An Oxford University study has shown that taking a 300 mg dose of aspirin daily overa  period of five years can lower the incidence of bowel cancer by more than a third. The cancer generally affects the large intestine in the lower end of the gastro-intestinal tract, in the form of malignant tumours. Researchers in the Lancet have also warned, however, that continuous use could lead to side effects such as stomach bleeding and should not be recommended to the public as a preventative medication.