Why Do I Have Pain on My Left Side When Running?

Left-side pain during a run is most often a side stitch, formally known as exercise-related transient abdominal pain, or ETAP. It affects up to 70% of runners in a given year and, while not dangerous, can be sharp enough to force you to slow down or stop entirely.¹ The cause is not as straightforward as many runners assume, and the left side in particular has a few quirks worth understanding.

What Is Actually Happening Inside Your Abdomen

Despite how familiar the side stitch is, researchers still debate its exact cause. The leading explanation points to irritation of the parietal peritoneum, a thin membrane that lines your abdominal wall and the underside of your diaphragm. When this tissue gets aggravated, it produces the sharp, localized pain that runners know well. The evidence for this theory is strong: stimulating the parietal peritoneum in clinical settings produces pain that closely matches the character and location of a stitch, and the pain worsens with movement, just as a stitch does during running.² The peritoneum spans the entire abdominal wall, which explains why stitches can strike almost anywhere in your torso, not just on one side.

What triggers that irritation in the first place? One mechanism involves what is happening in your gut during a run. Researchers found that when the stomach and intestines are heavy with fluid, the bouncing motion of running causes the gut to tug on the ligaments that suspend it, transferring stress to the peritoneum.³ If you have ever noticed that a stitch is worse after drinking a large amount of water or a sports drink shortly before a run, this is the likely reason. The portion of the peritoneum beneath the diaphragm is connected to the phrenic nerve, and when that area is irritated, the result can be a referred pain in the shoulder tip, something some runners report alongside a stitch.²

Why the Left Side Specifically

Side stitches are actually more common on the right, where the liver sits and adds weight that pulls on the diaphragm’s peritoneal lining.⁴ So if yours reliably shows up on the left, a few things could be at play.

On the left side of your abdomen, your stomach, spleen, and the descending colon all sit relatively close together. After a meal or large drink, the stomach becomes heavier on the left, and the splenic flexure of your colon, the sharp bend where the transverse colon turns downward, can trap gas. Both of these factors can create localized pulling or distension that the repeated impact of running magnifies. Left-side stitches after eating tend to follow this pattern.

There is also a breathing-related explanation. Some running coaches have long observed that most people naturally exhale when their right foot strikes the ground. If you happen to be someone who consistently exhales on your left footstrike instead, the theory is that the diaphragm is in a relaxed, upward position at the exact moment impact travels through the left side of your trunk. Researchers have speculated that this repeated synchronization of exhalation with ipsilateral footstrike may irritate the peritoneum on that side, contributing to stitch development.¹ The evidence here is still circumstantial, but the pattern is familiar to enough runners and coaches that it is worth experimenting with.

Blood Flow Redistribution and Gut Distress

Running shifts your blood supply away from your digestive organs and toward your legs and heart. During intense or prolonged effort, blood flow to the gut can drop by 80% or more.⁵ This “splanchnic hypoperfusion” starves the intestinal lining of oxygen, which can trigger cramping, bloating, and pain anywhere along the GI tract. On the left side, the descending colon and sigmoid colon are the organs most affected by this oxygen drop, and cramping there produces pain in the left lower abdomen that is easy to confuse with a classic stitch.

Dehydration makes this worse. When you are low on fluids, your blood volume drops, and your body pulls even more blood away from the gut to maintain circulation to your muscles and brain.⁶ At extreme intensities, this reduced blood flow can cause GI ischemia severe enough to produce diarrhea, delayed stomach emptying, or even minor bleeding in the gut lining.⁷ Most recreational runners will not reach that severity, but the milder version, a dull ache or crampy sensation in the left abdomen during a hard run, is quite common and resolves once you slow down and blood flow redistributes.

The practical takeaway is that if your left-side pain feels deep and crampy rather than sharp and localized, and it worsens during hard efforts or on hot days when you sweat more, reduced gut blood flow is a more likely culprit than a straightforward stitch.

How Posture and Core Strength Factor In

Your spine may play a surprisingly large role. Runners with increased thoracic kyphosis, the rounded upper-back posture common in people who sit at a desk all day, are significantly more likely to experience side stitches. Among people already susceptible, the degree of both kyphosis and lumbar lordosis influences how severe the pain gets.⁸ Body type itself, whether you are tall, short, lean, or heavy, does not appear to matter. It is the alignment of the spine that makes the difference.

Why would posture affect abdominal pain? One explanation is mechanical: an exaggerated thoracic curve changes how forces travel through the ribcage and diaphragm during running, increasing tension on the peritoneum. Another relates to how your trunk rotates. During running, your lumbar spine bends laterally with each stride, reaching peak displacement on each side shortly after that foot hits the ground.⁹ If your deep core muscles are weak, they cannot adequately control that oscillation, and the compensation required from other muscles increases spinal loading.¹⁰ More uncontrolled trunk movement means more peritoneal tugging with every step.

If your left-side pain tends to appear later in a run when you are fatigued, this pattern fits neatly. Early in a run your core is fresh and stabilizes your trunk well. As miles accumulate and those muscles tire, lateral trunk sway increases, the peritoneum gets jostled more, and a stitch creeps in.

When It Is Not a Stitch at All

Not every left-side pain during running is a side stitch. A few musculoskeletal conditions can produce similar symptoms, and they are worth knowing about because they do not resolve with the same strategies.

One is a side strain, which involves a tear in the internal oblique muscle where it attaches to one of the lower ribs or its cartilage.¹¹ This is more common in sports that involve explosive rotation, like cricket bowling or baseball pitching, but it can happen in runners who ramp up speed or hill work aggressively. The key difference from a stitch is that a side strain does not go away quickly when you stop running. It lingers, gets sore to the touch, and hurts when you twist or cough.

Another is slipping rib syndrome, in which one of the lower “floating” ribs shifts slightly and pinches the intercostal nerve underneath the rib above it.¹² Running is one of the activities frequently associated with this condition.¹³ The pain is intermittent and linked to posture or movement, sometimes accompanied by a clicking sensation at the costal margin.¹⁴ A doctor can check for this with a simple maneuver called the hooking test, where they hook their fingers under the lower rib margin and pull forward. If it reproduces your pain and produces a click, slipping rib syndrome is likely.

If your left-side pain recurs in the same spot run after run, does not respond to the usual stitch-relief strategies, or persists well after you finish running, one of these conditions is worth investigating.

What to Do When It Hits Mid-Run

The traditional advice of “push through it” is not great. The classic stitch responds well to a few specific maneuvers, and the faster you act, the faster it resolves.

The most effective immediate strategies involve increasing pressure on the abdominal wall while changing your breathing pattern. Bending forward at the waist while tightening your core muscles was found to relieve stitches within seconds. Breathing through pursed lips at higher lung volume, essentially taking a deep breath and letting it out slowly through a small opening, also provided rapid relief. Interestingly, simply trying to relax your abdominal muscles or stomping harder with your footstrike did not help.¹⁵

A belt or compression garment tight around the waist also alleviated pain quickly in the same research, likely because it reduces the range of motion of abdominal contents and limits peritoneal tugging. You might not want to run in a tight belt, but pressing your hand firmly into the painful area while slowing your pace works on the same principle.

If the pain is crampy and deep rather than sharp, suggesting it is more of a blood-flow issue than peritoneal irritation, slowing to a walk and sipping small amounts of water is usually enough to let splanchnic blood flow recover.

Prevention Strategies That Actually Have Evidence Behind Them

Because researchers still argue about the root cause, prevention advice is not perfectly settled. But the following approaches are consistent with the available evidence and are low-risk to try.

Time your meals: A full stomach increases the weight pulling on your visceral ligaments and peritoneum. Leaving at least two hours after a large meal before running reduces both stitch incidence and GI symptoms in general. Stomach pain and side stitches were among the most commonly reported GI complaints in endurance runners, affecting roughly one in five.¹⁶
Control fluid intake: Large volumes of fluid before or during a run worsen stitches, and hypertonic or sugary drinks seem to persist longer in the stomach, creating more drag.³ Small, frequent sips are better than gulping a full bottle at once.
Work on thoracic mobility: Given the link between kyphosis and stitch susceptibility, exercises that extend and mobilize the upper back, such as foam rolling the thoracic spine and performing chest-opening stretches, could help over time.⁸
Strengthen your deep core: If trunk instability worsens peritoneal stress, training the muscles that resist lateral trunk sway, particularly the deep spinal stabilizers, should reduce the forces your peritoneum absorbs with every stride.¹⁰ Planks and side planks are a start, but anti-rotation exercises like Pallof presses and single-leg deadlifts more directly target the stabilizers that control trunk sway.
Experiment with breathing rhythm: If your stitch is always on the left, try consciously exhaling when your right foot strikes for a few minutes and see if it shifts or resolves. The theoretical basis for this is the footstrike-exhalation synchronization hypothesis.¹ Even if the mechanism is not fully confirmed, changing your breathing pattern has no downside.

Why Newer Runners Get More Stitches

If you are relatively new to running or returning after a break, side stitches are far more common for you than for experienced runners, and the reasons are partly mechanical. Newer runners tend to have weaker deep core musculature, less efficient breathing patterns, and more trunk movement per stride. They also tend to have less intuitive pacing and are more likely to go out too fast, pushing into higher intensities where gut blood flow drops sharply. Children and young athletes experience stitches at higher rates too, in part because they have a proportionally larger peritoneal surface area relative to their body size.²

The encouraging news is that frequency tends to decline with consistent training. As your core strengthens, your breathing becomes more rhythmic, and you develop a better feel for pacing, the conditions that produce a stitch become less common. Most seasoned runners still get the occasional stitch, but it transitions from a regular annoyance to an infrequent one.

When Left-Side Pain During Running Needs Medical Attention

The vast majority of left-side running pain is benign and self-limiting. But certain patterns warrant a visit to a doctor rather than another internet search.

Left-side chest pain that radiates to the arm, jaw, or back, or that comes with dizziness, nausea, or shortness of breath beyond what your effort level explains, needs immediate evaluation. While cardiac events during running are rare in younger people, they are not impossible, and left-sided chest or upper abdominal pain is the classic presentation. Pain that persists for more than an hour after finishing a run, especially if it is accompanied by tenderness that you can localize with a finger, points to a musculoskeletal injury like a side strain or a rib problem rather than a stitch. Recurrent left-lower-abdominal pain with changes in bowel habits after running could suggest exercise-induced GI ischemia that warrants investigation, particularly in endurance athletes training at high volumes.⁷

The simplest screening question is whether the pain behaves like a stitch: does it appear during running, get worse with movement, and resolve quickly once you stop? If the answer to all three is yes, you are almost certainly dealing with ETAP or transient gut distress. If any of those three characteristics are missing, particularly if the pain lingers or changes character over time, a sports medicine evaluation is a reasonable next step.

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