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Simple Summary: Tamoxifen is prescribed to premenopausal patients with estrogen-receptor-positive breast cancers for a period of 5–10 years after cancer diagnosis. This prolonged treatment regimen, though effective at preventing cancer recurrence, is often associated with unwanted cognitive and affective symptoms. To understand the clinical side effects of tamoxifen, animal studies investigating the effect of tamoxifen on the brain must model the chronic nature of tamoxifen therapy. This study describes a novel method of tamoxifen administration in female rats conducive to long-term administration of tamoxifen. Blood samples from treated rats showed levels of tamoxifen similar to levels in humans. Brain samples revealed tamoxifen-induced changes of a neurotrophic factor in the hippocampus, a structure critical to cognitive and affective processing. This study, therefore, suggests a potential mechanism that may underlie the cognitive side effects reported in patients. Tamoxifen, a selective estrogen receptor modulator (SERM), is commonly used as an adjuvant drug therapy for estrogen-receptor-positive breast cancers. Though effective at reducing the rate of cancer recurrence, patients often report unwanted cognitive and affective side effects. Despite this, the impacts of chronic tamoxifen exposure on the brain are poorly understood, and rodent models of tamoxifen exposure do not replicate the chronic oral administration seen in patients. We, therefore, used long-term ad lib consumption of medicated food pellets to model chronic tamoxifen exposure in a clinically relevant way. Adult female Long-Evans Hooded rats consumed tamoxifen-medicated food pellets for approximately 12 weeks, while control animals received standard chow. At the conclusion of the experiment, blood and brain samples were collected for analyses. Blood tamoxifen levels were measured using a novel ultra-performance liquid chromatography–tandem mass spectrometry assay, which found that this administration paradigm produced serum levels of tamoxifen similar to those in human patients. In the brain, brain-derived neurotrophic factor (BDNF) was visualized in the hippocampus using immunohistochemistry. Chronic oral tamoxifen treatment resulted in a decrease in BDNF expression across several regions of the hippocampus. These findings provide a novel method of modeling and measuring chronic oral tamoxifen exposure and suggest a putative mechanism by which tamoxifen may cause cognitive and behavioral changes reported by patients. [ABSTRACT FROM AUTHOR]