What Causes Menstrual Cramps?

Here's my best memory of a doctor's explanation: When your period starts, the extra blood vessels and tissue that have built up in the lining of your uterus (endometrium) have to break loose from the uterine wall so they can come out (that's the blood and tissue that is your period). The chemicals that do this are called prostaglandins. I don't know where the first prostaglandins in this process come from, but as the connections of the endometrium to the uterus break down, more prostaglandins are produced, so there's a cascade of prostaglandins going on. Some types of prostaglandins cause pain. You can also get cramps when your uterus tries to expel a clot that's having a hard time fitting through your cervix (whose opening is very small if you're not in the process of giving birth!).

This is why ibuprofen works well for many women's cramps: It's not just a pain reliever like Tylenol -- it's also a prostaglandin inhibitor. So it slows down the prostaglandin cascade, lessening the pain you feel. And it works best if you take it before the prostaglandin cascade really gets started -- in other words, before the cramps get bad.