A diverticulitis flare-up is an episode of inflammation or infection in one or more small pouches (diverticula) that have formed along the wall of your colon. These pouches are common and usually harmless, but when one becomes inflamed, it can cause sudden, significant abdominal pain that typically concentrates in the lower left side. About 75% of flare-ups remain uncomplicated and resolve with conservative treatment, but up to 25% develop complications like abscesses, perforations, or fistulas.
What Happens Inside Your Colon During a Flare-Up
Your colon wall has natural weak spots where blood vessels pass through the muscle layer. Over time, pressure inside the colon can push the inner lining outward through these weak spots, forming small balloon-like pouches. Having these pouches is called diverticulosis, and most people with them never have symptoms.
A flare-up starts when one of these pouches develops a tiny tear or perforation. This can happen when hardened stool or food particles erode the thin wall of the pouch, or when increased pressure inside the colon damages it. Once the wall is breached, bacteria from the colon leak into the surrounding tissue, triggering inflammation and infection. Changes in the gut microbiome and immune response also appear to play a role in why some pouches become inflamed while others don’t.
When the body’s immune system walls off the infection successfully, the flare-up stays mild. The surrounding fat and tissue essentially contain the small perforation, keeping it localized. But if the perforation is larger or the infection spreads, it can form an abscess (a walled-off pocket of pus), or in rare cases, rupture into the abdominal cavity, which is a medical emergency called peritonitis.
What a Flare-Up Feels Like
The hallmark symptom is abdominal pain, usually moderate to severe, that comes on suddenly. Most people feel it in the lower left abdomen, though the location can vary. The pain may start mild and build over hours, or it may hit intensely from the start. Your abdomen will likely feel tender to the touch.
Other common symptoms include:
- Fever
- Nausea or vomiting
- Changes in bowel habits, including sudden constipation or diarrhea
- Bloating or a visibly distended abdomen
- Rectal bleeding (less common but possible)
Symptoms that suggest a more serious complication include blood in your stool, facial paleness or weakness, frequent or painful urination (which can signal the infection is affecting nearby organs), or an abdomen that feels rigid and extremely sensitive. These warrant immediate medical attention.
How Flare-Ups Are Diagnosed
A CT scan of the abdomen is the gold standard for confirming a diverticulitis flare-up. It’s highly accurate, with sensitivity and specificity reaching 97% when contrast is used. The scan can show thickening of the colon wall, inflammation in the surrounding fat, abscesses, or free air that would indicate a perforation.
Blood tests often show an elevated white blood cell count, signaling infection. However, 20% to 40% of people with confirmed diverticulitis have a normal white blood cell count, so normal blood work alone doesn’t rule it out. The CT findings are what drive the diagnosis and help determine severity.
Treatment for Uncomplicated Flare-Ups
Most mild flare-ups are managed at home. Your doctor will likely recommend starting with clear liquids only for a few days to give your colon time to heal. That means broth, pulp-free fruit juice, water, tea, coffee without cream, gelatin, and ice pops. You should start feeling better within two to three days on this approach. Don’t stay on clear liquids for more than a few days, as it can lead to weakness and nutritional problems.
As symptoms improve, you’ll gradually transition to low-fiber foods before eventually returning to your regular diet. The pace of this transition depends on how quickly your symptoms resolve.
The role of antibiotics has shifted in recent years. A growing body of evidence suggests antibiotics aren’t necessary for otherwise healthy people with uncomplicated diverticulitis. An updated Cochrane review found that skipping antibiotics is a feasible approach, though the overall evidence quality remains low. In practice, adoption has been slow: a survey of surgeons found only 26% had stopped routinely prescribing antibiotics for uncomplicated cases. Antibiotics are still recommended for people with significant underlying health conditions, weakened immune systems, or signs of systemic infection.
When Complications Develop
Up to 25% of diverticulitis episodes become complicated, meaning the infection has spread beyond the immediate area. Complications include abscesses (contained pockets of infection), fistulas (abnormal tunnels between the colon and adjacent organs like the bladder), bowel obstruction, and perforation into the abdominal cavity.
Smaller abscesses can sometimes be drained with a needle guided by imaging. Larger abscesses, fistulas, or perforations typically require surgical intervention. If an abscess ruptures into the abdominal cavity, it causes peritonitis, a life-threatening condition that requires emergency surgery. The severity of complicated diverticulitis is graded on a four-stage scale, from a localized abscess (stage I) up to widespread fecal contamination of the abdominal cavity (stage IV).
Risk Factors That Trigger Flare-Ups
One of the more actionable risk factors is the use of anti-inflammatory pain relievers. Non-aspirin NSAIDs like ibuprofen and naproxen increase the risk of a diverticulitis flare-up by about 72% compared to people who don’t take them. Regular aspirin use raises the risk by roughly 25%, and daily aspirin use by 46%. These medications may weaken the colon wall or interfere with its ability to repair small tears, making a perforation more likely.
Other factors that increase flare-up risk include a low-fiber diet, obesity, smoking, and a sedentary lifestyle. Age is also a factor: diverticula become more common after 40, and the risk of inflammation increases with the number and size of pouches present.
Recovery Timeline
For a straightforward, uncomplicated flare-up, most people notice improvement within two to three days of starting a clear liquid diet (with or without antibiotics). Full recovery, meaning a return to normal eating and daily activities, typically takes one to two weeks. Your doctor will guide you through a staged return to higher-fiber foods as your symptoms allow.
Complicated cases take longer. If an abscess needs to be drained, recovery may take several weeks. Surgical cases can require a hospital stay followed by weeks of restricted activity. After a complicated flare-up, your doctor may recommend a colonoscopy once the inflammation has fully resolved (usually six to eight weeks later) to rule out other conditions and assess the extent of diverticular disease.
Recurrence is common. After a first episode, many people will have another flare-up at some point. Maintaining a high-fiber diet once you’ve fully recovered, staying physically active, and avoiding regular NSAID use are the most evidence-backed strategies for reducing your risk of a repeat episode.