Salicylic acid has been known as a therapeutic substance since antiquity. Since its inception, acetylsalicylic acid, i.e. its original acetylated derivative, has become one of the most popular analgesic substances. Over the years, it has gained the recognition of cardiologists, and its potential antineoplastic effects have been recently studied. By inhibiting cyclooxygenase 2 (COX-2), the drug reduces the formation of prostaglandins, which relieves inflammation, eases swelling and lowers fever. Moreover, aspirin has an anticoagulant effect owing to its inhibition of cyclooxygenase 1 (COX-1). The prophylactic outcome of acetylsalicylic acid in patients with cardiovascular diseases is undeniable, but its benefits in individuals without cardiac disease are controversial. However, research is ongoing to identify the group of beneficiaries of primary prevention precisely, and the results are expected in the near future. Another field of action of acetylsalicylic acid which is currently under investigation is related to the potential antineoplastic effect of the drug. Nowadays, most of the data indicate its positive effects in patients with a positive history of colorectal proliferative diseases. The antineoplastic activity of the drug is based on the inhibition of COX-2, an enzyme stimulating cell divisions. Therefore, the effect of the medicine may be visible in tumours characterised by an overexpression of this enzyme. A positive response to aspirin also depends on the human genome and specific mutations in cancer cells. By acting on platelet COX-1, aspirin reduces the metastatic potential of cancer, as platelet-derived substances promote the formation of metastases. To sum up, only a selected group of patients exhibit sensitivity to the anticancer effects of the drug. Nevertheless, the use of aspirin could turn out to be dangerous in people with a high risk of complications, so each patient should be treated individually.