Warfarin Institute of America
Dedicated To Your Health
LEVONORGESTREL INTERACTION WITH WARFARIN
Brand names: Plan B, Norplant
Plan B is an oral, emergency contraception tablet. Norplant is an implantable, long-term contraception system. There is a case report of a woman who took Plan B tablets and had her INR rise from 2.1 to 8.1 over three days.
Norplant has been on the market for many years and even the case report cited above acknowledges that Wyeth, the manufacturer of both products, has received no previous reports of this interaction.
The authors of the case report do not state if they sought other sources of the increased INR. It seems rather odd that this interaction would happen with only two oral tablet doses and not when the device is implanted and functioning over months.
The authors of the case report also express some concern because there are proposals to make Plan B available without prescription.
Management of this interaction will be somewhat difficult because by the time the elevated INR is apparent, the seemingly offending agent (Plan B) will have been stopped for the past two days. My experience has been that there has been little harm done by someone having an INR of 8.1 for a day or two. Possibly, the prescriber of Plan B would want to have a woman hold one dose of warfarin when taking Plan B. This may prevent the INR from going very high. It has been four years since this report was published and there have been no others as of November 13, 2004, so it seems likely that there was another cause in this case.
Reference:
Ellison J et al. Apparent interaction between warfarin and levonorgestrel used for emergency contraception. Brit Med J 2000;321:1382.
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Last updated November 14, 2004