Scientists have learned how aspirin interferes with a protein enzyme. One form of the enzyme makes the prostaglandin that causes pain and swelling. Another form of the enzyme creates the protective kind of compound. So, while aspirin can reduce pain and swelling in damaged tissues, it also can harm the lining of the stomach and small intestine. Still, aspirin does things that other drugs for treating pain cannot do.
Source: tatoeba (12265861)