There are different theories regarding the causes of debilitating side stitches, such as tension in the diaphragm, cramping in the abdominal muscles and irritation of structures within the abdominal cavity.
There are a few things you can do to attempt to alleviate the problem – but to get the most benefit you should reduce your pace while you try them. Gently push your hands into the area of discomfort, just under your ribs, to help ease the cramping.
Also, try changing your breathing pattern for a few breaths, taking in a very deep breath, then exhaling sharply. It may be helpful to synchronise your breathing with your running – breathing out when you footstrike on the opposite side to your pain is said to encourage the tension in the diaphragm to relax. If these strategies don’t work, stop running and try stretching: first, stretch to your side away from the site of the pain and then bend forward.
There are some measures you can take that may prevent a stitch from developing in the first place. It could be that your stitch is being caused by a bad breathing pattern, postural problems or weak abdominals. It’s also important to review your pre-run or pre-race meal – an irritated stomach can cause a side stitch to develop (among other troubles...). And make sure you always warm up properly, to ease your body into more intense exercise.