considers non-oral hormonal and long-acting methods more reliable during treatment¹⁴.

What if pregnancy has already happened?

Animal studies point to possible fetal risks¹⁴. The available human data show no large teratogenic signal, but they cannot rule one out either:

• Nordic, US and Israeli cohort of women with type 2 diabetes (JAMA Intern Med 2024): major congenital malformations 8.3% with GLP-1s (938 pregnancies), aRR 0.95 (0.72–1.26) vs insulin⁴.
• Target-trial emulation with 3,572 pregnancies (Ann Intern Med 2026), continuing vs stopping the GLP-1 in the first trimester: major malformations PR 1.21 (0.83–1.82); the authors conclude the estimates are compatible with both no increased risk and clinically relevant differences⁵.
• Denmark (2026): preterm birth was raised only in users with diabetes (aOR 1.84 with semaglutide), not in weight-management users (aOR 0.71; 0.30–1.70), which the authors say is compatible with the underlying diabetes, rather than the drug, being the cause⁶.