Why are birth control pills 91% effective?

That’s a good question, sometimes I believe it’s because other medications could be interfering with its mechanisms; also not to account for failure necessarily, it is to prevent a law suite. If the claim was 100% effective and for some unknown reason someone became pregnant, the company who manufactures and markets that contraceptive could be held liable.

I hope this helps :)

When use perfectly correct birth control pills can prove to be 99% effective but people often mess up their pill schedule. So birth control pills often prove to only be 91% effective because of people taking them at the wrong times or missing days and just overall inconsistency.