The review included 139 case-control and cohort (including nested case-control) studies.
Aspirin users had a significantly reduced risk of digestive tract cancers, including colorectal cancer (RR 0.73, 95% CI 0.67 to 0.79; 30 studies), squamous cell/not otherwise specified (NOS) oesophageal cancer (RR 0.61, 95% CI 0.50 to 0.76; 11 studies), oesophageal and gastric cardia adenocarcinoma (RR 0.64, 95% CI 0.52 to 0.78; 11 studies) and gastric cancer (RR 0.67, 95% CI 0.54 to 0.83; 13 studies). For all these analyses there was statistically significant heterogeneity (Χ² p-values ranged from 0.060 to <0.001). For outcomes other than colorectal cancer, risk reductions were not significant when analysis was restricted to cohort studies. There was evidence of publication bias for studies of colorectal cancer (p=0.003) and squamous cell/NOS oesophageal cancer (p<0.001).
Aspirin users had a significantly reduced risk of lung cancer (RR 0.91, 95% CI 0.84 to 0.99; 20 studies), breast cancer (RR 0.90, 95% CI 0.85 to 0.95; 32 studies) and prostate cancer (RR 0.90, 95% CI 0.85 to 0.96; 24 studies), with significant heterogeneity (p<0.001). There was no significant difference between users and non-users for the outcomes of pancreatic cancer (10 studies), ovarian cancer (15 studies), bladder cancer (nine studies) and kidney cancer (10 studies).
There was significant heterogeneity (p 0.060 to <0.001) for analyses of all outcomes except pancreatic, endometrial, bladder and kidney cancer. There was evidence of significant publication bias for analyses of colorectal cancer (Egger's test p=0.003), squamous cell/NOS oesophageal cancer (p<0.001), lung cancer (p<0.003) and breast cancer (p<0.032).
Results of subgroup analyses were reported in the review.