Twenty four populations across 23 studies were included in the review: 14 case control (n=24,001, range 225 to 5,785); and nine cohort studies (n=652,810, range 10,671 to 267,400). Follow-up in the cohort studies ranged from four to 35 years. Fourteen studies reported colon cancers, eight reported rectal cancers and 18 reported colorectal cancers.
Never use versus ever use: Compared to never oral contraceptive users, ever users had a statistically significantly lower risk of colorectal cancer (RR 0.81, 95% CI 0.72 to 0.92; 18 studies), colon cancer (RR 0.85, 95% CI 0.79 to 0.93; 15 populations, 14 studies) and rectal cancer (RR 0.80, 95% CI 0.70 to 0.92; nine populations, eight studies). These findings were similar to those derived from case-control and cohort studies seperately; results for rectal cancer from cohort studies was not statistically significant.
Duration: Studies that compared risks of these outcomes against duration of oral contraceptive use did not identify a statistically significant difference between those who used oral contraceptives for up to five years and those who used oral contraceptives for five years or longer.
Recency: Women who had last used oral contraceptive less than 10 years ago had a statistically significantly lower risk (RR 0.51, 95% CI 0.35 to 0.74) of developing colorectal cancer than women who had last used oral contraceptives10 years or more ago (RR 0.77, 95% CI 0.60 to 0.99; three studies).
Heterogeneity was observed only for colorectal cancer assessed in case-control studies. There was no evidence of publication bias; results of the Egger's test were not reported. No results of the sensitivity analyses were reported.