Story at-a-glance Aspirin helps prevent cancer metastasis by boosting your immune system's T cells, which attack cancer cells that try to spread throughout your body Metastasis causes 90% of cancer deaths as cancer cells travel through blood or lymph systems to form new tumors in organs like lungs, liver or bones Aspirin blocks a substance in your body that normally slows down T cells, allowing them to move and attack cancer cells more effectively Low doses of aspirin (75 to 300 milligrams daily) appear effective, with benefits increasing after 5 to 7.5 years of regular use Willow bark is a natural alternative to aspirin; it contains a compound that your body turns into the same active ingredient as in aspirin