Some antibiotics make it hard for your body to absorb the hormones in your birth control pills. Other antibiotics make your body get rid of the hormones faster. If either happens, the lower levels of hormones in your body may allow you to get pregnant when you do not want to. Antibiotics that may do this include:
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rifampin;
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penicillins, such as penicillin, ampicillin and amoxicillin;
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tetracylines, such as tetracyline, minocycline and doxycycline;
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co-trimoxazale (trimethoprim and sulfamethoxazole);
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trimethoprim; and
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others.
Other drugs may also make your birth control pills NOT work.
When you have to take prescription drugs, keep taking your birth control pills (unless your doctor tells you not to) BUT also use a barrier form of birth control. You can use:
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a condom;
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a diaphragm; or
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a cervical cap.
If you use a barrier form of birth control as well as the pill when you are taking antibiotics, you will reduce the risk of getting pregnant by accident.
Continue to use the barrier form of birth control for 7-14 days after you finish the antibiotics.
You can read more here.
http://www.brown.edu/Student_Services/Health_Services/Health_Education/sexual_health/ssc/bcps.htm
There are also some studies that state antibiotics will not interfere with BCP's. So it's considered a controversy topic. Either way.. if you are not wanting to become pregnant, take other means while on antibiotics.
Good luck, if you need anything else please feel free to ask.