The Mounjaro and Zepbound prescribing information includes a specific instruction: people using oral hormonal birth control should switch to a non-oral method, or add a barrier method, for 4 weeks after starting the peptide and for 4 weeks after each dose increase.

The mechanism is documented in pharmacokinetic language. A single 5 mg dose of tirzepatide reduces the peak concentration of ethinyl estradiol (the estrogen in many birth-control pills) by about 59%. That number is not a pregnancy-risk percentage.

Semaglutide labels describe delayed gastric emptying and the potential to affect absorption of oral medications, but they do not carry the same explicit oral-contraceptive backup instruction.

"GLP-1 medications affect birth control" collapses two different label situations into one. For tirzepatide, the 4-week backup window is real and labeled. For semaglutide, the same oral-contraceptive instruction is not on the label, even though oral-medication absorption remains a label caveat. Neither label states a pregnancy-risk percentage.