In the largest study to date of the potential for drug-drug interaction to reduce tamoxifen’s protection against breast cancer recurrence, we examined whether Danish breast cancer patients with estrogen-receptor positive tumors who were treated with tamoxifen for one, two, or most often for five years had a higher rate of recurrence if they were concomitantly taking the SSRI citalopram or its s-stereoisomer (“citalopram” from here forward) than if they were not. The gene discussed is ESR1; the disease is breast cancer.