Antibiotics and Birth Control: Potential Impact & What to Watch For

Question
I am 19 years old, have no children, and have been on birth control for the last 8 months. My last placebo pills began on July 31st with bleeding beginning on the 2nd of August. I started my next set of placebo pills this past Sunday, August 28th and have not begun seeing my period yet. My question is: on August 8th I took a 2000 mg does of Ampicillin prior to a dental cleaning, and on August 23rd I took a 2000 mg dose of amoxicillin prior to a filling (for cardiac reasons.) I have had unprotected sex every weekend since the first dose and also on the 17th of August. Do I have any reason to worry that I might have become pregnant in this time? The ampicillin bottle had a warning label against it's altering birth control effects.

Answer
Some antibiotics will lower the efficacy of birth control pills. Therefore, we usually suggest that, if you are on antibiotics, use an additional method of birth control (condoms, diaphram). HOwever, in actually, Ampicillin or Amoxicillin has not been shown to lower the effectiveness of the pill. Therefore, since you did not miss any pills, you were still protected when you had sex. If you do not get your period when expected, wait an additional week and take a pregnancy test. If it is negative, you are not pregnant.