 - Note for subjects using a hormonal contraceptive method: No formal drug interaction studies have been carried out with Rebif. As interferons have been reported to exert an inhibitory activity on hepatic microsomal enzymes, it is unlikely that the clearance of oral contraceptives would increase and result in decreased efficacy. In over 10,000 patient years of clinical trial experience with Rebif, there has never been any indication of an interaction with oral contraceptives. 