Diverticulitis is inflammation or infection of small bulging pouches that form in the wall of your large intestine. These pouches, called diverticula, are extremely common as people age and usually cause no problems on their own. But when one or more of them becomes blocked and inflamed, the resulting condition can cause significant pain, most often in the lower left side of your abdomen.
Diverticulosis vs. Diverticulitis
The distinction matters. Diverticulosis simply means you have diverticula present in your digestive tract. Many people do and never know it. The pouches form at weak spots in the colon wall, typically where blood vessels pass through the muscle layer. They’re most common in the sigmoid colon, the S-shaped section just above the rectum on your left side.
Diverticulitis is the complication. It occurs when one or more of those pouches becomes inflamed or infected. Only a fraction of people with diverticulosis ever develop diverticulitis, but when it happens, it tends to come on fast.
How Inflammation Starts
The exact chain of events isn’t fully settled, but the leading explanation involves blockage. Fecal material or undigested food particles collect inside a pouch, plugging the narrow opening. Once trapped, mucus secretion builds up and normal gut bacteria overgrow in the confined space. The pouch swells, blood flow to its thin wall gets compromised, and tiny tears (called microperforations) develop. Those small tears allow bacteria to leak into surrounding tissue, triggering inflammation and, in some cases, a localized infection.
An alternative theory focuses on mechanical wear. Increased pressure inside the colon, or hard food particles scraping against the pouch lining, gradually erodes the wall until it becomes inflamed and perforates. Either way, the end result is the same: a breach in the pouch wall that sets off an immune response in the surrounding tissue.
What Causes It and Who’s at Risk
No single factor causes diverticulitis. It develops from a combination of structural weakness in the colon wall, pressure dynamics, and lifestyle influences that accumulate over years.
Low-Fiber Diet
Fiber softens stool and helps it move through the colon more quickly, which reduces the pressure your colon has to generate to push things along. A diet low in fiber means harder stool and higher internal pressure, both of which encourage pouches to form and make blockages more likely. High-fiber diets are consistently associated with lower rates of diverticular disease.
Body Weight
Carrying excess weight raises your risk meaningfully. A large study of women found that those with a BMI of 35 or higher had a 42% greater risk of diverticulitis compared to women with a BMI under 22.5. Waist circumference and waist-to-hip ratio independently predicted risk as well. Women who gained 20 kg (about 44 pounds) or more from age 18 had a 73% increased risk compared to those who maintained their weight.
Genetics
There’s a heritable component. If close family members have had diverticular disease, your own risk is higher. The specifics of which genes are involved are still being worked out, but genetics influence the structural integrity of the colon wall and how the muscle layer functions.
Certain Pain Medications
Regular use of common over-the-counter pain relievers is linked to higher risk. In a study of men, regular use of non-aspirin anti-inflammatory drugs (like ibuprofen or naproxen) was associated with a 72% increased risk of diverticulitis and a 74% increased risk of diverticular bleeding. Regular aspirin use carried a smaller but still notable 25% increase in diverticulitis risk. These drugs can weaken the protective lining of the gut, making an already-thin pouch wall more vulnerable.
Age
Diverticula become more common with age as the colon wall naturally loses elasticity. Most cases of diverticulitis occur in people over 40, and the prevalence of the underlying pouches rises steadily through later decades.
What It Feels Like
The hallmark symptom is abdominal pain, most often in the lower left side. This makes anatomical sense because the sigmoid colon sits in the lower left abdomen, and that’s where diverticula most frequently form. The pain is usually sudden and intense, though in some people it starts mild and builds over several hours. The intensity can also fluctuate, easing and then worsening again.
Beyond the pain, you may notice fever, nausea, tenderness when the abdomen is touched, and changes in bowel habits. Some people develop sudden constipation, others diarrhea. The combination of lower left abdominal pain with fever is the classic presentation that prompts most people to seek medical attention.
How It’s Diagnosed
A CT scan is the primary diagnostic tool. It has a diagnostic accuracy of about 98% for diverticulitis and can simultaneously rule out other conditions that cause similar symptoms, like appendicitis or ovarian problems. The scan also reveals how severe the inflammation is and whether complications like abscesses have formed, which directly shapes treatment decisions. Ultrasound can also detect diverticulitis with over 90% sensitivity, though CT remains the standard in most settings.
Mild vs. Complicated Cases
Not all diverticulitis is the same, and severity determines what happens next. Doctors classify cases using a staging system that ranges from mild, localized inflammation to life-threatening complications.
In uncomplicated diverticulitis, the inflammation stays contained near the affected pouch. Most cases fall into this category. Recovery typically happens at home over a week or so, with a temporary shift to easily digestible foods while symptoms resolve..
Complicated diverticulitis means the inflammation has progressed. At the lower end, this involves a small abscess forming near the colon. More serious stages involve larger abscesses spreading into the pelvis or deeper abdominal spaces. The most severe cases involve a free perforation where infected material spills into the abdominal cavity, causing peritonitis. This is a surgical emergency.
Treatment Has Shifted
For decades, antibiotics were automatic for any diverticulitis diagnosis. That’s changed. Current guidelines from the American Gastroenterological Association recommend that antibiotics be used selectively rather than routinely for mild, uncomplicated cases in otherwise healthy people. A review of over 2,500 patients found no difference in recovery time, readmission rates, complication progression, or need for surgery between those treated with antibiotics and those managed without them.
This means that if you’re generally healthy and your case is mild, your doctor may recommend rest, a temporary diet modification, and monitoring rather than a course of antibiotics. Antibiotics are still recommended when symptoms are more severe, when bloodwork shows significant inflammation, or when you have a weakened immune system or other health conditions that raise the stakes of an untreated infection.
For complicated cases, treatment escalates. Small abscesses may be drained with a needle guided by imaging. Larger perforations or peritonitis require surgery to remove the damaged section of colon.
Preventing Flare-Ups
If you’ve had one episode of diverticulitis, the most effective long-term strategy is increasing your fiber intake. High-fiber foods soften stool and reduce the pressure your colon has to exert, which lowers the chance of pouches forming or existing ones becoming blocked. Fruits, vegetables, whole grains, and legumes are the main sources. Aim to increase fiber gradually, since a sudden jump can cause bloating and gas.
One outdated piece of advice you can safely ignore: you do not need to avoid nuts, seeds, or popcorn. For years, patients were told these foods could lodge in diverticula and trigger attacks. There’s no evidence this is true, and most gastroenterology guidelines no longer support this restriction.
Maintaining a healthy weight, staying physically active, and being cautious with regular use of anti-inflammatory pain medications all contribute to lowering your risk of recurrence. If you rely on ibuprofen or similar drugs for chronic pain, it’s worth discussing alternatives with your doctor given the documented increase in diverticular complications.