Microscopic colitis: the watery diarrhea that only the microscope sees
Last updated September 3, 2026.
Microscopic colitis is inflammation of the colon invisible to the camera (the colonoscopy looks normal: only the biopsy under the microscope shows it): causing chronic, watery, non-bloody diarrhea, often with urgency and sometimes at night. It strikes mainly the over-50s (women slightly more), it is linked to the smoking, the certain common medications, and the autoimmune conditions, and it is treatable: the budesonide course settles the great majority.
What does it feel like?
The persistent watery diarrhea (the several-times-daily, watery, non-bloody: sometimes ten-plus), often with the urgency (the dash), sometimes at night (the waking-from-sleep diarrhea is a genuine clue), with the cramping, the bloating, the weight drifting down, and the fatigue. It runs for months to years, often mislabeled as the irritable bowel (the camera tests being normal), and it fluctuates (the flares and the lulls). The two microscopic kinds (the collagenous and the lymphocytic) behave the same to the patient.
Why does it happen?
The colon's lining inflamed at the microscopic level: the cause unclear, but the associations are genuine: the age over 50, the smoking (the current smokers get it more and worse), the common medications (the acid-suppressing PPIs, the anti-inflammatory painkillers, the SSRIs: linked: the medication review is part of the treatment), and the autoimmune family (the celiac, the thyroid, the diabetes type 1: the immune tendency). It is common (one of the commonest causes of the chronic watery diarrhea in the over-50s) and under-diagnosed (the normal-looking colonoscopy requires the biopsies to be taken).
How is it treated?
- The biopsies diagnose it: the colonoscopy with the biopsies (the camera normal, the microscope positive): the only route.
- The triggers audited: the suspect medications swapped where possible (the PPI, the anti-inflammatory, the SSRI review: part of the cure), the smoking stopped (helps).
- The budesonide course: the gut-targeted steroid (the weeks' course: settling the great majority, with the taper).
- The maintenance for the relapsing: the low-dose budesonide or the alternatives (the relapse after the course is common: manageable).
- The bile-acid question: the overlap with the bile-acid diarrhea (the sequestrants helping some).
When is it urgent?
Microscopic colitis is clinic medicine; the red flags that need the different door: the blood in the stool, the high fever, the severe abdominal pain, and the marked weight loss with the unwellness (the inflammatory bowel diseases and the cancer question get the workup). The chronic watery diarrhea in the over-50s earns the colonoscopy-with-biopsies referral: the diagnosis changes the treatment. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper.
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Common questions
My colonoscopy was normal. How can I have colitis?
Because this colitis is microscopic: the colonoscopy camera sees the colon's surface (the gross appearance: normal in this condition: that is the definition), and the inflammation lives at the cellular level (visible only when the biopsies are taken and the pathologist looks down the microscope: the collagen band or the lymphocyte crowding appearing), so the normal-looking colonoscopy excludes the ordinary colitis and the cancer but says nothing about the microscopic kind unless the biopsies were taken, which is the genuine diagnostic trap (the guidelines recommend the biopsies in the chronic-diarrhea workup, but they are not always taken), and your question to the team (were biopsies taken?) is the pivotal one. If they were not, the referral back is justified: the diagnosis changes the treatment from the IBS coping to the genuine cure.
Could my omeprazole be causing this?
possibly, and it is one of the condition's genuine quirks: the acid-suppressing PPIs (the omeprazole family) are linked to the microscopic colitis (the mechanism unclear, the association genuine: alongside the anti-inflammatory painkillers and the SSRIs), so the medication review is part of the treatment (the PPI swapped to the alternative or the lowest dose where the stomach allows: tried for the weeks, and the diarrhea improving in some), balanced against the reason you take it (the reflux needs managing too: the swap is the gastroenterology-and-GP judgment, never the abrupt self-stop if the stomach needs it). The medication audit is one of the condition's actionable levers, alongside the smoking, and it is worth raising at the review.
Why does it wake me at night?
The nocturnal diarrhea is diagnostic gold: the irritable bowel sleeps (the functional gut settles overnight: the IBS diarrhea is the daytime, the post-meal kind), while the inflammatory diarrhea continues around the clock (the inflammation does not sleep), so the waking-from-sleep-to-the-loose-stool is one of the useful distinguishing features (pointing toward the organic causes: the microscopic colitis among them, against the functional label), which is exactly why your eight-month nocturnal pattern deserved the biopsy-level look rather than the IBS label. The other organic clues you carry: the weight drifting down and the sheer chronicity. Bring the nocturnal detail to the referral explicitly: it moves the needle.
What is budesonide, and is it a steroid?
The gut-targeted steroid, and the clever design: budesonide is a corticosteroid formulated to release in the gut and then be destroyed by the liver on the first pass (the gut gets the full anti-inflammatory effect; the rest of the body gets almost none: avoiding the ordinary steroid side effects at the standard course), and it is the best-evidenced treatment in this condition (the trials: the great majority of patients settling within the weeks' course, superior to the alternatives), taken as the daily capsule for the six-to-eight weeks with the taper. The honest caveats: the relapse after stopping is common (the half-or-more kind: manageable with the repeat courses or the low maintenance dose), and the long courses get the steroid-question review. For the misery of the eight months you describe, it is often the life-changer within the fortnight.
Is this the same as Crohn's or ulcerative colitis?
No, and the difference matters for the prognosis: the microscopic colitis is the separate condition (the inflammation microscopic-only, the colon structurally normal, no ulcers, no bleeding), and crucially the gentler one: it does NOT carry the Crohn's-and-colitis complications (no strictures, no fistulas, no surgery, and no increased bowel-cancer risk above the ordinary), it responds to the budesonide and the trigger-removal, and a genuine proportion remits over the years. The shared features (the chronic diarrhea, the immune involvement) explain the confusion, but the courses differ genuinely: the Crohn's-and-colitis are the structural, progressive, immunosuppression-needing kind; the microscopic colitis is the inconvenient-but-manageable kind. The name your biopsy would give it is the gentle one of the family.
Will it ever go away completely?
The genuine arc: the condition fluctuates (the flares and the lulls), the budesonide induces the remission (the great majority settle on the course), the relapse happens (common: the repeat courses or the low maintenance manage it), and the genuine long-term picture: a substantial proportion remit fully over the years (the condition burning out in the studies' follow-up), the trigger-removal helping the odds (the medication swap, the smoking stopped), and the quality-of-life between the flares normal for most. So the frame: treatable now (the budesonide fortnight ends the eight-month misery for most), manageable long-term (the maintenance light), and possibly-transient over the years. The losing-hope phase ends with the biopsy result and the first course.