1. ¹Department of Epidemiology, Boston University School of Public Health, 715 Albany Street, Boston, MA 02118, USA
2. ²Department of Clinical Epidemiology, Aarhus University Hospital, Olof Palmes Alle 43, Aarhus N 8200, Denmark
3. ³Department of Medicine, Boston University School of Medicine, 715 Albany Street, Boston, MA 02118, USA
4. ⁴Department of Biostatistics, Boston University School of Public Health, 715 Albany Street, Boston, MA 02118, USA
5. ⁵Institute of Pathology, Aarhus University, Noerrebrogade 44, Aarhus C 8000, Denmark
6. ⁶Department of Surgery, Aalborg Hospital, Aarhus University Hospital, Hobrovej 18-22, Aalborg 9100, Denmark
7. ⁷On behalf of the Danish Breast Cancer Co-operative Group (DBCG)
8. ⁸Department of Oncology, Odense University Hospital, Sdr. Boulevard 29, Odense C 5000, Denmark

Correspondence: Dr TL Lash, Department of Epidemiology, Boston University School of Public Health, 715 Albany St., TE3, Boston, MA 02118, USA. E-mail: tlash@bu.edu

Received 29 April 2008; Revised 16 June 2008; Accepted 30 June 2008; Published online 29 July 2008.

Abstract

Tamoxifen remains an important adjuvant therapy to reduce the rate of breast cancer recurrence among patients with oestrogen-receptor-positive tumours. Cytochrome P-450 2D6 metabolises tamoxifen to metabolites that more readily bind the oestrogen receptor. This enzyme also metabolises selective serotonin reuptake inhibitors (SSRI), so these widely used drugs – when taken concurrently – may reduce tamoxifen's prevention of breast cancer recurrence. We studied citalopram use in 184 cases of breast cancer recurrence and 184 matched controls without recurrence after equivalent follow-up. Cases and controls were nested in a population of female residents of Northern Denmark with stages I–III oestrogen-receptor-positive breast cancer 1985–2001 and who took tamoxifen for 1, 2, or most often for 5 years. We ascertained prescription histories by linking participants' central personal registry numbers to prescription databases from the National Health Service. Seventeen cases (9%) and 21 controls (11%) received at least one prescription for the SSRI citalopram while taking tamoxifen (adjusted conditional odds ratio=0.85, 95% confidence interval=0.42, 1.7). We also observed no reduction of tamoxifen effectiveness among regular citalopram users (30% overlap with tamoxifen use). These results suggest that concurrent use of citalopram does not reduce tamoxifen's prevention of breast cancer recurrence.