Ulcerative Colitis Symptoms and Flares: What to Watch For

By Anna Curran, RN, BSN, PHN · Last reviewed October 2026

The most common ulcerative colitis symptoms are diarrhea with blood or mucus, belly cramps and a sudden, urgent need to use the bathroom. Symptoms tend to come in flares, then calm down during remission. Severe pain, heavy bleeding, a high fever or a swollen belly are emergency signs that need care right away.

Knowing your usual pattern helps you spot a flare early and call your care team before it gets worse. Here is what to watch for.

Common Ulcerative Colitis Symptoms

Ulcerative colitis (UC) inflames the lining of the colon and rectum. NIDDK and Mayo Clinic list these common symptoms:

Diarrhea, often with blood, mucus or pus
Rectal bleeding or blood in the stool
Belly cramps and pain
Urgency, a sudden need to have a bowel movement
Tenesmus, the feeling that you need to go even when your bowel is empty
Rectal pain
Waking up at night to have a bowel movement

With more severe or widespread disease, you may also have:

Fatigue
Fever
Nausea or loss of appetite
Weight loss
Slow growth in children

Symptoms depend on where UC is

Your symptoms often match how much of your colon is inflamed. With ulcerative proctitis, which affects only the rectum, rectal bleeding and urgency may be the only signs. Mayo Clinic notes that some people with proctitis even feel constipated.

When more of the colon is involved, bloody diarrhea, belly pain and weight loss are more common. Learn more about the types in our guide to what ulcerative colitis is.

Symptoms Outside the Gut

UC can affect other parts of the body too. Mayo Clinic and NIDDK list:

Joint pain and swelling
Eye redness, pain or irritation
Skin sores or rashes
Liver and bile duct problems
Anemia, which can make you tired, pale or short of breath
A higher risk of blood clots

Tell your care team about any of these. They are part of your UC care, not a separate issue to ignore.

What Is a Flare?

A flare is when symptoms come back or get worse after a calmer period. Remission is the time when symptoms ease or go away. According to NIDDK, remission can last weeks or years.

Flares can start slowly over days or come on suddenly. Many people notice small early signs first, such as:

More bowel movements than usual
Looser stools
A little blood or mucus
More urgency or cramping
Feeling more tired than usual

Call your gastroenterologist when you notice these changes. Treating a flare early may help keep it from getting worse.

Mild vs. Severe Flares

Flares range from mild to life-threatening. Doctors look at how many bowel movements you have, how much blood you see and how you feel overall. Here is a simple guide based on NIDDK and Cleveland Clinic:

Flare level	What it may look like
Mild	Up to about four bowel movements a day. You may feel mostly well.
Moderate to severe	Four or more bowel movements a day, blood or pus in the stool, fever, weight loss. NIDDK describes severe UC as more than six bloody bowel movements a day.
Fulminant (very severe)	More than 10 bloody bowel movements a day. This is rare and usually needs hospital care.

Cleveland Clinic notes that about half of people with UC have mild symptoms during flares.

These numbers are only a guide. If your symptoms feel worse than your usual flare, trust that feeling and call your care team.

What Can Trigger a Flare?

A flare can happen even when you do everything right. Still, the Crohn’s & Colitis Foundation lists some common triggers:

Missed doses of your UC medicine, even during remission
NSAIDs, such as ibuprofen, naproxen and aspirin
Antibiotics, which can sometimes set off a flare
Gut infections, such as C. difficile
Stress, which can worsen symptoms like diarrhea and belly pain

If you take aspirin for your heart or another reason, do not stop it on your own. Ask your doctor what is safest for you. For pain relief, ask whether acetaminophen is a good choice.

When you have a flare, your doctor may check a stool sample for infection. This is because a gut infection can look just like a UC flare.

Tracking Your Symptoms

A simple log can help your care team see patterns. Each day, you might write down:

How many bowel movements you had
Whether you saw blood or mucus
Your pain level from 0 to 10
Any fever
Your energy level
What you ate and drank
Any missed medicine doses
New stress, illness or travel

Bring your log to your appointments. It gives your doctor a clearer picture than memory alone.

When to Call Your Doctor

MedlinePlus advises contacting your provider if you have:

Cramps or pain in your lower belly
Bloody diarrhea, often with mucus or pus
Diarrhea that does not get better with diet changes and medicine
Rectal bleeding, drainage or sores
A fever that lasts more than 2 or 3 days, or an unexplained fever higher than 100.4°F (38°C)
Nausea and vomiting that lasts more than a day
Skin sores that do not heal
Joint pain that keeps you from your daily activities
Little warning before you need a bowel movement
Waking up at night to have a bowel movement
Side effects from any UC medicine

Also call if you take a medicine that weakens your immune system and you get a fever or signs of infection.

Emergency Signs: Call 911 or Go to the ER

Some UC complications are life-threatening. Call 911 or go to the emergency room if you have:

Heavy rectal bleeding, passing large amounts of blood or blood clots
Severe or constant belly pain
A high fever with belly pain
A swollen, tight or very tender belly
A fast heartbeat, dizziness or fainting
Signs of severe dehydration, such as very little urine, extreme thirst, confusion or feeling lightheaded when you stand
Vomiting that keeps you from holding down liquids

Know the signs of toxic megacolon

Toxic megacolon is a rare but serious complication. The colon swells quickly and can stop working. If it is not treated, the colon can tear, which can lead to a dangerous infection in the blood.

MedlinePlus lists these signs:

A painful, swollen belly
Fever
Diarrhea, usually bloody
A fast heart rate

If you have severe belly pain along with any of these, go to the emergency room right away. Do not wait to see if it gets better on its own.

Mayo Clinic notes that anti-diarrhea medicines may raise the risk of toxic megacolon when inflammation is severe. Check with your doctor before taking loperamide or any other over-the-counter medicine for diarrhea.

Frequently Asked Questions

What does an ulcerative colitis flare feel like?

Many people notice more frequent, looser stools with blood or mucus, cramps and urgency. Fatigue and low appetite are also common. Flares can be mild or severe and may feel different each time.

How long does a UC flare last?

It varies. A flare can last days to weeks, and treatment can help shorten it. Call your gastroenterologist when a flare starts so you can get the right care early.

Is blood in the stool always a sign of a flare?

Not always. Blood can also come from hemorrhoids, a tear or other conditions. Still, call your doctor about any new or increased bleeding, and go to the ER for heavy bleeding.

Can I have ulcerative colitis without diarrhea?

Yes. Some people, especially those with ulcerative proctitis, have rectal bleeding or urgency without much diarrhea. Some even have constipation.

The Bottom Line

Ulcerative colitis symptoms often include bloody diarrhea, cramps and urgency, and they tend to come in flares. Tracking your symptoms helps you and your care team spot changes early. Call your doctor for new or worsening symptoms, fever or medicine side effects. Go to the ER for heavy bleeding, severe belly pain, a swollen belly, a high fever or signs of dehydration.