The effects of transdermal and oral oestrogen replacement therapy on colorectal cancer risk in postmenopausal women

The aim of this study was to examine the effects of oral and transdermal oestrogen replacement therapy on the risk of colorectal cancer. Data from a nested case–control study, designed to investigate the effect of hormone replacement therapy (HRT) on colorectal cancer were analysed. New cases of colorectal cancer, diagnosed between 1992 and mid-1998 (N=1197), were identified using the Saskatchewan Cancer Agency cancer registry. Women ⩾50 years of age, eligible for coverage by the Saskatchewan Prescription Drug Plan, were included in the study. Four controls per case were age matched to cases, using incidence density sampling. The outpatient prescription drug plan database was used to ascertain oestrogen prescriptions. Women were classified according to history of transdermal (TDE) and oral (OE) oestrogen use. Conditional logistic regression was used to estimate odds ratios (ORs) and 95% confidence intervals (CIs). Compared with women who had never used HRT, ORs for <3 and ⩾3 years of TDE use and colorectal cancer were 0.69 (95% CI: 0.43–1.10) and 0.33 (95% CI: 0.12–0.95), and for OE use were 0.90 (95% CI: 0.73–1.01) and 0.75 (95% CI: 0.60–0.93), respectively. The risk reduction for colorectal cancer with TDE may be greater in magnitude than that which has been reported for oral HRT.