It is common to hear that antibiotics, including penicillin and ampicillin, have the potential to interfere with the effectiveness of birth control pills; however the only antibiotic shown to decrease effectiveness of the pill in studies is rifampin.

As for other antibiotics, the evidence for birth control pill being less effective when you take other antibiotics is less clear.

In theory, the penicillin and tetracycline category of antibiotics, as well as some others, could impair the effectiveness of the pill. Early studies done 15 to 20 years ago found evidence of such an interaction; however, more recent studies have been inconclusive. This is because there doesn’t appear to be an increased rate of pregnancy when taking common antibiotics while on the birth control pill.

Luckily, rifampin isn't commonly prescribed in the United States (it is usually used to treat tuberculosis). Researchers are continuing to investigate the matter, and it is not necessary to use a back up method of birth control while on antibiotics. This is because antibiotics don't lower the levels of hormones in your blood to the point that you would be at risk for pregnancy.

The exception would be if you are taking rifampin (as noted above) or if your doctor has diagnosed an underlying medical condition that interferes with absorption.
Now, if you are vomiting and unable to keep food or pills down because of your illness, you may end up missing your daily dose of birth control. Whether or not you're on antibiotics, if you miss one of your pills due to vomiting (or if you miss a pill for any other reason), you'll want to call your health care provider to get their recommendation as to whether and for how long you need back up.
Here are some other guidelines to keep in mind when using antibiotics:

• Always take the entire dosage prescribed by your doctor. This means that if you are given a seven day prescription and you feel better after two days, continue to take the antibiotics for the full seven days to insure that the infection is cleared.
• Avoid alcohol when you are on antibiotics, as there is an increased possibility of getting physically ill with this combination.
• Talk to your doctor about any possible side effects, and monitor your symptoms carefully.