A side stitch is a sharp, intense pain under the lower edge of the ribcage, more often on the left side. They're common for all runners, but especially for those just starting out who tend to breathe more quickly and shallow.
What causes side stitches? The actual cause is a topic that's been studied and debated, and there's still no definite answer. Some research has shown that drinking high-sugar beverages before exercise increases the likelihood of stitches. One study found a link between side stitches and increased curvature of the spine. Contrary to popular belief, drinking while running does not cause side stitches.
Regardless of what actually causes side stitches, there are some things you can do to help prevent them. If you tend to suffer from side stitches, try to avoid eating within one hour of running. Stick to plain water for pre-hydration -- avoid sugary, especially carbonated, drinks. Make sure that you always warm up before your runs.
The most effective way to prevent side stitches is to avoid shallow breathing. Make sure you're breathing in and out through your mouth when you're running. Breathe deeply from your belly, not your chest. Deep belly breathing allows you to take in more air. Also, try to keep your posture upright while running as that allows you to take in more air.
You can get rid of a side stitch like this: tighten your abs like someone is going to punch you. Then lean over the side with the stitch. So, if it's on your right side, lean over your right side. Some people get rid of stitches by raising their arms above their head and leaning from side to side. Others press on the stitch until it goes away. Try these suggestions and prevention techniques and hopefully your stitches will be a thing of the past!