Ibuprofen is thought to work by blocking the enzyme that makes prostaglandins, chemicals that drive pain, fever, and inflammation. With less prostaglandin, there is less pain and swelling. In menstrual cramps, it lowers prostaglandin levels in menstrual fluid and reduces the pressure and frequency of contractions of the uterus. Exactly how it works is not completely understood.
It is absorbed quickly, reaching peak blood levels in 1 to 2 hours, and the body clears half of it in about 2 hours. It also makes platelets less sticky, but this effect is weaker, shorter, and reversible compared with aspirin. Because some people's kidneys rely on prostaglandins to keep blood flowing, it can harm the kidneys, and it can cause bleeding or ulcers in the stomach or intestines without warning. Like other NSAIDs, it raises the risk of heart attack and stroke, which can begin early in treatment.