The association between oral contraceptive use and colorectal cancer have yielded conflicting results. The analysis from a multicenter case-control study, conducted in 6 Italian regions in 1992-96 with data from a 1985-91, yielding a total of 803 women with colon cancer (median age, 61 years), 429 cases of rectal cancer (median age, 62 years), and 2793 controls (median age, 57 years)showed that the protection conferred by oral contraceptives (HC) use was similar when the origin of the neoplasm was in the ascending, transverse,or descending colon.  An inverse association was also found between use of HCs and rectal cancer (OR, 0.66), but there was no association with duration of OC use.

For colon and rectal cancers combined,a 36%  reduction in cancer risk was present among combined oral contraceptive (COC)  users (OR,0.64).

These findings are consistent with the descriptive epidemiology of colorectal cancer,  and experimental findings on estrogen receptors and the colorectal cancer pathway. ⁽⁶⁴⁾ 

Other researchers reported that oral contraceptive use showed no significant influence, while users of hormone replacement therapy had a reduced risk of rectal cancer (OR = 0.56). Thus, the association of colorectal cancer with reproductive and menstrual factors is neither strong nor consistent ⁽⁶⁵⁾.

Similar results were obtained from a large study on 118.404 women which supports as the current or past of oral contraceptives use did not appreciably alter the risk of colorectal cancer ⁽⁶⁶⁾. Adenomatous polyps (adenomas)  are precursors of colorectal cancer.

Parity, history of spontaneous or induced abortion, infertility, type of menopause, age at menopause,  use   of   oral   contraceptives,  and   use   of   menopausal   hormone replacement therapy were not associated statistically,  with significant adenoma risk, although some possible trends were observed ⁽⁶⁷⁾.

As recognized precursor lesions to colorectal cancer, colorectal adenomatous polyps have been studied to enhance knowledge of colorectal cancer etiology.  Although most of the known risk factors for colorectal cancer are also associated with the occurrence of colorectal adenomas;  cigarette smoking has had a strong,consistent relationship with colorectal adenomas but is generally not associated with colorectal cancer.

The explanation for this paradox is unknown ⁽⁶⁸⁾. It is also suggest that the major effect of smoking on the colorectal adenoma-carcinoma sequence occurs in the earlier stages of the formation of adenoma and the development of carcinoma in situ.