Crohn Disease: How It Behaves, What Controls It, and the One Habit That Changes Everything

Crohn disease can affect any part of the digestive tract from mouth to anus, most often the last part of the small intestine and the start of the colon. Two features define how it behaves. It appears in patches with normal bowel in between, and the inflammation goes through the full thickness of the wall rather than staying on the surface.

That depth explains the complications that distinguish it from ulcerative colitis. Inflammation through the wall heals with scar, and scar narrows the tube, producing strictures. Inflammation that tunnels outward creates fistulas, abnormal channels to another loop of bowel, the bladder, the vagina or the skin. Neither happens in ulcerative colitis, where inflammation stays superficial.

The disease runs in flares and remissions, and the aim of modern treatment is not simply to make a flare feel better. It is to achieve mucosal healing, because bowel that looks normal on endoscopy predicts far fewer complications and operations than bowel that merely feels quiet.

🔍 What it looks like, and how it differs from ulcerative colitis

Typical symptoms are abdominal pain, often in the right lower abdomen and worse after eating, diarrhoea that can wake you at night, fatigue out of proportion to anything, unintended weight loss and low-grade fever. Night-time diarrhoea and weight loss are the two features that separate this from irritable bowel syndrome, which does not cause either.

The disease is not confined to the gut. Joint pain, red painful eyes, mouth ulcers, and skin lesions such as tender red nodules on the shins or deep ulcers can all accompany it, and occasionally appear before the bowel symptoms.

🧪 The test that replaces guesswork

Diagnosis otherwise rests on colonoscopy with biopsies, imaging of the small bowel by MRI or CT enterography, blood tests including CRP, full blood count, ferritin, B12 and vitamin D, and stool cultures to exclude infection. Perianal disease needs pelvic MRI.

💥 Treating a flare

Corticosteroids control a flare and do not maintain remission. That distinction is the source of a great deal of avoidable harm, because people left on steroids long term accumulate bone loss, diabetes, cataracts and infection risk without gaining protection against the next flare.

• 💊 Prednisolone is typically started around 40 mg daily and tapered over six to eight weeks, using Deltasone (Prednisolone) or Predniheal (Prednisolone)
• 🎯 Budesonide with controlled release in the ileum is preferred for mild to moderate disease limited to that area, because most of it acts locally and little reaches the rest of the body. This form is not currently in our catalogue
• 🚫 Never stop a steroid course abruptly, and never treat a flare with steroids repeatedly without adding maintenance treatment. Needing two or more steroid courses in a year is the signal to step up, not to prescribe a third
• 🧫 Calcium and vitamin D alongside steroid courses, with bone density assessed where courses are repeated

Mesalamine, available as Asacol (Mesalamine), and sulfasalazine, as Azulfidine (Sulfasalazine), act on the bowel surface. Their place in Crohn disease is narrow: mild disease confined to the colon, where sulfasalazine has some evidence. Neither is effective for small bowel disease or for maintaining remission, and relying on them for moderate disease is a common reason treatment appears to fail.

🛡️ Keeping remission: the drugs that actually do it

This is where Crohn disease is won or lost, and it needs a different class from the flare treatment.

🔹 Azathioprine

Imuran (Azathioprine 50 mg) is the conventional maintenance drug. It suppresses the proliferating immune cells driving the inflammation, allowing steroids to be withdrawn and keeping remission for years in those who respond.

• 📅 It works slowly. Full effect takes three to four months, which is why it is started while steroids are still tapering rather than at the point of crisis
• 📏 Dosed by weight, typically 2 to 2.5 mg per kg daily, built up gradually
• 🍴 Taken with food to reduce nausea

🔹 Monitoring and longer-term risks

• 🩸 Full blood count and liver enzymes weekly for the first month, then fortnightly, then every three months indefinitely
• 🌡️ Any fever, sore throat, mouth ulcers or unexplained bruising means stopping and having an urgent blood count
• ☀️ Skin cancer risk rises, so daily sun protection and prompt review of any changing skin lesion. This is a real and often unmentioned issue
• 🧬 A small increase in lymphoma risk, which is outweighed by the benefit in moderate to severe disease but is part of an honest conversation
• 💉 Live vaccines are avoided; influenza and pneumococcal vaccines are given, ideally before starting
• 🤰 Azathioprine is generally continued in pregnancy, because active disease poses more risk to a pregnancy than the drug does

🔹 The alternatives

Methotrexate is the other conventional maintenance option, particularly where azathioprine is not tolerated. It is taken once weekly, never daily, with weekly folic acid on a different day; our guide to methotrexate covers the practical detail, and it must be stopped before conception in both partners.

Beyond these, biologic therapy with anti-TNF agents and newer classes is the backbone of modern moderate to severe Crohn disease, achieving mucosal healing at rates the older drugs do not reach. These are injected or infused, require screening for tuberculosis and hepatitis B first, and are hospital-supplied rather than something we stock.

🚫 What makes Crohn disease worse

🥗 Nutrition, which differs by situation

There is no diet that treats Crohn disease in adults, and the advice that matters is situational rather than general.

During a flare

Smaller frequent meals, reduced fibre and reduced fat, limited lactose if it provokes symptoms, and attention to fluid and salt with diarrhoea. This is temporary, not a permanent restriction.

With a known stricture

A low-fibre diet becomes a mechanical necessity rather than a comfort measure. Nuts, seeds, popcorn, raw vegetables, sweetcorn, citrus pith and skins can physically lodge at a narrowing and cause obstruction. Food is chewed thoroughly and the fibrous items avoided.

In remission

A varied unrestricted diet. Unnecessary long-term restriction causes malnutrition and is itself a problem; a dietitian is more useful here than an elimination protocol.

In children

Exclusive enteral nutrition, a liquid formula diet for six to eight weeks, is as effective as steroids at inducing remission without affecting growth, and is used first-line in paediatric practice.

🧫 The deficiencies to check for

• 🍄 Vitamin B12 — absorbed in the terminal ileum, precisely where Crohn disease concentrates. After significant ileal disease or resection, deficiency is usual and replacement is by injection, lifelong, since oral supplements will not be absorbed
• 🩸 Iron — anaemia is very common. Oral iron is poorly absorbed in active inflammation and frequently worsens gut symptoms, so intravenous iron is often preferred, which is worth asking about rather than persisting with tablets that make things worse
• ☀️ Vitamin D — commonly low, and relevant to both bone and immune function
• 🦴 Calcium and bone density — after repeated steroid courses
• 🍃 Folate — particularly on sulfasalazine or methotrexate
• 💊 Zinc and magnesium — with prolonged diarrhoea or after extensive resection

🔬 Complications and surveillance

Roughly half of people with Crohn disease need surgery at some point, usually for a stricture that no longer responds to drugs or for an abscess or fistula. Surgery is not a failure of treatment and it is not a cure: recurrence at the join is common, which is why maintenance treatment continues afterwards and why stopping smoking matters so much around an operation.

Perianal fistulas need a combined approach: imaging with pelvic MRI, antibiotics such as Flagyl (Metronidazole) or Cipro (Ciprofloxacin), drainage of any abscess, and usually biologic therapy. They rarely close on tablets alone.

Colorectal cancer surveillance by colonoscopy begins about eight to ten years after the onset of disease involving the colon, at intervals set by how extensive the inflammation is.

🚨 When to seek urgent care

• Severe abdominal pain with vomiting and no bowel movement or wind — possible obstruction
• Fever with severe abdominal pain and a tender rigid abdomen — possible abscess or perforation
• Heavy rectal bleeding, or dizziness and palpitations with bleeding
• More than six bloody stools a day with fever and a fast pulse
• A painful swelling near the anus with fever — abscess needing drainage
• Fever, sore throat or mouth ulcers on azathioprine or methotrexate — urgent blood count
• Rapid weight loss, or inability to keep fluids down
• Passing faeces or air through the vagina, or urine that smells faecal — suggests a fistula

❓ Frequently asked questions

📑 Sources and editorial

• Gastroenterology guidelines on the management of Crohn disease, including induction, maintenance and treat-to-target with mucosal healing
• Studies of smoking and relapse, surgical risk and postoperative recurrence in Crohn disease, and the contrasting effect in ulcerative colitis
• Evidence on faecal calprotectin for distinguishing inflammatory bowel disease from functional disorders and for monitoring activity
• Prescribing information for azathioprine, including TPMT and NUDT15 testing, monitoring schedules and the allopurinol interaction
• Trials of exclusive enteral nutrition in paediatric Crohn disease
• Guidance on iron and vitamin B12 replacement in inflammatory bowel disease
• Related reading: methotrexate in practice, acute diarrhea and antidiarrhoeals
• Related products: Imuran (Azathioprine), Asacol (Mesalamine), Deltasone (Prednisolone), Methotrexate, anti-inflammatories category
• RXshop Editorial Team — reviewed by Emily Chen, MD, MPH — Internal Medicine Specialist