Failure of oral contraceptives occur when used with any of these except (AIIMS Nov 2009)
Oral contraceptive failure can occur with:
a. Enzyme inducers like phenytoin, phenobarbitone, rifampicin, carbamazepine etc
b. Drugs inhibiting the enterohepatic cycling e.g ampicillin, tetracycline etc.
c. Aspirin does not interfere with oral contraceptives.