Cimetidine inhibits cytochrome P450 and reduces hepatic blood flow. As a result it inhibits the metabolism of OCP's and enhance their blood levels and increases their effectiveness. Phenytoin and Griseofulvin induces cytochrome P450 enzymes and thereby enhance the metabolism of oral contraceptive pills. Ampicillin suppresses the intestinal flora and decreases enterohepatic circulation of OCP. Ref: Essentials of Medical Pharmacology By K D Tripathi, 5th Edition, Page 590