Why Do Constipation and Diarrhoea Occur? Causes, Stool Changes, Self-Care and When to See a Doctor

Many people experience bowel-related concerns such as “I haven’t had a bowel movement for several days”, “my stool is hard and difficult to pass”, “I suddenly have loose stools”, or “I alternate between constipation and diarrhoea”.

Constipation and diarrhoea may seem like completely opposite conditions, but both can be considered changes in normal bowel habits.

However, constipation is not defined simply as “not having a bowel movement every day”, and diarrhoea is not determined only by how often you go to the toilet.

Stool consistency and shape, ease of passing stool, bowel movement frequency, and how long the change has continued are all important factors to consider.

Temporary changes in diet or daily routine may sometimes play a role, while medicines or digestive conditions may also be involved.

When constipation and diarrhoea repeatedly alternate, conditions associated with changes in bowel habits, such as irritable bowel syndrome (IBS), may sometimes be involved.

In this article, we explain the basics of constipation and diarrhoea, what stool appearance can tell us, common causes, why constipation and diarrhoea may alternate, everyday factors to consider, and signs that may warrant medical advice.

Constipation and diarrhoea are both changes in bowel habits

Constipation and diarrhoea may appear to be opposite conditions.

With constipation, stool may be difficult to pass, while diarrhoea involves loose or watery stools.

However, both can be viewed as changes in bowel habits involving factors such as bowel movement frequency, stool form and ease of passing stool.

Having a bowel movement every day is not necessarily the definition of “normal”

Bowel movement frequency varies from person to person.

Some people have a bowel movement every day, while others may go once every few days without discomfort or difficulty.

For this reason, not having a bowel movement once a day does not automatically mean that someone is constipated.

Constipation may involve features such as:

• Hard or pellet-like stools
• Straining during bowel movements
• Difficulty passing stool
• A feeling that the bowel has not completely emptied
• Fewer bowel movements than usual

These features can be just as important as bowel movement frequency.

With diarrhoea, the number of bowel movements may increase, but loose or watery stool consistency is also an important feature.

The Bristol Stool Form Scale

The Bristol Stool Form Scale is widely used in healthcare and research as a way of describing stool consistency more objectively.

It classifies stool into seven types based on appearance.

In general, Types 1–2 represent harder stools, while Types 6–7 represent loose or watery stools.

Types 3–4 are relatively well formed, but stool appearance alone cannot determine a person’s overall health.

It is also important to consider whether a change in stool continues and whether symptoms such as abdominal pain are present.

Stool consistency can change from day to day

Food, fluid intake, travel, changes in daily environment and general physical condition can all be associated with temporary changes in stool.

Having one hard or loose stool does not necessarily mean that there is an underlying disease.

However, if changes in bowel habits continue for a prolonged period or occur together with severe abdominal pain, blood in the stool or other concerning symptoms, it is important to investigate the possible cause.

What is constipation? Common causes and stool characteristics

Constipation is often thought of simply as “not having a bowel movement for several days”, but bowel movement frequency alone does not define it.

Hard stools that are difficult to pass, the need to strain, and a feeling of incomplete evacuation can also be features of constipation.

Having fewer than three bowel movements per week is one commonly used indicator, but a person’s usual bowel pattern and stool characteristics also need to be considered.

The speed at which stool moves through the large intestine can play a role

As stool moves through the large intestine, water is absorbed and the stool gradually becomes more formed.

If stool takes longer to move through the colon, more water may be absorbed and the stool can become harder.

However, not all constipation is caused simply by slow movement through the large intestine.

The mechanics of passing stool may also be involved

Passing stool requires coordinated activity involving the rectum, anus and muscles of the pelvic floor.

When these functions do not coordinate effectively, passing stool can become more difficult.

For this reason, constipation cannot always be explained simply as “reduced intestinal movement”.

Diet, fluids and physical activity may also be associated

A number of everyday factors may be associated with changes in bowel habits.

Examples include:

• A diet relatively low in foods containing dietary fibre
• Not drinking enough fluids
• Low levels of physical activity
• Frequently ignoring the urge to have a bowel movement
• Changes in routine, such as during travel
• Ageing

These factors may contribute to changes in bowel habits in some people.

However, it is not accurate to assume that “drinking lots of water will cure constipation” or that “increasing fibre will always solve the problem”.

The most appropriate approach can differ depending on the underlying cause and individual circumstances.

Medicines can sometimes contribute to constipation

Some medicines and supplements can be associated with constipation.

Examples may include:

• Certain pain medicines
• Medicines with anticholinergic effects
• Certain blood pressure medicines
• Iron supplements
• Certain antidepressants

These can affect bowel habits in some people.

However, even if constipation begins after starting a medicine, necessary treatment should not be stopped without medical advice.

If you are concerned, speak with the doctor who prescribed the medicine or a pharmacist.

Constipation can sometimes be associated with an underlying medical condition

Constipation can occur in association with conditions affecting the digestive tract or other parts of the body.

Possible examples include conditions that narrow or obstruct the bowel, as well as diabetes, thyroid disorders and certain neurological conditions.

If constipation continues for a long period or your bowel habits change significantly from what is normal for you, it is important not to assume that it is simply “your usual constitution”.

What is diarrhoea? Acute, persistent and chronic diarrhoea

Diarrhoea generally refers to the passage of loose or watery stools.

Bowel movement frequency may also increase, but stool consistency is an important part of the definition.

Diarrhoea can also be considered according to how long it lasts, not only what caused it.

Acute diarrhoea

Acute diarrhoea begins suddenly and is usually present for a relatively short period.

Possible causes include viral or bacterial gastroenteritis, food poisoning, temporary dietary changes and medicines.

Because diarrhoea can lead to the loss of fluids and electrolytes, dehydration is an important concern, particularly when symptoms are frequent.

Persistent diarrhoea

When diarrhoea does not settle quickly and continues for more than two weeks but less than four weeks, it may be described as persistent diarrhoea.

If symptoms continue for this length of time, it is important not to assume that they are simply a temporary digestive upset and to seek medical advice when appropriate.

Chronic diarrhoea

Japanese clinical guidelines on bowel disorders define chronic diarrhoea as loose or watery stools that persist or repeatedly occur for four weeks or longer.

Long-term diarrhoea can be associated with a range of causes, including problems with digestion or absorption, medicines, irritable bowel syndrome and inflammatory bowel disease.

If diarrhoea continues for a prolonged period, it is important not to rely only on self-directed dietary restriction and to investigate the underlying cause.

Diarrhoea caused by infections or food poisoning

Infections caused by viruses, bacteria or parasites are among the common causes of acute diarrhoea.

Other symptoms such as nausea, vomiting, abdominal pain and fever may also occur.

If symptoms are severe or occur together with blood in the stool, fever, repeated vomiting, dehydration or other concerning signs, seek medical advice promptly.

Components in certain foods may also play a role

Difficulty digesting or absorbing certain carbohydrates can sometimes lead to diarrhoea, bloating or other digestive symptoms.

One well-known example is lactose intolerance.

When the activity of lactase, the enzyme that digests lactose, is low, lactose in milk and other dairy products may not be fully broken down. This can result in diarrhoea, bloating and gas in some people.

Sugar alcohols can also loosen stools when consumed in larger amounts by some individuals.

Medicines can sometimes cause diarrhoea

As with constipation, some medicines can be associated with diarrhoea.

For example, bowel habits may change during or after the use of antibiotics.

Many other medicines can also affect bowel habits, so if symptoms begin after starting a new medicine, discuss the possibility with a doctor or pharmacist.

Do not stop prescribed medicines without appropriate medical advice.

Why do constipation and diarrhoea sometimes alternate?

Some people experience both constipation and diarrhoea — for example, several days of constipation followed by suddenly loose stools.

There is no single explanation for why this happens.

Temporary changes associated with diet or medicines can affect stool consistency, but when alternating constipation and diarrhoea continue, conditions such as irritable bowel syndrome (IBS) may sometimes be involved.

What is irritable bowel syndrome (IBS)?

Irritable bowel syndrome is a condition in which abdominal pain and changes in bowel habits repeatedly occur in association with one another.

Diagnosis involves considering the pattern and duration of symptoms, ruling out other possible conditions where appropriate, and carrying out tests when necessary.

IBS is commonly classified according to stool pattern as:

• IBS with predominant constipation
• IBS with predominant diarrhoea
• Mixed IBS
• Unclassified IBS

Mixed IBS can involve both constipation and diarrhoea

With mixed IBS, periods of harder stools and periods of loose or watery stools may both occur.

For this reason, some people who feel unsure whether they are “more prone to constipation or diarrhoea” may have a mixed bowel pattern.

However, alternating constipation and diarrhoea does not automatically mean that someone has IBS.

Other digestive conditions and medicines can also alter bowel habits, so persistent symptoms should be medically assessed when appropriate.

IBS does not have one single known cause

IBS was once sometimes explained simply as a condition caused by stress.

However, the mechanisms involved in IBS are not yet fully understood, and a number of possible factors continue to be investigated.

These include:

• Intestinal motility
• Gut sensitivity
• Interactions between the brain and digestive tract
• Changes following gastrointestinal infections
• Diet
• Gut microorganisms
• Psychosocial factors

Research continues into how these factors may be involved.

The brain and gut communicate with each other

The brain and digestive tract communicate through the nervous system and other biological pathways. This relationship is often referred to as the gut-brain axis.

Feeling an urgent need to use the toilet when nervous, or noticing changes in your stomach or bowel habits during periods of emotional strain, are familiar examples that illustrate this connection.

However, it is not appropriate to think of IBS as simply “being all in your head”.

Multiple factors are thought to contribute to symptoms, and medical management can differ depending on the type and severity of symptoms.

Everyday factors to consider with constipation and diarrhoea

When constipation or diarrhoea occurs, many people wonder, “What should I eat?” or “How much should I drink?”

However, constipation and diarrhoea have different causes and characteristics, so the same approach will not be appropriate for everyone.

If symptoms are severe or continue for a prolonged period, it is important not to rely only on lifestyle changes and to investigate the possible cause.

Constipation and dietary fibre

Dietary fibre is one of the dietary components that has been widely studied in relation to bowel habits.

It is found in foods such as vegetables, fruit, legumes, whole grains, root vegetables, mushrooms, seaweed, nuts and seeds.

However, suddenly increasing fibre intake by a large amount can cause bloating or gas in some people.

In particular, if you already experience abdominal bloating or pain, it may be better to adjust fibre intake according to how your body responds rather than simply assuming that “more fibre is always better for constipation”.

Fluid intake can also be related to stool consistency

Low fluid intake can be one factor associated with harder stools.

If you are not drinking enough during the day, it may be helpful to pay attention to regular fluid intake alongside your meals.

However, this does not mean that drinking very large amounts of water will necessarily resolve constipation.

Constipation can have many different causes, so fluid intake should be considered together with diet, physical activity and other individual factors.

Physical activity and bowel habits

The relationship between physical activity and bowel habits has also been studied.

If you spend long periods sitting, you may be able to include more movement in everyday life by walking more often or standing up regularly.

There is no need to begin intense exercise. The aim is to include physical activity that is appropriate for your fitness and health.

Try not to repeatedly ignore the urge to have a bowel movement

Work, school or other circumstances can sometimes make it tempting to postpone going to the toilet.

When thinking about bowel habits, it can be helpful not to repeatedly ignore a natural urge to have a bowel movement.

At the same time, there is no need to sit on the toilet for a long period or strain simply because you feel that you “must go every morning”.

Bowel habits vary between individuals, so paying attention to your own natural signals is important.

Be aware of dehydration during diarrhoea

Diarrhoea can cause the loss of fluids and electrolytes.

If bowel movements are frequent, taking small amounts of fluid regularly can be important.

When significant amounts of electrolytes are also being lost, oral rehydration solutions may sometimes be used when appropriate.

Infants, older adults and people with certain underlying medical conditions may be more vulnerable to dehydration and require extra care.

Food choices during diarrhoea depend on the individual

Not everyone with acute diarrhoea needs to avoid exactly the same foods.

If you have an appetite, you can usually eat according to your tolerance without forcing yourself.

However, symptoms may become more noticeable for some people after foods high in fat, alcohol, drinks containing large amounts of caffeine or foods containing sugar alcohols.

Rather than assuming that one food is “always good for diarrhoea” or another food is “always forbidden”, it is more appropriate to consider your symptoms and their possible cause.

How to interpret probiotics research

When looking for information about bowel habits and digestive health, you may frequently encounter the term “probiotics”.

In scientific terminology, probiotics are defined as live microorganisms that, when administered in adequate amounts, confer a health benefit on the host.

This is the scientific definition of the term “probiotic” and does not mean that all foods or health supplements containing microorganisms have the same effects.

Research findings do not apply equally to every probiotic

Probiotics are being studied in a variety of areas, including bowel habits and digestive symptoms.

However, research findings can differ depending on the people studied, the species and strain used, the amount administered, the duration of the study and the research design.

For this reason, findings from one study cannot automatically be applied to all probiotics, foods or health supplements.

The strain matters, not only the species

In probiotic research, the specific strain used can be important even when microorganisms belong to the same species.

Different strains within the same species can have different characteristics.

Results obtained with one particular strain therefore cannot automatically be applied to another strain.

Higher microorganism counts are not automatically better

When evaluating research, it is important to consider not only the number of microorganisms used but also the specific strain, the dose and the conditions under which it was studied.

It is not appropriate to assume simply that “more bacteria means better”.

Foods and health supplements are different from medical treatment

Foods and health supplements are not positioned in the same way as medicines.

If constipation, diarrhoea or other symptoms continue, it is important not to rely on health supplements alone to determine the cause or manage persistent symptoms. Seek medical advice when appropriate.

Signs that constipation or diarrhoea may need medical attention

Constipation and diarrhoea are common symptoms, but depending on the cause, medical assessment or treatment may sometimes be necessary.

Pay particular attention to bowel changes that are clearly different from your usual pattern or that occur together with other symptoms.

Blood in the stool or black stools

Red blood mixed with the stool or black, tarry stools can sometimes be associated with bleeding in the digestive tract.

Bleeding may sometimes come from haemorrhoids or other causes, but it is not possible to determine the cause simply from appearance.

If you notice blood in your stool or black, tarry stools, seek medical advice.

Severe or persistent abdominal pain

Severe abdominal pain or pain that continues for a prolonged period should also be taken seriously.

If your abdomen is very swollen, is very painful to touch, or you are unable to pass stool or gas, seek medical attention promptly.

Fever or repeated vomiting

Diarrhoea accompanied by fever or repeated vomiting can have a number of causes, including infections.

If you are unable to keep fluids down because of repeated vomiting, the risk of dehydration also increases.

Signs of possible dehydration

Dehydration is an important concern when diarrhoea continues.

Possible signs include:

• Intense thirst
• Passing less urine than usual
• Very dark urine
• Dizziness or light-headedness
• Marked weakness or fatigue

These symptoms may indicate dehydration.

Infants and older adults can become dehydrated more quickly, so early medical advice may be especially important.

Unintentional weight loss

If constipation or diarrhoea occurs together with weight loss that is not explained by dieting or another known reason, it is worth discussing with a healthcare professional.

Changes in bowel habits that continue

If newly developed constipation persists, or loose or watery stools continue or repeatedly occur for four weeks or longer, the underlying cause should be investigated.

Similarly, if constipation and diarrhoea repeatedly alternate, do not automatically assume that it is simply “your usual constitution” if the pattern continues.

Summary | Constipation and diarrhoea are about more than frequency

Constipation and diarrhoea are both common changes in bowel habits, but neither can be judged by bowel movement frequency alone.

With constipation, hard stools, difficulty passing stool, straining and a feeling of incomplete evacuation can all be important. With diarrhoea, stool consistency and how long loose or watery stools continue are also important.

Diet, fluid intake, physical activity, medicines and living environment may all be associated with changes in bowel habits, while digestive or other medical conditions can sometimes be involved.

Alternating constipation and diarrhoea may sometimes occur with conditions such as irritable bowel syndrome, but symptoms alone are not enough to make a diagnosis.

Being familiar with your usual bowel habits and stool consistency can help you notice meaningful changes. If changes continue or occur together with concerning symptoms, seek appropriate medical advice.