It's brought on by runners who exert themselves too hard at the beginning. Your body can't take in enough oxygen which causes rapid breathing and brings on the side pain.
"To avoid side pain you start your exercise very slowly. When you breathe, breathe so the air goes down into your lower lungs -- belly breathe so your stomach is rising and falling," said Jeff Galloway, author of "Running Until You're 100."
Jeff Galloway says the best advice is to start out slow, but if you feel the side cramp coming on slow down from whatever pace you're doing.