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There is a "caution advised" status for the interaction of these two medications because it may decrease the active metabolite levels of tamoxifen (the amount in the blood stream, which may in turn decrease it's efficacy). She needs to talk to the doctor who gave her the tamoxifen to see if they were aware of the concern, if they have any reason to believe the benefits of using the combo outweighed the risks, and if they have any reason to believe the tamoxifen has been working less strongly than expected.
Her doctor may also be able to help her understand the research behind this warning and help her better understand what the real risks are in her specific case. For example, the decrease in efficacy might not be related to the two drugs, but only when both drugs are present in someone who already has a poorly functioning liver. Or only under specific doses for each medication are these drops in efficacy starting to happen, if both medications need the same organs/chemicals/receptors in the body to work properly and there isn't enough resources to handle high doses of both medications.
Then after she's heard what her doctor has to say, she'll be in a better place to make new decisions (such as changing the paroxetine to something else, or seeking legal avenues).
With medication interactions it is always important to realize that it takes a small percentage of "problems" (usually 2% or higher of the population having a symptom/side-effect gets reported, even if they can't completely narrow down that the drug caused the issue in the first place) to get reported. She has most likely been given a warning (like the don't use hairdryer in shower warning), not a death sentence (hairdryers aren't lethal when used properly). If these medications were showing a common or strong interaction they would be listed a lot more strongly than "caution advised" by the FDA.
I would be happy with your answer if this was medication for something other than cancer but Tamoxifen is given to lower the risk of death i was told by the GP taking Tamoxifen would give me a 91% chance of survival as apposed 24% if i was to not take tamoxifen as paroxitine has absorbed all the Tamoxifen this will of given me a 91% chance therefore i need to know what steps to take next?
There are still conflicting research on this issue. One study found the effects mostly affecting older women (66+) after five years of taking both medications, another study found no link, and one study found a link of a higher occurrence rate of cancer after 2.5 years (7.5% recurrence with Tamoxifen and 15% recurrence with Tamoxifen + Paroxetine, which is still a much lower recurrence rate than no Tamoxifen at all). This is all new research (coming out within the last year) and still with very small groups of subjects involved. Therefore, it's really hard to know what the direct link is (also these studies aren't looking at different doses of paroxetine to find out if there is a safe dose-range yet).
There is new research that indicates that "weaker" SSRI's such as Celexa and Lexapro may not affect Tamoxifen. So the first thing would be to switch to one of those medications if you and your doctor believe that you do in fact need an SSRI.
As for the lawsuit, I'm not a lawyer, but I seriously doubt you can prove malpractice based on new research that is still considered "hypothetical" by the Journal of the National Cancer Institute. If this research continues to show links between paroxetine and other SSRIs significantly causing impairments of tamoxifen, then doctors will be at risk of malpractice. But right now, the decision still rests in your hands and your doctor's suggestions. For example, someone with severe depression that has not responded well to other antidepressants may decide that it's in their best interest to stay on paroxetine if it works, and to accept the decreased tamoxifen efficacy as a trade-off.
Talk to your doctor, see if you can switch to a new SSRI, if they refuse to do so then you have grounds to complain about negligence or malpractice. If nothing else, if they refuse to let you change drugs then it's time to find a new doctor!
But so far I can't find any research that says that paroxetine totally negates the use of tamoxifen. Only that it hypothetically could for some patients because of the underlying mechanism that allows the body to break down tamoxifen into it's useful metabolites.
So take it a serious issue (to change the SSRI or remove it), but really try to let go of the emotional panic. The research is just not that solid yet. But when it comes to your cancer treatment, everything is important to look at.