Why does my side hurt when I run?

That sharp, cramping pain under your ribs, usually on the right side, is a side stitch. It’s extremely common in new runners, it’s harmless, and it’s largely preventable once you know what tends to trigger it.

The honest position is that the exact mechanism isn’t settled. The leading explanations involve irritation of the membranes lining the abdominal cavity, or strain on the ligaments connecting the diaphragm to the organs below it. What is well established is which behaviours make it more likely, and those are the useful part.

How to get rid of one mid-run

If you have a stitch right now:

Slow to a walk. Don’t try to push through it. The stitch will usually ease within a minute or two of dropping the intensity.
Breathe deeply and slowly. Push your belly out as you inhale rather than lifting your chest. Shallow, fast chest breathing tends to make it worse.
Press into the painful spot with your fingers while bending slightly forward.
Try exhaling as your opposite foot lands. If your stitch is on the right, breathe out as your left foot strikes.
Raise the arm on the affected side and lean gently away to stretch the area.

Once it settles, resume at a slower pace than before. Going straight back to the pace that produced it usually brings it straight back.

What tends to cause it

Eating too close to the run. The most common trigger by a distance. A full stomach adds weight and pressure in exactly the wrong place. Leave two to three hours after a full meal, and thirty to sixty minutes after a small snack. There’s more detail in What should you eat before a run.

Drinking a lot immediately beforehand. Sipping is fine; a large volume of fluid right before you set off sloshes around and contributes to the same problem. Sugary drinks and fruit juice seem particularly prone to it.

Shallow breathing. Fast, shallow chest breathing doesn’t let the diaphragm move through its full range. Beginners often default to it because the effort feels high. Deeper belly breathing is the counter.

Starting too fast. Stitches love an aggressive first few minutes. Easing into the run gives your breathing time to settle into a rhythm.

Weak core and postural muscles. Nothing dramatic needed: planks, dead bugs and side planks a couple of times a week tend to reduce how often stitches show up.

How to stop them recurring

Leave a proper gap between eating and running
Sip rather than gulp in the half hour before you set off
Start every run slower than feels necessary
Practise deep belly breathing while walking, so it’s available when you run: see How to breathe while running
Add a small amount of core work to your week

Most people find stitches become rare on their own within a few weeks as their breathing pattern and core strength adapt to running. If you’re getting them constantly, the eating and pacing variables are the two to examine first.

Pain that isn’t a stitch

A stitch is sharp, localised under the ribs, tied to running, and disappears within a few minutes of stopping. Anything that doesn’t match that description deserves attention rather than a walk break.

Stop running and seek medical advice if you have:

Chest pain, or pain radiating to your arm, neck or jaw
Pain with shortness of breath disproportionate to your effort, dizziness, or nausea
Side pain that persists long after you’ve stopped, or that appears at rest
Severe pain in the upper right abdomen, or pain accompanied by fever
Any pain that feels different from the stitches you’re used to

If in any doubt, particularly with chest symptoms, treat it as urgent rather than something to run off.

This is general information, not medical advice. If you have recurring or unusual pain, please see a doctor: especially anything involving your chest or breathing.

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