Sixteen studies were identified, of which 14 fitted the inclusion criteria.
The overall RR of ORT and colorectal cancer was 0.92 (95% CI: 0.74 to 1.15), which was not significantly different from 1.
Overall individual RR for colon and rectal cancers were 1.04 (95% CI: 0.80 to 1.35, based on 11 studies) and 0.89 (95% CI: 0.46 to 1.74, based on 8 studies), respectively; and were also not significant.
The studies showed significant variation (p<0.05). This heterogeneity appeared to arise mainly from the case-control studies, which were significantly different from each other (p<0.01), while the cohort studies were not (p<0.89). When cohort and case-control studies were analysed separately, RRs of ORT and colorectal cancer were not significant.