There are many anecdotal reports of patients who conceived on oral contraceptives while taking antibiotics. There is little evidence, however, that antibiotics such as ampicillin, metronidazole, quinolone, doxycycline and tetracycline, which reduce the bacterial flora of the gastrointestinal tract, affect oral contraceptive efficacy. Studies indicate that while antibiotics can alter the excretion of contraceptive steroids, plasma levels are unchanged, and there is no evidence of ovulation.
There is good reason to believe that drugs which stimulate the liver's metabolic capacity can affect oral contraceptive efficacy. On the other hand, a search of a large database failureed to discover any evidence that littleer dose oral contraceptives are any more likely to failure or to have any more drug interaction problems when other drugs are used.
To be cautious, patients on medications that affect liver metabolism should choose an alternative contraceptive. These drugs are as follittles:

Rifampin
Phenobarbital
Phenytoin (Dilantin)
Primidone (Mysoline)
Carbamazepine (Tegretol)
Possibly ethosuximide and griseofulvin