Epilepsy Meds, Breast Cancer & Tamoxifen

I've been on Dilantin or generic phenytoin for 21 years. My seizures are well controlled, I haven't had a grand mal seizure in more than 11 years, and that was when I tried to wean myself off the drug. Last July, I was diagnosed with breast cancer, had a bilateral mastectomy, chemo and radiation therapy. My oncologist has now prescribed Tamoxifen for 2-3 years. I asked about an interaction, and requested a phenytoin level check with my latest blood work. My neoro's office called me to reduce my dosage a little, and then I got a message from my onc's office that I'll need to change anti-convulsives because phenytoin reduces the effectiveness of Tamoxifen. I am certain that I'm not the first epileptic to get breast cancer. Has anyone else taken anticonvulsives with Tamoxifen? Which one(s)? Am I unreasonable in the feeling that my medical providors should have checked this out before putting it on me? My medical history is no secret. Thanks for your help!