A Crohn’s flare-up is a period when the chronic inflammation in your digestive tract becomes active again after a stretch of feeling relatively normal. During remission, symptoms quiet down and the intestinal lining may partially heal. During a flare, your immune system ramps up its attack on the gut wall, producing a cascade of inflammatory signals that cause pain, diarrhea, fatigue, and a range of other symptoms that can last days to weeks.
What Happens Inside Your Gut During a Flare
Crohn’s disease is driven by an overactive immune response in the intestinal wall. During a flare, immune cells release a surge of inflammatory signaling molecules, most notably TNF-alpha, along with several interleukins that amplify the response. These signals recruit more immune cells to the area, creating a cycle of escalating inflammation.
What makes Crohn’s especially damaging is what happens after the acute inflammation. The body attempts to repair the injured tissue, but in Crohn’s disease, the repair process goes wrong. Cells called myofibroblasts overproduce scar tissue (fibrosis) in the intestinal wall, and the normal breakdown-and-cleanup system that should dissolve excess scar tissue doesn’t work properly. Over time, repeated flares can cause the intestinal wall to thicken and stiffen. This is why managing flares matters beyond just controlling symptoms: each episode of active inflammation carries a risk of permanent structural damage, including narrowing of the intestine (strictures) that can eventually cause blockages.
Common Symptoms of a Flare
Flares can come on gradually over several days or hit suddenly without warning. The core symptoms are digestive:
- Diarrhea, often persistent and urgent
- Abdominal pain and cramping, typically in the lower right area but variable depending on where the disease is active
- Blood in the stool
- Reduced appetite and weight loss
- Fatigue that feels out of proportion to your activity level
- Fever
- Mouth sores
- Pain or drainage near the anus, which can indicate a fistula (an abnormal tunnel between the intestine and skin)
The severity varies enormously from person to person and from flare to flare. Some people experience mild diarrhea and low-grade cramping. Others are unable to keep food down and lose significant weight over a short period.
Symptoms Beyond the Gut
Crohn’s flares frequently affect parts of the body that seem unrelated to digestion. Joint pain and swelling are among the most common, particularly in the knees, ankles, and wrists. Skin problems can appear, including tender red bumps on the shins or painful ulcers. Eye inflammation is another possibility. One type, called episcleritis (redness and soreness on the white of the eye), tends to track directly with active gut inflammation, meaning it flares and calms alongside your digestive symptoms.
Repeated or prolonged flares can also lead to iron deficiency and anemia from blood loss and poor nutrient absorption, kidney stones, and bone thinning. In children, poorly controlled disease can delay growth and puberty.
What Triggers a Flare
Flares often seem to come out of nowhere, but several known factors raise the risk. Stopping or inconsistently taking maintenance medication is one of the most common and preventable triggers. Nonsteroidal anti-inflammatory drugs like ibuprofen and naproxen can irritate the gut lining and provoke inflammation. Infections, whether a stomach bug or something unrelated like an upper respiratory illness, can also destabilize the immune balance in the gut.
Stress doesn’t cause Crohn’s disease, but it reliably worsens it. Chronic psychological stress alters immune function and gut permeability in ways that can tip a quiet disease into an active one. Smoking is another well-established trigger; people with Crohn’s who smoke have more frequent flares, more complications, and a higher likelihood of needing surgery. Antibiotics, by disrupting the gut microbiome, can also set off a flare in some people.
How Long a Flare Typically Lasts
There’s no single answer here because flare duration depends on the severity of inflammation, how quickly treatment starts, and how your body responds. Mild flares caught early and treated promptly may resolve within a few days to a couple of weeks. More severe flares, especially those involving deep ulceration or complications like strictures, can take weeks to months to bring under control. Some people cycle in and out of low-grade flares without ever reaching full remission, which is a sign that the current treatment plan needs adjustment.
How Doctors Confirm a Flare
Your symptoms tell part of the story, but inflammation can be brewing before you feel it, and some symptoms (like diarrhea) can have causes other than active Crohn’s. Two lab markers help clarify what’s happening.
Fecal calprotectin is a protein released by immune cells in the gut, and it’s measured through a stool sample. During remission, levels typically sit below about 75 micrograms per gram. In mild disease, the median rises to around 165, and in moderate-to-severe flares, it jumps to roughly 445. A reading below 75 predicts clinical remission with about 89% sensitivity, making it a reliable way to confirm that things are genuinely calm or genuinely inflamed.
C-reactive protein (CRP) is a blood marker of body-wide inflammation. In Crohn’s, a level of 10 mg/L or higher signals active inflammation. A CRP above 19 mg/L suggests moderate-to-severe disease activity. CRP is less specific to the gut than fecal calprotectin, since infections and other conditions can raise it, but a high reading in someone with known Crohn’s disease strongly supports the presence of a flare.
Eating During a Flare
When inflammation is active, the gut has a harder time processing certain foods. The general approach is to temporarily reduce fiber and residue, giving the irritated intestinal lining less mechanical work to do. This is especially important if you have or are suspected of having a stricture, where bulky food can cause a painful backup.
Foods commonly restricted during a flare include:
- High-fiber grains: whole wheat, brown rice, wild rice, quinoa, wheat bran
- Raw vegetables and tough skins: broccoli, cauliflower, cabbage, corn, celery, potato skins
- Fruits with seeds, skins, or membranes: berries, figs, oranges, dried fruits
- Legumes: beans, lentils, peas
- Nuts, seeds, popcorn, and coconut
- Juices with pulp and prune juice
Bananas and peeled potatoes are generally tolerated even during active symptoms. White rice, refined bread, eggs, lean poultry, and fish tend to be easier to digest. The goal isn’t a permanent restriction. Once inflammation settles, foods should be reintroduced gradually based on tolerance. Prolonged unnecessary restriction can lead to nutritional deficiencies, which are already a risk with Crohn’s disease.
Warning Signs That Need Urgent Attention
Most flares can be managed by contacting your care team and adjusting treatment. Some situations, however, require immediate medical attention:
- A high fever or chills that don’t resolve
- Inability to keep any food or liquids down
- Significant blood in the stool
- Feeling weak, dizzy, or on the verge of passing out
- Severe abdominal pain that is constant and worsening, which could indicate a bowel obstruction or perforation
Symptoms lasting more than a few days without improvement also warrant a call to your gastroenterologist, even if they don’t feel like an emergency. Letting a flare smolder increases the risk of the kind of deep tissue damage and fibrosis that becomes permanent.