Crohn’s disease and ulcerative colitis: What’s the difference?

28 Min Read

Problems affecting the digestive system can have so many causes. The gut is a highly sensitive organ, and there are several factors that can affect its function, such as tiredness, stress, certain foods, alcohol consumption, or infections.

This is perhaps partly the reason why many people might choose not to dwell on symptoms such as diarrhea, or pain when going to the toilet when they arise. In the vast majority of cases, such symptoms will tend to settle on their own once the instigating factor has either passed (in the case of tiredness or an infection) or been addressed (stress, alcohol, or food).

In cases where these problems persist, someone may still be hesitant to approach their doctor. The nature of such symptoms can make them difficult to discuss, and people tend to have very individual habits when going to the toilet; so a definition what is of ‘normal’ isn’t always clear.

It’s unsurprising then that chronic conditions affecting the bowel, namely Crohn’s disease and ulcerative colitis, will for long periods go undiagnosed. Mild cases of diarrhea or intestinal pain might not seem like issues worthy of attention; and even where symptoms seem more pronounced, it’s easy to presume they will pass.

But if you’re experiencing prolonged or significant episodes of diarrhea on a regular basis, it’s important to see a doctor. Some cases may indeed be caused by passing factors, but others may be indicative of an illness requiring medical attention.   

In this post, we’ll be discussing the two most common forms of inflammatory bowel disease, Crohn’s disease, and ulcerative colitis, and examining:

  • what the bowel does
  • what the conditions are, and the differences between them
  • what symptoms they can cause
  • who it affects
  • what causes the conditions
  • what diagnosis involves
  • how the conditions are managed
  • and what treatment may involve

What does the bowel do?

It is perhaps important first to explain what the small and large bowels do, in order to understand how inflammatory bowel disease (or IBD) can affect their function.

The gastrointestinal tract is made up of the mouth, esophagus, stomach, small intestine, large intestine, and anus.

When we eat, food is passed down through the esophagus into the stomach, where it is partially digested by stomach acids. It then passes into the small intestine (or small bowel) where juices generated from inside of this organ, as well as from the pancreas and liver, further digest the food. Here, water and nutrients are absorbed through the lining of the small intestine, and taken up into the bloodstream.

Following this, waste from the small bowel along with any food which hasn’t been digested is passed into the large bowel, which soaks up any remaining nutrients, and takes up water to help form waste into the more solid stool.

This stool is then passed into the rectum and stored, before being excreted in bowel movements.

What is inflammatory bowel disease? How is it different from IBS?

There are two main types of inflammatory bowel disease: Crohn’s disease and ulcerative colitis. They’re both chronic conditions, which tend to go through periods of flare and remission.

It’s important to note that IBD (Crohn’s and ulcerative colitis) is different from IBS, or irritable bowel syndrome (a condition which we’ve written about before). IBS is more common than IBD and is not characterized by inflammation and ulceration. Consequently, IBS is not likely to show up on tests and does not cause symptoms such as rectal bleeding or bloody stools, or damage to the bowel. Due to the nature of symptoms, IBS is categorized more commonly as a ‘functional disorder’; whereas IBD is categorized as a disease.

What’s the difference between Crohn’s and ulcerative colitis?

In ulcerative colitis, the inflammation affects the large bowel and the rectum. This can cause the tissues lining these areas to become irritated and develop ulcers. These ulcerations can then bleed and expel mucus.

Similarly, Crohn’s disease is characterized by tissues in the bowel becoming inflamed, again causing ulcers that bleed and leak mucus; however, this inflammation may not be localized to just the large bowel and rectum, as it is in ulcerative colitis.

Crohn’s can also affect the small intestine and other parts of the gastrointestinal tract.

The typical pattern of inflammation is also a potential difference between the two conditions:

  • in ulcerative colitis, inflammation may be present in longer, uninterrupted sections of the large intestine;
  • whereas in Crohn’s, inflammation is more likely to be intermittent and appear in patches (but may appear in multiple sections of the GI tract)

Where someone has disease characteristics that could suggest either, they might be said to have ‘indeterminate colitis’, as a firm distinction cannot necessarily be made.

Diagnosis and management are subjects we’ll come to later, but a doctor caring for a patient with indeterminate colitis will need to do so taking the possibility of both conditions into consideration.

What are the symptoms?

There are many similarities between the two conditions, but because they tend to affect different regions of the GI tract, some patients may notice a difference in symptoms.

The most common symptoms of both are:

  • persistent diarrhea. It depends on the severity of the flare, but someone with the mild or moderate flaring disease may have around five movements in a day, whereas someone having a severe flare may have up to 15 or 20. Many patients will tend to have urgency (a desperate need to get to the toilet) during a flare.
  • blood and mucus in the stool, or rectal bleeding. In many cases, blood may also appear separately in the bowl and is likely to be on tissue paper when wiping.
  • pain in the abdomen and stomach. People with inflammation in the rectal area (proctitis) will also tend to have pain when passing stool.

The more frequent bowel movements occur, the more likely someone might be to experience:

  • weight loss
  • and fatigue.

In the midst of a flare, someone may also develop:

  • irritated eyes
  • joint pain
  • mouth ulcers
  • constipation
  • or a high temperature

However, these symptoms are not as common.

In some cases, Crohn’s can also lead to other issues such as swelling in the legs (erythema nodosum), nausea, and vomiting.

Fistulas (or anal fissures) can also occur if someone has had severe inflammation for a long time. This is where small tears develop in damaged tissue, creating tunnels that are prone to infection. The conditions usually go through periods of flare and remission. Some people may have several years between episodes; whereas others may have flares more frequently.

How common are Crohn’s and ulcerative colitis?

The charity Crohn’s and Colitis UK states that in the UK, roughly 115,000 people are affected by Crohn’s disease, and a further 146,000 by ulcerative colitis.

Both forms of IBD are more commonly seen in white Europeans (and particularly so in people of Jewish descent).

According to the NHS, between 15 and 25 is the most commonly diagnosed age group for ulcerative colitis; for Crohn’s, it’s between 16 and 30.

What causes it?

Experts aren’t entirely sure, but there are theories.

Both ulcerative colitis and Crohn’s were, until recently, thought to be the result of the immune system mistakenly attacking the bowel, and inducing inflammation (as occurs with other autoimmune disorders, such as rheumatoid arthritis or lupus, where the immune system attacks healthy tissue).

But there is growing evidence to suggest that in addition to this, a combination of other factors might play a role too.

These include:

  • genetics (WebMD reports that one-fifth of patients with IBD have a known family history of it)
  • the immune system responding to bacteria in the intestine
  • or an environmental trigger, such as diet, smoking, or stress, prompting an immune response.

There has been some discussion about how diseases respond to smoking, or could even be caused by smoking.

Evidence suggests that Crohn’s disease is more common in smokers and exacerbated by smoking); and that ulcerative colitis is more common in non- or ex-smokers.

However, health experts have explicitly warned against smoking as a form of therapy for ulcerative colitis, due to the numerous other health risks it can pose.

How are Crohn’s and ulcerative colitis diagnosed?

Due to the nature of symptoms, Crohn’s and ulcerative colitis can sometimes be mistaken for irritable bowel syndrome (IBS, which also causes unpredictable bowel movements and abdominal pain), gastroenteritis (which causes diarrhea), hemorrhoids (rectal pain and bleeding), and even appendicitis (abdominal pain and diarrhea).

Colorectal cancer can also cause symptoms that are very similar to inflammatory bowel disease, so it’s important to get checked out if you develop persistent diarrhea or blood in the stool.

There are several tests available that can help doctors to determine the presence of IBD, but sometimes a definite diagnosis may take some time.

When someone with symptoms suggestive of IBD first sees a doctor, they may start by ruling out other potential causes, such as a viral or bacterial infection, by asking questions about diet or recent travel. They may also perform an examination to check for internal hemorrhoids.

If after ruling these possibilities out, a doctor then suspects IBD might be the cause of symptoms, they may refer the patient for testing.

This may entail:

Blood tests

When taking a blood sample from a patient, a doctor will be able to tell from the results if there is inflammation present in the body, using a marker called C-reactive protein (CRP). A higher concentration of CRP suggests inflammation is present.

A full blood count (or FBC) is used to check for anemia, as this can be another marker for IBD. High platelet levels, for instance, can indicate that bleeding is taking place; and anemia may also be indicated by a low red blood cell (RBC) count.

Stool tests

A doctor can also use a stool sample, or fecal calprotectin test, to identify whether inflammation is present in the bowel (and if so, to what extent). This is important in helping to determine whether it is IBD or a non-inflammatory condition, such as IBS, causing symptoms. High levels of calprotectin are indicative of inflammation.

Colonoscopy or sigmoidoscopy

These are tests where a camera is used to inspect the lining of the bowel, via a flexible tube inserted into the anus. They enable a doctor to see the degree of inflammation, and in some cases can also be used to take biopsies (small tissue samples) from the bowel. Biopsies can be looked at to see if cell changes consistent with those normally seen in IBD are present.

  • A colonoscopy involves the whole large bowel and usually enters into the first part of the small intestine;
  • whereas a sigmoidoscopy does not go as far, reaching only into the left side of the large bowel (the sigmoid or ‘descending’ colon).

Sigmoidoscopies are more commonly used in ulcerative colitis (or suspected cases) due to the region that condition affects. A person with Crohn’s (or a suspected case) is more likely to have a colonoscopy, as the ascending large bowel and small bowel will need to be looked at too.

Someone having an endoscope test will usually be instructed not to eat for a certain period beforehand (up to 48 hours, depending on the test being undertaken), and only consume clear liquids. They’ll also be given a drinkable laxative to help clear the GI tract of waste.

The test isn’t painful but can cause discomfort. Patients are typically offered sedation to help them relax when the test is taking place.  

Capsule endoscopy

This is a small capsule containing a camera, which is swallowed by the patient. The camera then passes through the GI tract, taking pictures and transmitting these to a recorder which is worn on the patient’s person. Generally, this type of test is used if someone has suspected IBD, but a colonoscopy has produced little or no evidence of disease.


This type of test utilizes sound waves to produce an image and is conducted using a sensor on the skin. It can help a doctor to see how inflamed the bowel walls are. An ultrasound can also be used to detect complications of bowel disease, such as strictures or fistulae.

MRI and CT scans

Magnetic resonance imaging (MRI) and computerized tomography (CT) scans can also help to give doctors an idea of what is happening inside the bowel.

A CT scan involves a series of x-rays, taken using a machine that looks like a large white ring. The patient passes through this on a moveable bed.

An MRI scan is similar but uses magnetic and radio waves instead of x-rays to take images. The machine is a tunnel, which again the patient passes into lying down on a moveable platform.

Before these tests, the patient will either be asked to drink a contrast agent or have a contrast agent given intravenously (or both) to enable better imaging of the bowel wall.

How is IBD managed?

Most patients with Crohn’s or ulcerative colitis will be reviewed in the hospital on an outpatient basis by a specialist consultant, as well as an IBD nurse. Testing and decisions on treatment may also involve a patient’s GP.

There is no evidence at present to suggest that any particular food or dietary factor can cause IBD. However, most patients find that certain foods or drinks make symptoms worse during a flare-up.

Some examples of items that might exacerbate symptoms are:

  • high fiber foods, such as whole grain cereals, brown rice, or brown bread
  • spicy foods
  • dairy products
  • nuts and seeds
  • caffeine
  • or alcohol or fizzy drinks

Every patient is different, so what aggravates one person may not necessarily cause problems in another.

But generally, a low residue diet that is high in protein and low in saturated fat is often recommended, as it can help to reduce stool frequency and diarrhea.

It’s important not to omit whole food groups, as this can cause vitamin or nutrition deficiencies; your doctor or specialist nurse will be able to give you more detailed advice on what foods to include (and to avoid) if you have IBD. They may also suggest (or even prescribe) a vitamin or mineral supplement to offset the risk of deficiency.

Other factors, such as tiredness or stress, can also significantly impact symptoms; so getting enough sleep and taking measures to reduce stress are crucial in disease management.

Undertaking a program of light exercise can help with this, but caution should be exercised when considering more vigorous forms of exercise, as this can exacerbate symptoms. Once more, it’s useful to speak to your consultant or IBD nurse for guidance on physical activity.

What does treatment involve?

For both conditions, the treatment issued depends on the severity of the disease, and whether or not the patient is in a state of flare or not. The goal of treatment is to reduce the severity and length of a flare if one is in progress and increase the length of remission.


When a patient is flaring, oral corticosteroid tablets such (as prednisolone) are often used to help get symptoms under control. These will usually be started at a higher dose, which is tapered off gradually when symptoms start to subside. Steroids work by inhibiting the immune system, and this helps to settle inflammation.

Oral steroids can cause several side effects, so patients will only be instructed to take them for a limited time.

In some cases, steroid medication may be given as an enema or suppository. These aren’t as likely to cause side effects as oral steroids, as they work locally and the body doesn’t absorb as much of the active ingredient.


Once steroids have been successful at settling symptoms, a consultant or specialist nurse will typically issue a continuous maintenance treatment. This can take on various forms and may differ depending on what type of IBD is present. Again, there are different levels of treatment, depending on the severity of the disease.

Aminosalicylate medication, or 5-ASAs, such as mesalazine, can help to reduce inflammation in the large bowel. This type of medication can be given in various forms, such as oral modified-release tablets, granules, suppositories, or enemas. It may be issued in both types of IBD, but is thought to be more effective when used in ulcerative colitis. It is taken long-term, usually on a daily basis.

Azathioprine and Methotrexate

Other forms of maintenance treatment also work by inhibiting the function of the immune system, to help stop and prevent inflammation.

Azathioprine is a tablet treatment used in several conditions. It works by reducing the number of white blood cells, and this helps to suppress the immune system, lowering inflammation in autoimmune conditions such as IBD. The effects of the drug can take time (up to several months) to develop.

Methotrexate works in a similar manner but is more commonly used in Crohn’s than it is in ulcerative colitis. This can be administered as an injection, but may also be given as a tablet once remission has been achieved.

Both of these medicines can have an adverse effect on liver function, so regular blood monitoring will be required in patients taking them. Initially, a patient will need to have weekly blood tests; if blood levels remain satisfactory after the first few months, then testing may be reduced to once every two or three months.  


Biological treatments are becoming more widely used in various inflammatory conditions, and again work by inhibiting different processes in the immune system to help induce remission.

In some cases, they may be given in addition to azathioprine (this is known as combination therapy). It can take some time for them to have an effect; so it may be weeks or months before a noticeable benefit is seen.


Infliximab (known as Remicade) is one such option. This is given intravenously, usually in a hospital clinic. The infusion typically takes around two hours, but during the early stages of treatment, patients will need to remain in the clinic for a couple of hours longer to be monitored.

In most cases, a second infusion will be given two weeks after the first, and a third four weeks after that. Then the patient will then move onto 8-weekly infusions.

The drug works by limiting the activity of a protein called TNF-alpha in the immune system (a high presence of which is thought to contribute towards inflammation).


Adalimumab (Humira) is a drug that works in a very similar way to infliximab. However, this is not delivered intravenously. It is a subcutaneous injection (an injection pen), which can be self-administered by a patient at home (although the first administration will need to be supervised by a nurse).

The injections need to be kept at a refrigerated temperature. Typically, a patient will initiate on a high dose, which is then tapered down to a maintenance dose in subsequent weeks. Most patients will administer one injection a fortnight, but this might be increased to weekly administrations in some instances.


Known also as Entyvio, this drug is at present considered for patients who have not responded to anti-TNF medicines such as Humira or Remicade.

Like infliximab, it is administered intravenously but works in a slightly different way in the body. It is a ‘gut selective integrin blocker’, which functions by limiting the production of white blood cells, and their activity in the intestine.

Again, an injection is given in the clinic, then a second after two weeks and a third four weeks after that; before settling on 8-weekly infusions.


One of the newer treatments to be approved for use in Crohn’s is ustekinumab. At the time of writing, this is a medicine a consultant might consider if anti-TNFs have been insufficient in inducing remission.

It works by inhibiting interleukin-13 and -23, which again are immune proteins sometimes responsible for inflammation. The treatment is a subcutaneous injection; a doctor may give the initial dose, which is then followed by self-administered doses.


Where medication does not succeed, or if a consultant team feels as though a patient may benefit from it, surgery may be recommended.

The procedure someone has will depend on whether they have Crohn’s or ulcerative colitis, and what part of the bowel is affected.

For instance, in the case of ulcerative colitis where the large bowel is affected, a procedure called a colectomy may be used. This involves the removal of all or part of the large bowel, and may be undertaken in two or more stages:

  • Firstly, the affected part of the colon will be removed, and the small intestine will be directed out of the abdomen through a ‘stoma’ (usually near the belly button), which is connected to a bag that collects waste. This is called an ileostomy.
  • In suitable cases, the procedure can be ‘reversed’ a few months later, by creating an internal pouch out of the end of the small intestine.
  • This is pouch then connected to the anus, which enables the patient to pass stools normally.

In Crohn’s, a procedure called a resection may be used. This is similar to the above, again involving the creation of an ileostomy; where the inflamed part of the colon is removed, and later stitched together.

In some cases of IBD, the risks of symptoms returning may outweigh the benefits of reversal surgery; so a permanent ileostomy may be recommended.

Surgery doesn’t completely eliminate the chances of symptoms coming back in Crohn’s, as it can affect any portion of the GI tract.

However in ulcerative colitis, the affected part of the bowel is removed during a colectomy, so symptoms cannot return afterward.

There are some risks involved with surgery, and it can take some time to adapt to life with an ileostomy; however, for the vast majority of patients, surgery can have a significant improvement in quality of life in the long term.

You can find more helpful information on IBD at the Crohn’s and Colitis UK site.

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Tom Perry, M.D., attended Tulane University and graduated Magna Cum Laude with a B.S. degree in Parasitology. He received his M.D. degree in 1983 from the University of Virginia School of Medicine, where he gained extensive research experience, including studies conducted through the National Institutes of Health.