Warfarin Institute of America
Dedicated To Your Health
Possible Effect of Raloxifene on Warfarin
Brand Names: Raloxifene is Evista. Levothyroxine is Synthroid, Levothyroid and it has many other names.
PLEASE NOTE CAREFULLY THAT THIS PAGE IS NOT ABOUT RALOXIFENE HAVING A DIRECT EFFECT ON WARFARIN. IT APPEARS THAT THIS WILL ONLY HAPPEN IF YOU TAKE BOTH RALOXIFENE AND LEVOTHYROXINE AT THE SAME TIME. THE TIME OF DAY THAT YOU TAKE WARFARIN WOULD NOT MATTER.
If this is true, it could go a long way toward explaining why warfarin is so difficult to manage. This is not an interaction with warfarin itself but a possible interaction between two drugs, one of which is known to interact with warfarin. To make matters more difficult, it only occurred when the two medications were taken at the same time, not when they were taken at different times of the day. It is important to netw that the woman in the case presented was not taking warfarin, but this case could apply to people who take all three drugs.
Siraj et al (Arch Intern Med. 2003;163:1367-70) report on a 79-year-old woman who was taking levothyroxine and had raloxifene added to her medications. She started taking these two medications at the same time of the day. Shortly after this, her low-thyroid condition was out of control. On two occasions, the doctors had her separate the raloxifene and levothyroxine doses by twelve hours. When this was done, her thyroid condition came back into control. They also tested her blood for levels of the levothyroxine and discovered that it was not being absorbed very well when it was taken at the same time as raloxifene.
So what are the implications for people taking warfarin? Levothyroxine reacts with warfarin in a dose-dependent relationship. If you lower the dose of levothyroxine, the INR will go down so more warfarin will be required. If you raise the dose of levothyroxine, the INR will go up so less warfarin will be required. If what happened in this case happens to everyone who takes raloxifene, levothyroxine and warfarin then there are many implications.
1. Starting raloxifene in someone already taking warfarin and levothyroxine could upset a stable INR, particularly if they were taken at the same time.
2. Not being consistent in the times that levothyroxine and raloxifene were taken could affect the INR.
3. Forgetting to take raloxifene or running out of it could affect the INR, even if the warfarin dose were constant. This could be especially critical in a person who runs out of raloxifene a few days before they can afford to get a prescription refilled. The INR could then be off, causing an adjustment of the warfarin dose which would then be off again if the raloxifene were restarted.
This makes me wonder if there are other drugs, vitamins, herbals, supplements etc that interact with something that interacts with warfarin causing the "unexplainable" INR fluctuations often seen in warfarin patients. Possibly something as simple as skipping a supplement could throw off the warfarin as much as a week later. It really illustrates why consistency is the name of the game in warfarin management.
THE IMPORTANT MESSAGE HERE IS THAT YOU SHOULD NOT TAKE RALOXIFENE (EVISTA AND LEVOTHYROXINE (SYNTHROID/LEVOTHYROID) AT THE SAME TIME. TAKE THE LEVOTHYROXINE IN THE MORNING AND THE RALOXIFENE AT NIGHT. THE TIME OF DAY THAT YOU TAKE THE WARFARIN WILL NOT BE IMPORTANT.
To read about the effects of levothyroxine dosage changes on warfarin, click here
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Last updated November 20, 2004