What Is Microscopic Colitis? Symptoms, Causes & Treatment

Microscopic colitis is a type of inflammatory bowel disease in which the lining of the colon becomes chronically inflamed, causing persistent watery diarrhea, but the inflammation is invisible during a standard colonoscopy. The colon looks completely normal to the naked eye, and the condition only reveals itself under a microscope after tissue samples are taken. That paradox is what gives the disease its name and what makes it so frequently missed. It affects roughly 10 to 11 people per 100,000 each year, hits women far harder than men, and responds well to treatment once correctly identified.

What Microscopic Colitis Feels Like

The hallmark symptom is chronic, watery, non-bloody diarrhea. In a study of patients who had been misdiagnosed with irritable bowel syndrome before their microscopic colitis was caught, every single patient had this type of diarrhea. But the symptom picture doesn’t stop there. Night-time diarrhea was common, appearing in the majority of patients, along with abdominal pain, bloating, gas, and mild weight loss.¹ Some people have a few loose stools a day; others have ten or more. The diarrhea can be so urgent and unpredictable that it reshapes daily routines, limits travel, and causes genuine anxiety about being far from a restroom.

What the diarrhea is not is bloody. The absence of visible blood in the stool is one reason the disease flies under the radar. Doctors may attribute the symptoms to stress, diet, or irritable bowel syndrome, particularly when colonoscopy images look unremarkable. Fatigue, joint pain, and general malaise sometimes accompany the bowel symptoms, likely reflecting the autoimmune dimension of the disease discussed below.

Who Gets It

Microscopic colitis is primarily a disease of middle-aged and older adults. A large Swedish cohort study found the average age at diagnosis was about 60, and the condition was far more common in women, who accounted for roughly 72% of cases.² People over 65 are more than five times as likely to be diagnosed compared with those under 65.³ The female skew is strong: women are roughly four times more likely than men to develop the condition overall, and the gap is even wider for the lymphocytic subtype.

That said, younger people can and do develop microscopic colitis. It simply occurs at far lower rates. The lifetime risk of developing the disease is under 1% for women and about a third of a percent for men, so while it is not rare, it is not common enough for most people to have heard of it before they are diagnosed.²

Why It So Often Gets Missed

The single biggest diagnostic pitfall is that microscopic colitis looks exactly like diarrhea-predominant irritable bowel syndrome from the outside. Both conditions cause recurring, non-bloody, watery diarrhea. Both produce a colon that looks normal on colonoscopy. The only way to tell them apart is by taking biopsies of the colon’s lining and examining them under a microscope.¹ Without that step, microscopic colitis is invisible.

In one study of 247 patients who had been told they had diarrhea-predominant IBS, about 6% actually had microscopic colitis once biopsies were finally performed.¹ A case series of older adults found delays between symptom onset and correct diagnosis ranging from two months to six years, with every patient having first been told they had IBS.⁴ That delay matters, because microscopic colitis responds well to specific treatment that IBS-targeted therapies do not address.

How It Is Diagnosed

Diagnosis requires a colonoscopy with biopsies, and the biopsies need to come from the right places. Because the inflammation in microscopic colitis can be patchy, sampling only one segment of the colon risks a false-negative result. Most experts recommend biopsying at least the ascending colon (right side) and the descending colon (left side).⁵ A study found that combining biopsies from these two locations caught 100% of microscopic colitis cases, matching the detection rate of more extensive protocols that sample four sites. A systematic review recommended taking at least six biopsy specimens, split between the ascending and descending colon, to ensure high diagnostic certainty.⁶

The inflammation tends to be more pronounced in the right colon and less severe in the rectum, which is another trap. Biopsies confined to the rectum alone, as might happen in a limited sigmoidoscopy, frequently miss the disease.⁷

The Two Main Subtypes

Under the microscope, microscopic colitis comes in two flavors. The distinction between them depends on what the pathologist sees in the tissue sample, not on how the patient feels, since the symptoms of both are essentially the same.

Collagenous colitis: The defining feature is a thickened band of collagen beneath the surface lining of the colon. Normally that band is about 5 micrometers thick; in collagenous colitis it reaches 10 micrometers or more. The band has a characteristic ragged, irregular lower edge and often traps tiny blood vessels and inflammatory cells within it.⁸ Collagenous colitis is more strongly linked to female sex and tends to be diagnosed at a slightly older age.
Lymphocytic colitis: There is no thickened collagen band. Instead, what stands out is a sharp increase in lymphocytes (a type of immune cell) within the surface lining of the colon. In healthy tissue, there are about five of these cells per 100 surface cells; in lymphocytic colitis the count rises to 20 or more.⁸ A population-based study in Minnesota found that lymphocytic colitis was the more common subtype, diagnosed at a rate of about 5.5 per 100,000 compared with 3.1 per 100,000 for collagenous colitis.⁹

Despite the histological differences, the two subtypes share many similarities, including inflammatory patterns in the deeper tissue layers.¹⁰ Some researchers have argued they may be stages or variants of the same disease rather than truly separate conditions. There is also a third, less established category called incomplete microscopic colitis, discussed later in this article.

Causes and Risk Factors

The exact cause of microscopic colitis remains unclear, but several risk factors are well established. The picture that emerges is of a disease triggered by an abnormal immune response in the colon, set off or amplified by drugs, smoking, and the patient’s broader immune profile.

Medications

Certain common medications are linked to a meaningfully higher risk. Proton pump inhibitors (PPIs), the acid-suppressing drugs widely prescribed for heartburn and reflux, carry the strongest association. A systematic review and meta-analysis found that PPI users had roughly two and a half times the odds of developing microscopic colitis.¹¹ NSAIDs (common painkillers like ibuprofen and naproxen) and SSRIs (a class of antidepressants) each roughly doubled the odds.¹² The combination of a PPI and an NSAID at the same time was especially risky, raising the odds roughly fivefold. Continued use for several months further increased the danger.

This is why one of the first steps in managing a new diagnosis often involves reviewing the patient’s medication list and, where feasible, discontinuing or substituting suspected culprits.¹³ In a minority of patients, stopping the offending medication alone is enough to resolve symptoms.

Smoking

Cigarette smoking is another significant risk factor. A large prospective study of U.S. women found that current smokers had about two and a half times the risk of developing microscopic colitis compared with people who had never smoked. Past smokers retained a moderately elevated risk, but it declined over time after quitting. More than five years after stopping, the risk dropped substantially.¹⁴ A separate case-control study found even steeper numbers: current smokers faced a more than fourfold increase in risk.¹⁵ The risk rose with cumulative pack-years of smoking, reinforcing the dose-response relationship.

Autoimmune Connections

Microscopic colitis clusters with other autoimmune conditions. A large Danish case-control study found that about 22% of patients with microscopic colitis had at least one other autoimmune disease, compared with 11% of the general population. After adjusting for other factors, the odds of having a coexisting autoimmune condition were roughly doubled.¹⁶ The associations were strongest for gastrointestinal and endocrine autoimmune diseases and for connective tissue disorders.

Celiac disease is a particularly notable overlap. A meta-analysis found that celiac disease was roughly eight times more common in people with microscopic colitis than in the general population, with about 6% of microscopic colitis patients having concurrent celiac disease.¹⁷ Because celiac disease also causes chronic diarrhea, an undetected overlap could complicate treatment if only one condition is addressed.

Gut Microbiome

Emerging research suggests the gut microbiome plays a role. One study found that patients with microscopic colitis had a less diverse community of gut bacteria compared with healthy individuals. Their microbiomes were depleted of certain beneficial bacteria and enriched for others, with predicted increases in immune-activation pathways. When lab-grown colon cells were exposed to products from microscopic colitis patients’ stool, the cells showed reduced expression of a protein involved in maintaining the gut barrier and increased signaling for immune cell recruitment.¹⁸ Whether these microbial changes are a cause or a consequence of the inflammation is still being worked out.

Treatment With Budesonide

The go-to drug for microscopic colitis is budesonide, a corticosteroid that acts primarily in the gut with limited absorption into the rest of the body. A randomized, placebo-controlled trial in collagenous colitis found that about 87% of patients on budesonide achieved clinical remission, compared with roughly 14% on placebo. Improvement in the tissue under the microscope accompanied the symptom relief.¹⁹ A population-based study confirmed similarly strong numbers: about 80% of patients had a complete response to budesonide induction, and another 14% had a partial response.²⁰

The catch is relapse. When budesonide is stopped after an initial course, relapse rates can run as high as 80%.²¹ Many patients therefore need low-dose maintenance therapy. In the population-based study, every patient placed on budesonide maintenance responded, and none relapsed while continuing the drug.²⁰ Long-term budesonide use does carry some concerns, particularly around bone density, but it is far better tolerated than systemic steroids like prednisone.

When Budesonide Is Not Enough

A small percentage of patients either do not respond to budesonide or cannot tolerate it. For these steroid-refractory cases, several alternatives exist, though none have the same depth of evidence. Bismuth subsalicylate (the active ingredient in Pepto-Bismol), anti-diarrheal medications like loperamide, and bile acid sequestrants like cholestyramine are all used.²²

Biologic drugs are an emerging option for difficult cases. A meta-analysis of patients with steroid-refractory disease found that vedolizumab, a drug that targets gut-specific immune pathways, produced clinical remission in roughly 64% of patients. Infliximab achieved remission in about 58%, while adalimumab performed less well at around 39%.²³ These numbers come from relatively small pooled study populations, so they should be treated as encouraging signals rather than definitive benchmarks. A multicenter cohort from France showed that some patients needed to try a second or even third biologic agent before finding one that worked, which speaks to how individualized treatment can become in refractory cases.²⁴

Bile Acid Malabsorption and Its Overlap

A related but distinct problem that often travels alongside microscopic colitis is bile acid malabsorption. Normally, bile acids released during digestion are reabsorbed before they reach the colon. When that process fails, excess bile acids irritate the colon and worsen diarrhea. One study found that about 43% of microscopic colitis patients had concurrent bile acid malabsorption, with the rate being substantially higher in the lymphocytic subtype (60%) than in collagenous colitis (27%). Cholestyramine, a bile acid-binding medication, induced remission in nearly all of those patients who had the overlap.²⁵

A more recent study using a blood test for bile acid synthesis found a lower prevalence (about 12%) but confirmed that budesonide treatment normalized bile acid levels in stool even in those patients.²⁶ The spread in prevalence figures likely reflects different diagnostic thresholds and testing methods. The practical takeaway is that if diarrhea persists despite standard treatment, bile acid malabsorption is worth investigating as a contributing factor.

Does Microscopic Colitis Raise Cancer Risk?

Unlike ulcerative colitis and Crohn’s disease, microscopic colitis does not appear to increase the risk of colorectal cancer. In fact, multiple studies point in the opposite direction. A large Danish cohort study found that microscopic colitis patients were diagnosed with colorectal cancer at only about half the rate expected based on the general population.²⁷ A U.S. retrospective study found similar odds of polyps in microscopic colitis patients compared with colonoscopy controls, and no increase in colon cancer rates compared with national cancer data.²⁸ Another analysis found that the overall risk of colorectal cancer and adenomas was actually lower in people with microscopic colitis.²⁹

Some of that seemingly protective effect may be an artifact of surveillance: people with microscopic colitis tend to get colonoscopies, and polyps found early can be removed before they turn cancerous. Regardless of the mechanism, this is reassuring news for patients who worry that their condition might lead to something more serious. The majority of patients experience what researchers describe as an indolent disease course.

The Emerging Category of Incomplete Microscopic Colitis

Not every biopsy fits neatly into the collagenous or lymphocytic box. Some patients have symptoms that match microscopic colitis perfectly, but their tissue samples show borderline changes: a collagen band thicker than normal but not quite reaching the diagnostic threshold, or an elevated lymphocyte count that falls short of the cutoff. This gray zone has been termed incomplete microscopic colitis.³⁰

A large study of over 200 patients with incomplete microscopic colitis found that their demographics, medication exposures, and treatment responses were strikingly similar to those of patients with full-blown disease. About 73% had a complete response to treatment, and roughly 29% were helped by budesonide. After stopping therapy, about 36% relapsed, and about 14% eventually progressed to meet the full criteria for microscopic colitis.³¹ These findings have prompted some experts to argue that the definition of microscopic colitis should be broadened.³² For patients, the practical message is that borderline biopsy results in the presence of classic symptoms should still be taken seriously, not dismissed as inconclusive.

Living With the Condition

Even beyond the physical symptoms, microscopic colitis takes a real toll on daily life. A qualitative study of patients’ experiences found a recurring theme of struggling with what they called an “invisible, disabling disease.” Participants described not just the burden of unpredictable diarrhea but associated problems: fatigue, disrupted sleep, social withdrawal, and persistent anxiety about bowel function. That anxiety lingered even during periods of remission, creating a background hum of worry about the next flare.³³

In older adults, the impact can extend to functional independence. One case report documented how untreated microscopic colitis left a patient increasingly dependent on caregiving, and how successful treatment reversed that trajectory, preserving her ability to live independently.³⁴ Because the disease disproportionately affects older adults and can mimic the bowel changes people sometimes write off as part of aging, it deserves clinical suspicion rather than a shrug. The invisibility of the condition, both on a colonoscopy screen and in social life, is precisely what makes prompt diagnosis and effective treatment so important.

1
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PubMed Central. Smoking is Associated with an Increased Risk of Microscopic Colitis: Results From Two Large Prospective Cohort Studies of US Women
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PubMed. Current and past cigarette smoking significantly increase risk for microscopic colitis
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PubMed. Autoimmune diseases in microscopic colitis: A Danish nationwide case-control study
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