Birth control has side effects but other medications can effect that. Antibiotics can render birth control's function as nonexistent. I'm actually surprised how many pharmacists don't mention this when people pick up their antibiotics--even when asked if the medication could have effect other medication. Really, I am speaking from personal experience. I once asked my pharmacist directly after my consult whether it could interact with my other medications.
He said, "No."
I said, "Um . . . what about my birth control?"
"Oh, well, yeah."
Considering I had just been at an alternative school and taught a class of teenage mothers and heard them discuss how many were pregnant because they didn't realize antibiotics negated birth control, it came as no surprise the local pharmacist was negligent. Though, I have sense learned no pharmacist ever mentions the causation between birth control, antibiotics and pregnancy.