Eighteen epidemiologic studies of postmenopausal hormone therapy and colorectal cancer were included (n>5197). Four studies that investigated the association of hormone therapy and colorectal adenoma (n>1695) were also included and four studies that investigated the use of tamoxifen (n>85,411).
Thirteen out of 18 studies reported lower risks of colon cancer for ever users of postmenopausal hormones that nonusers (RR 0.80, 95% CI: 0.74 to 0.86). However, there was significant heterogeneity among the studies (p=0.02). When five studies in which premenopausal women were part of the reference group of never users were excluded the summary RR was 0.76 (95% CI, 0.70 to 0.82) and there was no heterogeneity among the studies (p=0.32).
Ten studies investigated the relation between ever using postmenopausal hormone therapy and cancer of the rectum. The summary RR was RR 0.81 (95% CI: 0.72 to 0.92), and there was significant heterogeneity among the studies (p=0.03). When studies that were not strictly of postmenopausal women (n=3) were excluded the summary RR was unchanged (RR 0.81), but the test for heterogeneity was of borderline significance (p=0.06).
For current postmenopausal hormone use the data on colon and rectal cancers were combined. Of the ten studies with data on current hormone use, RR 0.66, 95% CI: 0.59 to 0.74. There was no evidence of heterogeneity among the studies (p=0.89).
Five studies provided data on the duration of current use. Summary RRs were: short duration RR 0.61, 95% CI: 0.48 to 0.79; long duration RR 0.67, 95% CI: 0.56 to 0.79).
Only four investigations were found to have examined postmenopausal hormone therapy and colorectal adenomas. Three reported significant results where the incidence of adenoma was lower among women taking or having ever used postmenopausal hormone therapy. It is not clear why a summary RR was not presented.
Although data are sparse, preliminary evidence suggests that bowel cancer incidence may be increased among women taking tamoxifen therapy, which has both estrogenic and antiestrogenic effects. No association was found between combined estrogen and progestin use and colorectal cancer