Crohn’s disease is a chronic inflammatory condition that can affect any part of the digestive tract, from the mouth to the anus, though it most commonly targets the last section of the small intestine and the beginning of the colon. It belongs to a group of conditions called inflammatory bowel disease (IBD), which affects an estimated 2.4 to 3.1 million people in the United States alone. Unlike a stomach bug or food poisoning, Crohn’s doesn’t resolve on its own. It’s a lifelong condition that cycles between flare-ups and periods of remission.
How Crohn’s Differs From Other Gut Conditions
What sets Crohn’s apart is how deeply the inflammation penetrates. In many digestive conditions, irritation stays on the surface lining of the intestine. In Crohn’s, inflammation cuts through the entire thickness of the intestinal wall. This “transmural” inflammation is what drives many of the disease’s serious complications, including narrowing of the intestine, abnormal tunnels between organs, and abscesses.
Crohn’s also behaves differently from ulcerative colitis, the other main type of IBD. While ulcerative colitis produces continuous inflammation limited to the colon, Crohn’s creates a patchwork pattern. Diseased segments sit right next to perfectly healthy tissue, a feature doctors call “skip lesions.” Under close examination, the inflamed areas develop a bumpy, cobblestone-like texture as ulcers deepen into the intestinal wall over time.
What Causes It
No single cause has been identified. Crohn’s appears to develop when the immune system in the gut overreacts to normal intestinal bacteria in someone who is genetically predisposed. More than 200 genetic locations have been linked to inflammatory bowel disease, but one of the strongest and earliest discovered associations involves a gene called NOD2. This gene helps cells detect and respond to bacteria. When it carries certain variations, bacterial recognition is impaired, which may trigger the chronic immune overreaction that defines the disease.
Genetics alone aren’t enough. Environmental factors play a significant role, and smoking is the most well-established modifiable risk. Current smokers are roughly twice as likely to develop Crohn’s as nonsmokers. Smoking impairs both the immediate and long-term branches of the immune system, increasing vulnerability to microbial imbalances in the gut. Interestingly, smoking also appears to interfere directly with the NOD2 gene’s function in intestinal cells, compounding the genetic risk.
Common Symptoms
Crohn’s often starts subtly. Early symptoms can be vague enough to be mistaken for irritable bowel syndrome or a lingering stomach virus. The hallmark symptoms include persistent diarrhea, abdominal pain (often in the lower right side), fatigue, and unintended weight loss. Some people experience bloody stools, fever, or a reduced appetite. Because symptoms can be nonspecific, diagnosis is frequently delayed.
What surprises many people is that Crohn’s doesn’t stay confined to the gut. Between 9% and 53% of patients experience joint pain, making it the most common symptom outside the digestive tract. This typically affects large joints like the knees and tends to flare alongside intestinal symptoms. Some people develop a type of spinal inflammation that causes morning back stiffness, which improves with movement.
Skin problems show up in 2% to 34% of patients. The most recognizable are tender, reddish nodules on the shins and painful skin ulcers with purplish borders that can appear on the legs or elsewhere. Mouth ulcers on the inner cheeks and lips are another common complaint. Eye involvement is less frequent, occurring in up to 5% of patients, but can include redness, burning, light sensitivity, and blurred vision.
How It’s Diagnosed
There’s no single test that confirms Crohn’s. Diagnosis relies on a combination of blood work, stool tests, imaging, and direct visualization of the intestine. A colonoscopy or endoscopy allows doctors to see the characteristic skip lesions and cobblestone pattern and take tissue samples for examination under a microscope.
A stool test measuring a protein called fecal calprotectin is increasingly used as a first step. Normal levels fall below 50 micrograms per gram. Levels above 250 are a strong signal of moderate to severe intestinal inflammation, with about 92% sensitivity for distinguishing active disease from mild or quiet disease. Blood tests for C-reactive protein (normal below 5 mg/L) can flag inflammation but are less precise on their own. MRI is the most commonly used imaging tool, employed in about 39% of cases to assess disease activity, while CT scans are used in about 14%.
Complications Over Time
Because inflammation extends through the full intestinal wall, Crohn’s can cause structural damage that goes beyond symptoms. Chronic inflammation leads to scar tissue, which narrows the intestinal passage (strictures) and can cause partial or complete blockages. When inflammation burrows all the way through, it can create abnormal tunnels called fistulas that connect the intestine to other organs, the skin surface, or the area around the anus.
Perianal fistulas are particularly common, developing in 23% to 38% of people with Crohn’s over their lifetime. These can be classified as simple, involving a single opening without surrounding infection, or complex, involving multiple openings and potentially connecting to the bladder or vagina. Complex fistulas often require both medication and surgical intervention.
About 8% of newly diagnosed patients require surgery within the first year. That number rises to roughly 14% at five years and 19% at ten years. While these rates are significant, they have been declining over the past two decades, largely because of earlier and more aggressive medical treatment.
Treatment Options
Treatment aims to reduce inflammation, control symptoms, and prevent complications. What you’re prescribed depends on the severity and location of your disease.
For moderate to severe Crohn’s, biologic therapies have transformed outcomes since the late 1990s. These are medications delivered by injection or infusion that target specific parts of the immune response. The main categories include:
- TNF blockers, the oldest class, which neutralize a key inflammatory protein. These have been available since 1998 and remain a cornerstone of treatment.
- Integrin blockers, which prevent inflammatory white blood cells from reaching the gut lining.
- Interleukin blockers, which target specific immune signaling molecules (IL-12 and IL-23) involved in driving chronic inflammation. Newer, more selective versions of these drugs have been approved as recently as 2022.
- JAK inhibitors, oral pills that block inflammation-promoting enzymes inside immune cells. These offer the convenience of a pill rather than an injection or infusion.
The choice between these options depends on factors like prior treatment history, other health conditions, and patient preference. Many people cycle through more than one class over the course of their disease if a medication loses effectiveness or causes side effects.
The Role of Diet
Diet doesn’t cause Crohn’s, but it can meaningfully influence flare-ups. One of the most studied dietary approaches is the Crohn’s Disease Exclusion Diet (CDED), which removes foods thought to promote intestinal inflammation, including processed foods, certain additives, and specific fats, while incorporating a partial liquid nutritional supplement.
The results from real-world studies are encouraging. In one study of adult patients, 77% achieved remission after six weeks on the diet, rising to 82% after twelve weeks. A study in children found a 69% remission rate at six weeks. In an observational study from a specialty center, 63% of patients with active disease reached remission within six to twelve weeks, and among those already in remission, 83% maintained it at twelve weeks. These are notable numbers for a dietary intervention, though they work best as a complement to medical treatment rather than a replacement.
No single “Crohn’s diet” works for everyone. Many people identify personal trigger foods through careful tracking. Common offenders include high-fiber raw vegetables during flares, dairy products, alcohol, and heavily processed foods. Working with a dietitian familiar with IBD can help you maintain nutrition while avoiding foods that worsen your symptoms.