Five RCTs were eligible (17,285 participants; 987 had a new solid cancer diagnosed during mean in-trial follow-up of 6.5 years). Four RCTs were placebo-controlled and double-blinded.
Aspirin treatment reduced risk of cancer with definite distant metastasis (all cancers HR 0·64, 95% CI 0·48 to 0·84). The result remained statistically significant for adenocarcinoma (HR 0.54, 95% CI 0.38 to 0.77), but not for other solid cancers (HR 0.82, 95% CI 0.53 to 1.28). Aspirin increased the risk of cancer with local disease only (HR 1.24, 95% CI 1.01 to 1.53).
Aspirin reduced the risk of adenocarcinoma with metastasis at initial diagnosis (HR 0.69, 95% CI 0.50 to 0.95) and the risk of metastasis on subsequent follow-up in patients without initial metastasis (HR 0.45, 95% CI 0.28 to 0.72), particularly in patients with colorectal cancer (HR 0.26, 95% CI 0.11 to 0.57) and patients who remained on trial treatment up to or after diagnosis (HR 0.31, 95% CI 0.15 to 0.62, all adenocarcinomas).
Aspirin treatment reduced death due to cancer in patients who developed adenocarcinoma, especially in patients without metastasis at diagnosis (HR 0.50, 95% CI 0.34 to 0.74) and reduced the overall risk of fatal adenocarcinoma (HR 0.65, 95% CI 0.53 to 0.82), but not the risk of other fatal cancers (HR 1.06, 95% CI 0.84 to 1.32). The effects were independent of age, sex and smoking status. Low doses of aspirin were as effective as higher doses.
Further results were reported.