What is the recommended step‑wise treatment for inflammatory bowel disease (Crohn’s disease and ulcerative colitis) based on disease type, severity, and extent?

Treatment of Inflammatory Bowel Disease

For ulcerative colitis, start with combination topical mesalazine 1 g daily plus oral mesalazine 2–4 g daily for distal disease, or oral mesalazine 2–4 g daily alone for extensive disease; for Crohn's disease, use high-dose mesalazine 4 g daily for mild ileocolonic disease, budesonide 9 mg daily for mild-to-moderate ileocecal disease, or oral prednisolone 40 mg daily for moderate-to-severe disease. 1, 2

Ulcerative Colitis Treatment Algorithm

Proctitis (Rectal Disease Only)

• Begin with mesalazine 1 g suppository once daily plus oral mesalazine 2–4 g daily, as this combination achieves significantly higher remission rates than either agent alone. 1, 3
• Suppositories deliver medication more effectively to the rectum than foam or enemas and are better tolerated. 1
• If topical mesalazine is not tolerated, switch to topical corticosteroid (budesonide 2–4 mg suppository or hydrocortisone enema) while continuing oral mesalazine. 1, 3
• Topical mesalazine is more effective than topical corticosteroids, so never use topical steroids as first-line therapy. 1, 3

Left-Sided Colitis

• Start with mesalazine enema ≥1 g daily plus oral mesalazine ≥2.4 g daily, which is more effective than either topical or oral therapy alone. 1
• Once-daily dosing is as effective as divided doses and improves adherence. 1
• For inadequate response after 2–4 weeks, add oral prednisolone 40 mg daily and taper gradually over 8 weeks. 1, 2, 3

Extensive Colitis (Mild-to-Moderate)

• Initiate oral mesalazine 2–4 g daily (once-daily dosing preferred) as first-line therapy. 1, 2, 3
• If no adequate response after 2–4 weeks, add oral prednisolone 40 mg daily with an 8-week taper. 1, 2
• Never taper prednisolone faster than 8 weeks, as rapid tapering precipitates early relapse. 1, 2

Extensive Colitis (Moderate-to-Severe)

• Initiate advanced therapy with infliximab, risankizumab, guselkumab, or ozanimod as first-line agents in biologic-naïve patients, as these demonstrate superior efficacy over adalimumab. 2, 3
• Combine TNF antagonists with thiopurines (azathioprine 1.5–2.5 mg/kg/day or mercaptopurine 0.75–1.5 mg/kg/day) rather than using TNF monotherapy, as combination therapy improves outcomes. 2, 3
• For non-TNF biologics (ustekinumab, vedolizumab, tofacitinib, ozanimod), evidence is insufficient to recommend routine combination with immunomodulators. 2

Severe Acute Ulcerative Colitis (Hospitalization Required)

• Admit immediately and start intravenous hydrocortisone 400 mg/day or methylprednisolone 60 mg/day without awaiting stool culture results. 1, 2, 3
• Provide supportive care: IV fluids, electrolyte replacement (potassium supplementation ≥60 mmol/day), blood transfusion to maintain hemoglobin >10 g/dL, and subcutaneous low-molecular-weight heparin for thromboprophylaxis (rectal bleeding is not a contraindication). 1, 2, 3
• Monitor daily: stool frequency, vital signs, complete blood count, CRP, albumin, and electrolytes. 2, 3
• By day 3, assess response: >8 stools/day or 3–8 stools/day with CRP >45 mg/L predicts ≈85% colectomy rate and signals need for rescue therapy. 1, 2
• Rescue options: infliximab 5 mg/kg IV at weeks 0,2,6 or ciclosporin 2 mg/kg/day IV. 1, 2, 3
• Urgent surgery is indicated for toxic megacolon not improving after 24–48 hours, colonic perforation, massive hemorrhage with hemodynamic instability, or failure of rescue therapy after 4–7 days. 1, 2

Maintenance Therapy

• Continue lifelong maintenance with oral mesalazine ≥2 g daily to reduce relapse risk and potentially lower colorectal cancer risk. 1, 2, 3
• For patients on combination therapy (TNF antagonist + thiopurine) in corticosteroid-free remission for ≥6 months, do not withdraw the TNF antagonist. 2, 3
• Discontinue 5-ASA in patients who have escalated to advanced therapies after documented 5-ASA failure. 2

Crohn's Disease Treatment Algorithm

Mild Ileocolonic or Colonic Disease

• Start with high-dose mesalazine 4 g daily as first-line therapy for mild ileocolonic Crohn's disease. 4, 1, 2
• Sulfasalazine 4–6 g daily is effective for active colonic disease but has higher side-effect rates; reserve for selected patients with reactive arthropathy. 4, 1, 2
• Topical mesalazine may be effective in left-sided colonic Crohn's disease of mild-to-moderate activity. 4, 1

Mild-to-Moderate Ileocecal Disease

• Use oral budesonide 9 mg daily as first-line therapy for isolated ileocecal Crohn's disease. 4, 1, 2
• Budesonide is marginally less effective than prednisolone but has a more favorable safety profile. 4, 1, 2
• Evaluate for symptomatic response between 4–8 weeks to determine need to modify therapy. 4
• Do not use budesonide for maintenance, as it does not sustain remission. 4, 2

Moderate-to-Severe Disease

• Initiate oral prednisolone 40–60 mg daily for moderate-to-severe Crohn's disease. 4, 1, 2
• Taper gradually over 8 weeks; rapid tapering (<8 weeks) is associated with early relapse. 4, 1, 2
• Evaluate for symptomatic response between 2–4 weeks to determine need to modify therapy. 4
• For steroid-dependent disease (requiring >1 course/year), add azathioprine 1.5–2.5 mg/kg/day or mercaptopurine 0.75–1.5 mg/kg/day as steroid-sparing agents. 4, 1, 2
• Thiopurines have a slow onset of action (8–12 weeks) and should not be used as monotherapy for active disease. 4, 2

Severe Disease (Hospitalization Required)

• Administer intravenous hydrocortisone 400 mg/day or methylprednisolone 60 mg/day plus intravenous metronidazole, because active disease is difficult to distinguish from septic complications. 4, 1, 2
• Evaluate for symptomatic response within 1 week to determine need to modify therapy. 4
• Infliximab 5 mg/kg is effective for severe disease but must be avoided in patients with obstructive symptoms. 4, 1, 2
• Screen for latent tuberculosis before initiating any anti-TNF therapy. 2
• Active sepsis (e.g., intra-abdominal abscess) is an absolute contraindication to anti-TNF therapy. 2

Fistulating and Perianal Disease

• Start with metronidazole 400 mg three times daily and/or ciprofloxacin 500 mg twice daily as first-line treatment for simple perianal fistulae. 4
• Azathioprine 1.5–2.5 mg/kg/day or mercaptopurine 0.75–1.5 mg/kg/day are potentially effective for simple perianal fistulae or enterocutaneous fistulae where distal obstruction and abscess have been excluded. 4
• Infliximab (three infusions of 5 mg/kg at 0,2, and 6 weeks) should be reserved for patients whose perianal or enterocutaneous fistulae are refractory to other treatment and should be used as part of a strategy that includes immunomodulation and surgery. 4
• Surgery (Seton drainage, fistulectomy, advancement flaps) is appropriate for persistent or complex fistulae in combination with medical treatment. 4

Maintenance Therapy

• Thiopurine monotherapy (azathioprine or mercaptopurine) can maintain remission achieved with corticosteroids in selected patients. 2
• Never use oral corticosteroids for long-term maintenance in Crohn's disease. 4, 2

Critical Monitoring and Treatment Targets

Ulcerative Colitis on Advanced Therapy

• Assess symptomatic response within 3 months of therapy initiation. 2
• Evaluate symptomatic and biochemical remission between 3–6 months. 2
• Confirm endoscopic improvement or remission within 6–12 months. 2

Treatment Response Assessment

• For corticosteroid therapy, evaluate clinical response within 2 weeks. 3
• For biologics, evaluate at 8–12 weeks. 3

Key Distinguishing Features Between UC and CD

• Ulcerative colitis responds well to aminosalicylates across all disease severities, making mesalazine the clear first-line choice. 1
• Crohn's disease has limited response to aminosalicylates, which are only appropriate for mild ileal/ileocolonic disease; corticosteroids are required earlier in the treatment algorithm. 4, 1
• Before escalating therapy in Crohn's disease, consider alternative explanations for persistent symptoms such as bacterial overgrowth, bile salt malabsorption, or fibrotic strictures rather than assuming active inflammation. 1, 2

Critical Pitfalls to Avoid

• Do not postpone corticosteroid therapy while awaiting stool microbiology results when severe inflammatory colitis is suspected. 1, 2, 3
• Never taper prednisolone faster than 8 weeks, as rapid tapering precipitates early relapse. 4, 1, 2
• Do not use infliximab in Crohn's disease patients with obstructive symptoms. 4, 1, 2
• Do not use topical corticosteroids as first-line therapy for ulcerative colitis; they are less effective than topical mesalazine. 1, 3
• Do not overlook topical therapy in ulcerative colitis—combination topical plus oral mesalazine is significantly more effective than oral alone. 1
• Avoid NSAIDs (ibuprofen, naproxen) during ulcerative colitis flares, as they can aggravate colonic inflammation. 3
• Address proximal constipation with stool-bulking agents or laxatives in ulcerative colitis, as constipation can exacerbate rectal symptoms. 3