Many women taking tamoxifen have been prescribed selective serotonin reuptake inhibitors (SSRIs), such as Paxil, to reduce hot flashes and other uncomfortable side effects or to treat depression. Recent research has shown that certain SSRI antidepressants interfere with the body's ability to metabolize tamoxifen. As a result, taking these medications can "cancel out" the cancer-preventing benefits of taking tamoxifen.

Here's how it works: Tamoxifen is broken down by 2D6 enzymes into a chemical called endoxifen. Endoxifen is the active agent that helps prevent hormone-positive breast cancer from recurring. It is 100 times more potent than tamoxifen itself.

If the enzymatic process that converts tamoxifen to endoxifen is interrupted – either because someone is a poor metabolizer or because they are taking SSRI antidepressants – the therapeutic effects of tamoxifen are significantly reduced.

If you are on tamoxifen and considering an SSRI antidepressant, it's important to work closely with your oncologist to determine treatments that won't compromise your breast cancer therapy.

Not all antidepressants affect the 2D6 enzyme in the same way, which means they don't all have the same affect on tamoxifen. If you are on Paxil, for example, and taking tamoxifen, you might talk with your doctor about switching to Effexor, which does not inhibit 2D6 enzymes.

Are you taking any other medications that may interfere with tamoxifen? See a list of drugs that inhibit the 2D6 enzyme, which is key to processing tamoxifen.