There is a lot of conflicting information surrounding this issue. In particular, your prescription insert will list "antibiotics" as decreasing pill effectiveness. However, unless your antibiotic or other medication is listed below, it does not reduce the effectiveness of the pill and does not require that you use a back up method, contrary to the product labeling.
The following medicines WILL DECREASE the effectiveness of the birth control pill: Topamax, Lamictil, Tegretol, Nevirapine, Trileptal, Phenobarbital, Dilantin, Mysoline, Rifampin, St. John's Wort, Provigil, Ethosuximine, Griseofulvin, Troglitazone, and Vigabatrin.
The following medicines will NOT DECREASE the effectiveness of the birth control pill: Ampicillin, Biaxin, Cipro, Doxycycline, Diflucan, Zaratontin, Keppra, Sabril, Zonegram, Lyrica, Klonopin, and Bafitril.