Dr Henry WeiGastroenterology & Endoscopy
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Crohn's disease: where it is and how we treat it

Crohn's disease causes inflammation in the digestive tract. It can affect different places and different layers of the bowel. Your tests help show which parts are involved and whether there are complications such as a narrowing. A complication described in this guide is something that can happen, not something everyone with Crohn's will get.

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In my practice

I match the treatment to your Crohn's pattern

My teaching on IBD treatment focuses on the disease pattern, safety and what is practical for the patient. In Crohn's, I check whether symptoms come from active inflammation, a narrowing, a fistula or another cause. Simply increasing immune treatment is not the answer to every new symptom.

I can look after your Crohn's disease privately, but some medicines have additional Pharmac funding criteria, and some infusion treatments may only be available through your local hospital. I am happy to help facilitate this if required. I check the current criteria when discussing treatment. We should also agree who reviews your blood tests, renews prescriptions and responds if symptoms worsen between appointments.

Guidance behind this advice
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Where and how Crohn's can affect the bowel

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Explanatory diagram
Crohn's can affect the digestive tract from mouth to anusFront view of the digestive tract. The mouth joins the food pipe, which enters the stomach in the upper left abdomen, on image right. The stomach outlet joins the C-shaped duodenum and then coiled small bowel inside the surrounding colon. The terminal ileum joins the caecum on the patient's right, on image left. The ascending, transverse and descending colon connect through the sigmoid colon to the rectum and anus. Separate rust highlights show examples in the terminal ileum and colon. Crohn's can affect any part from mouth to anus; highlights do not imply that every part shown is inflamed. MouthFood pipeStomachDuodenumColonSmallbowelTerminalileumRectumAnus Common sites: terminal ileum and colon. Highlighted examples shown.

From mouth to anus

Crohn's can affect any part of the digestive tract from the mouth to the anus. It most commonly affects the terminal ileum, the end of the small bowel, and/or the colon. The highlighted areas are examples, not a personal result.

Simplified illustrations, not to scale. These show an example, not your own anatomy or test result.

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From mouth to anus

Crohn's can affect any part of the digestive tract from the mouth to the anus. It most commonly affects the terminal ileum, the end of the small bowel, and/or the colon. The highlighted areas are examples, not a personal result.

Separate areas

Inflamed sections can be separated by areas that look less affected. These are often called skip areas. I use the full examination, biopsies and other tests to interpret the pattern.

Beyond the surface

Crohn's inflammation can extend through the bowel wall. This helps explain why a scan may be useful in addition to an examination of the lining. The depth varies between people and areas.

Narrowings and tunnels

A narrowing is different from a fistula, which is an abnormal tunnel. Inflammation, scar tissue or both can cause narrowing. These complications do not happen to everyone, and their treatment needs individual assessment.

Crohn's can affect more than the colon

The digestive tract runs from your mouth through the food pipe, stomach, small bowel, colon and rectum to the anus. Crohn's can occur anywhere along this route. It commonly affects the terminal ileum, the final section of small bowel, and/or the colon.

Inflamed patches may be separated by unaffected bowel. The inflammation can involve the full thickness of the bowel wall, unlike ulcerative colitis, which mainly affects the colon's inner lining.

Diarrhoea is common, but not everyone has it

Symptoms may include abdominal pain, urgency, diarrhoea, bleeding, weight loss and fatigue. Small-bowel Crohn's can cause pain or nutritional problems without diarrhoea. Mouth ulcers, joint pain, skin changes and eye inflammation can occur too.

Tell me about pain, swelling or discharge around the anus. Perianal Crohn's sometimes needs assessment by both gastroenterology and colorectal surgery. Fatigue also deserves assessment for iron deficiency, nutrition, inflammation and other causes, rather than being dismissed because your bowel frequency has improved.

Why you may need scans as well as colonoscopy

I combine the history, examination, blood tests and stool tests, including checks for infection. Calprotectin can support an assessment of inflammation but does not diagnose Crohn's alone. A low result does not exclude every case, particularly some small-bowel disease.

Colonoscopy examines the colon and usually the terminal ileum, with biopsies to assess the tissue. Gastroscopy may sometimes be useful to assess for Crohn's affecting the upper digestive tract. Colonoscopy does not examine most of the small bowel.

MR enterography or specialist bowel ultrasound helps map small-bowel involvement and assess the bowel wall. CT can be useful in urgent situations. Other tests are chosen for the clinical question.

What are narrowings and fistulas?

A stricture is a narrowed section of bowel. Swelling, scar tissue or both can contribute. Medicines treat inflammation, but extensive scarring with minimal active inflammation may need a different approach. Cramping after meals, increasing swelling or vomiting should prompt review.

A fistula is an abnormal tunnel from bowel to another organ or the skin. An abscess is a collection of infection. These can require medicines together with drainage or surgery. They are not the same as a superficial anal fissure.

A swallowed camera capsule is useful in selected cases, but can get stuck at a narrowing. Obstructive symptoms or known narrowing need imaging first. In established Crohn's, a dissolvable patency capsule is recommended before camera testing to check that it can pass.

Treat inflammation before it causes further damage

Treatment aims for sustained control without repeated steroids, with improvement in symptoms and evidence that inflammation has settled. The choice depends on location, severity, complications, previous treatment, other health conditions and your preferences. Biologic and other advanced treatments may be appropriate early for higher-risk disease.

Steroids are usually short-term treatment, not maintenance. Some medicines take longer to work or require blood monitoring. Nutrition treatment can help selected patients, particularly children, under specialist and dietitian supervision. Do not stop, restart or change prescribed treatment without your team's advice.

Surgery can be useful, but it does not cure Crohn's

Surgery may remove a badly affected section, relieve a scarred narrowing or treat a fistula or abscess. A planned operation can be an appropriate treatment choice rather than a failure. I discuss the options with you and the surgical team when relevant.

Crohn's can recur after surgery, including near the join in the bowel. Follow-up and sometimes preventive medicine are needed even when you feel well. The operation and your recurrence risk guide the monitoring plan.

Food, smoking and ongoing checks

Avoid unnecessary food restriction. A dietitian can help maintain energy, protein and nutrients. A narrowing may require individual advice about food texture and fibre, rather than the same diet for everyone. Smoking worsens Crohn's outcomes; ask your GP or our team for help to stop.

At follow-up I check your symptoms, blood tests, calprotectin and any scans or endoscopy you need. Ileal disease or surgery can affect vitamin B12 or bile-acid handling. Persistent diarrhoea is not always a flare.

Longstanding colonic Crohn's may need bowel cancer surveillance; isolated small-bowel disease does not by itself need the same colon surveillance. Your endoscopy and/or histology report will record the recommended follow-up once tissue results are available.

Symptoms that need urgent assessment

Seek urgent assessment for severe or worsening pain, a swollen abdomen with repeated vomiting or inability to pass wind, heavy bleeding, dehydration, or fever with marked illness. A painful swelling around the anus can be an abscess. Do not add fibre or laxatives for suspected blockage. A painful red eye or changed vision also needs urgent assessment.

Call 111 if you collapse, have heavy bleeding with faintness or are too unwell to travel safely. Do not wait for a routine appointment or an email reply.

Three things to remember

  • I check where Crohn's is affecting the bowel and whether it has caused a narrowing or another complication before recommending treatment.
  • Medicines, nutrition treatment and surgery have different roles. Surgery does not remove the need for follow-up.
  • New symptoms need assessment for inflammation, infection, narrowing or another cause before treatment is changed.

Related information

Sources and further reading

English clinical master checked 16 October 2026.

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