Antibiotics & Birth Control: Potential Interactions & What You Need to Know

Question
I am 20 years old and have no children. My last period began the 19th of July. I have been on birth control pills since i was 16 as per the orders of my dermatologist when i began accutane treatments. He has since then prescribed numerous antibiotics to help my acne and recently put my on the drug Myrac. The pill bottle came with a warning stating that this drug may alter the effectiveness of birth control. Although i remembered to take the myrac twice daily for about a month, i recently forgot to take the myrac about a week ago and haven't taken it since. However, during this month that i was taking it as directed, i was also just starting a new pack of pills. Yesterday was my 17th day (i am slated this sunday for my period if all goes well) and i am concerned that since i had unprotected sex last night (my 17th day) that i may become pregnant due  to the effects of this antibiotic. Quite simply, how possible is it that this antibiotic prevented my birthcontrol from working and if so, is it possible that i may become pregnant from unprotected sex on my 17th day. Thank you in advance!

Answer
Although antibitics sometimes lower the efficacy of birth control pills, in actually very few antibiotics alter the effectiveness of the pill. I doubt that you have anything to worry about. If you have been taking birth control pills every day and your last period started on July 19, you should have no problem. If you were not taking birth control pills you would have ovulated on cycle day #14 (August 1) and could only have conceived if you had unprotected sex on July 31, August 1,2, or 3. It is doubtful that you would have ovulated as late as cycle day #17.