Brufen 400 Tablet 20's contains 'Ibuprofen', which works by blocking the effect of a chemical messenger in the body. It is known as cyclo-oxygenase (COX) enzymes that make other chemical prostaglandins. These prostaglandins are produced at injury sites and cause pain and swelling. By blocking the COX enzymes' effect, fewer prostaglandins are produced, which reduces mild to moderate pain, inflammation, and fever.