You're talking about Tamoxifen and Paxil used at the same time, yes? It has been proven that certain SSRI's which is what Paxil is do stop the benefits of tamoxifen.
From Pharmacist's Letter's: July 2009, vol 25, no. 7
"You'll see new warnings not to combine tamoxifen with certain SSRIs..paroxetine, fluoxetine, and sertraline. .... Women have a higher risk of breast cancer recurrence if they take tamoxifen along with paroxetine, fluoxetine, or sertraline...