IBS develops through a combination of factors, but no single cause has been identified

Irritable bowel syndrome does not have one clear cause that doctors can point to and say "this is what triggers it." Instead, IBS appears to result from multiple things happening at once—how your gut muscles contract, how your nervous system processes signals from your intestines, what bacteria live in your colon, and what you eat. Different people develop IBS through different combinations of these factors, which is partly why treatment that works for one person may not work for another.

The condition is not caused by an infection, inflammation, or structural damage that shows up on imaging tests. This is why a colonoscopy or CT scan of your abdomen often comes back normal even when your symptoms are severe. IBS is a functional disorder, meaning your digestive system does not work the way it should, but the tissue itself appears healthy under a microscope.

Key Takeaways

  • IBS results from multiple overlapping factors including gut muscle contractions, nervous system sensitivity, gut bacteria, and diet—not from a single identifiable cause.
  • Infections, particularly bacterial gastroenteritis, can trigger IBS symptoms in some people, sometimes months after the infection has cleared.
  • Stress and emotional trauma appear to change how your gut responds to normal stimuli, making symptoms worse or more frequent.
  • Genetics play a role, so IBS is more common in people whose close relatives also have the condition.
  • Food sensitivities and changes in gut bacteria composition may contribute, though the exact mechanisms are still being studied.

How gut muscle contractions differ in people with IBS

Your intestines move food through your digestive system using coordinated muscle contractions called peristalsis. In people with IBS, these contractions are often stronger, last longer, or happen more frequently than normal. This can push food through too quickly, leading to diarrhea, or too slowly, leading to constipation. Some people experience both patterns at different times.

Researchers have measured these contractions using devices placed inside the colon and found that people with IBS-D (diarrhea-predominant) tend to have faster, more forceful contractions, while people with IBS-C (constipation-predominant) often have weaker or less coordinated ones. However, not every person with abnormal contractions develops IBS symptoms, and not every person with IBS symptoms shows abnormal contractions on testing. This suggests that muscle activity alone does not fully explain the condition.

The role of your nervous system in sensing gut signals

Your gut has its own nervous system—sometimes called the "second brain"—that communicates constantly with your central nervous system. In IBS, this communication appears to be amplified. Your gut may be sending normal signals, but your nervous system interprets them as painful or urgent when they should not be. This is called visceral hypersensitivity, and it means your threshold for noticing sensations in your intestines is lower than it is in people without IBS.

Brain imaging studies show that people with IBS process gut signals differently than others do. The same amount of intestinal stretching that causes mild discomfort in a person without IBS may cause significant pain in someone with IBS. This heightened sensitivity can develop after an infection, during periods of stress, or for reasons that are not yet clear. It is not imaginary—the nervous system is genuinely responding more strongly to the same stimulus.

Infections and post-infectious IBS

Some people develop IBS after a bout of food poisoning or traveler's diarrhea caused by bacteria like Campylobacter or Salmonella. Studies suggest that roughly 10 percent of people who have bacterial gastroenteritis go on to develop IBS symptoms, sometimes weeks or months after the infection has resolved. Researchers call this post-infectious IBS, and it appears to happen because the infection triggers lasting changes in gut bacteria and in how the nervous system responds to the gut.

The infection itself may damage the lining of the intestines or alter the balance of bacteria that normally live there. Even after the harmful bacteria are gone, the gut microbiome—the community of bacteria in your colon—may not fully return to its previous state. This altered microbiome may then contribute to ongoing symptoms. Not everyone who has an infection develops IBS, which suggests that genetics or other factors determine who is vulnerable.

Stress, trauma, and the gut-brain connection

Psychological stress does not cause IBS in the way that a virus causes a cold, but it strongly influences how severe symptoms become and how often they occur. During stress, your body releases hormones like cortisol and adrenaline that speed up gut contractions and increase sensitivity to pain. People with IBS often notice their symptoms worsen during stressful periods and improve during calmer ones.

Early-life trauma or chronic stress may also shape how your nervous system develops. Children who experience abuse or significant adversity have higher rates of IBS in adulthood. This does not mean the condition is psychological or "all in your head"—rather, stress appears to physically alter the nervous system in ways that persist. Treating the stress through therapy, medication, or other approaches can reduce IBS symptoms, which shows that the mind and gut are genuinely connected.

Genetics and family history

IBS runs in families. If one of your parents or siblings has IBS, your own risk is higher than average. Twin studies suggest that genetics account for roughly 50 percent of the variation in IBS risk, meaning inherited factors matter but are not the whole story. Researchers have not identified a single "IBS gene," but rather multiple genetic variations that each contribute a small amount to overall risk.

These genetic factors likely influence how sensitive your nervous system is, how your gut muscles contract, or how your immune system responds to bacteria in your colon. Having the genetic predisposition does not mean you will definitely develop IBS—environmental factors like infections, diet, and stress also play a role. This is why two siblings with the same genes may have very different experiences with IBS.

Gut bacteria and the microbiome

The bacteria living in your colon influence how your digestive system works and how your immune system responds. People with IBS often have a different balance of bacteria compared to people without the condition, though the exact differences vary from person to person. Some research suggests that people with IBS have fewer beneficial bacteria or an overgrowth of bacteria that produce gas, which could contribute to bloating and discomfort.

Changes in the microbiome can result from infections, antibiotics, diet, or stress. Once the balance shifts, it may not fully return to normal on its own, and the altered microbiome may then trigger or worsen IBS symptoms. Probiotics and dietary changes aimed at shifting the microbiome composition show promise in some studies, though results are mixed and benefits vary widely between individuals.

Food sensitivities and dietary triggers

Certain foods trigger or worsen IBS symptoms in many people, though the specific triggers differ widely. Common ones include high-fat foods, dairy, caffeine, alcohol, and foods high in fermentable carbohydrates called FODMAPs. However, food sensitivity in IBS is not the same as a food allergy—your immune system is not overreacting to the food, but rather your gut is more sensitive to the normal effects of digesting it.

A low-FODMAP diet, which limits foods that are harder to digest and more likely to ferment in the colon, reduces symptoms in roughly 50 to 70 percent of people who try it. This suggests that what you eat genuinely affects IBS, but it also shows that diet alone does not explain the condition. Some people with IBS have no obvious food triggers, and some people without IBS cannot tolerate high-FODMAP foods either.

Frequently Asked Questions

Can you catch IBS from someone else?

No, IBS is not contagious. You cannot catch it from a family member or friend who has it. However, if you share genes with someone who has IBS, your own risk is higher. And if you experience the same infection that triggered IBS in a relative, you might develop post-infectious IBS yourself—but that would be because of the infection, not because of contact with the person.

Does IBS get worse over time?

IBS does not progress or cause permanent damage to your digestive system the way inflammatory bowel disease does. However, symptoms can fluctuate significantly over months or years. Some people have long periods with few symptoms, then flare-ups triggered by stress, infection, or diet changes. Others have steady symptoms that remain relatively stable. The course varies widely between individuals.

Can you develop IBS at any age?

IBS can start at any age, though it most commonly begins in the teens or early adulthood. Some people have symptoms for years before seeking medical attention. Older adults can develop IBS, though it is less common than in younger people. If digestive symptoms start suddenly in someone over 50, doctors usually rule out other conditions first before diagnosing IBS.

Is IBS caused by anxiety or depression?

Anxiety and depression do not cause IBS, but they often occur together with it. Stress and emotional distress can trigger or worsen IBS symptoms, and living with chronic IBS symptoms can contribute to anxiety or depression. Treating the mental health condition may improve IBS symptoms, but the underlying cause of IBS is not psychological—it involves real changes in how your gut and nervous system function.

Can you prevent IBS if it runs in your family?

There is no may provide way to prevent IBS if you have a genetic predisposition. However, managing stress, maintaining a balanced diet, avoiding unnecessary antibiotics, and treating infections promptly may reduce your risk or delay onset. Since post-infectious IBS is one known trigger, recovering fully from gastroenteritis and supporting your gut health afterward may help. But some people with genetic risk will develop IBS regardless of these steps.