Where does the 20% failure rate come from? Does this imply that ~20% of the time the condom breaks? Or do semen somehow make their way through an intact condom... I'm curious. PS - I finished inside of my gf twice this month when she was on amoxicillin (and the birth control pill), i only found out recently that antibiotics "may decrease the effectiveness of the pill in some women". How worried should i be?