A retrospective analysis of data from thousands of cancer patients showed that lung and colon cancer patients who had been taking daily low-dose aspirin before diagnosis had smaller tumors. Their cancer was also 20% to 40% less likely to have spread to other parts of the body. It is still unclear whether aspirin indeed caused the less severe diagnoses or whether separate, independent factors make people both more likely to take daily aspirin and to have less advanced lung or colon cancers. Future studies may address this question. No association between daily aspirin and cancer severity was found in prostate or breast cancer.