Crohn's Disease: Symptoms, Complications, and Treatment Step by Step

Crohn's disease (CD) is a chronic, immune-mediated inflammatory bowel disease that can affect any segment of the digestive tract — from the mouth to the anus — though it most commonly settles in the terminal ileum and the beginning of the colon. Unlike ulcerative colitis, which we cover in a separate article, inflammation in Crohn's disease involves the full thickness of the bowel wall (it's transmural), and the affected areas are discontinuous — patches of healthy mucosa alternate with inflamed segments, a pattern described as "skip lesions."

Like ulcerative colitis, Crohn's disease results from a combination of genetic predisposition, an abnormal immune response to the gut microbiome, and environmental factors. One of the few well-documented, modifiable risk factors — for both developing the disease and for a more severe course — is cigarette smoking, which sets it apart from ulcerative colitis, where the relationship runs the opposite way or is unclear.

The disease course, similar to ulcerative colitis, alternates between flares and remission, but the transmural nature of the inflammation means Crohn's disease more often leads to structural complications than UC does — strictures narrowing the bowel, fistulas connecting the bowel to other organs or the skin, and intra-abdominal abscesses.

This variety of symptoms comes directly from the fact that Crohn's disease can affect any segment of the digestive tract — upper-tract involvement (stomach, duodenum) causes different complaints than ileal involvement, and colonic involvement can look similar to UC. This variability is one reason for delayed diagnosis, especially when extraintestinal or nonspecific symptoms like fatigue and weight loss dominate the picture.

Diagnosis relies on a combination of colonoscopy with ileoscopy (examining the terminal ileum) and biopsies, imaging studies of the small bowel (MR enterography or CT enterography), and lab tests, including fecal calprotectin as a marker of active inflammation. In some cases, especially with an unclear picture, small-bowel capsule endoscopy is helpful.

This is one of the more important practical differences from UC: while 5-ASA is the first-choice drug there, its efficacy in Crohn's disease is much more modest, which is why for most patients with active disease treatment starts with budesonide (a steroid acting mainly locally in the bowel, with lower risk of systemic side effects) or, in more severe cases, with systemic steroids or biologic treatment right away.

Biologic drugs — anti-TNF antibodies (infliximab, adalimumab), the anti-integrin antibody vedolizumab, and anti-IL-23 drugs (ustekinumab, risankizumab) — are today's standard for maintenance treatment in patients with moderate to severe disease, especially those with poor-prognosis features: young age of onset, deep ulcerations, perianal involvement, or an early need for steroids. Immunomodulators (azathioprine, mercaptopurine, methotrexate) are sometimes used alone or — more often — in combination with a biologic, which in some patients reduces the risk of developing antibodies against the biologic and extends how long it stays effective.

A key difference from UC also concerns the role of surgery: while removing the colon in UC eliminates the disease anatomically, resecting the affected segment of bowel in Crohn's disease doesn't cure it permanently — recurrence of inflammation elsewhere in the digestive tract, most often right above the surgical connection, is common. For this reason, surgery decisions in Crohn's disease are usually reserved for specific complications (strictures unresponsive to drug treatment, fistulas, abscesses) rather than treated as a way to cure the disease.

The transmural nature of the inflammation means that over time some patients develop fibrosis of the bowel wall and a stricture, presenting as recurring colicky pain, bloating, and vomiting after meals — a picture of partial obstruction. Deep ulcers can penetrate through the entire bowel wall, leading to a fistula (most often perianal, but also enterovesical or enterocutaneous) or an intra-abdominal abscess requiring drainage and antibiotics.

Involvement of the terminal ileum, typical of Crohn's disease, matters for another reason too: that's exactly where vitamin B12 is absorbed, so patients with disease involving that segment — or after its surgical removal — are at increased risk of B12 deficiency regardless of diet. We cover recognizing and understanding the consequences of that deficiency in our separate article on vitamin B12. Chronic inflammation and diarrhea also predispose to iron deficiency — a topic we cover in a separate article on iron.

Because small-bowel involvement and related absorption problems are common, assessing nutritional status and possible micronutrient deficiencies (iron, B12, vitamin D, zinc) should be a routine part of care for a Crohn's disease patient, regardless of the current disease phase.

Crohn's disease calls for a different approach than UC despite the apparent similarity of symptoms — the response to 5-ASA differs, the role of surgery differs, the typical complications differ. That makes an accurate diagnosis from the start, followed by regular gastroenterology care and vigilance for signs of structural complications, more important than in many other chronic diseases — a delayed diagnosis of a stricture or fistula means genuinely harder treatment down the line.