Diarrhoea in adults: causes, red flags and when to see a doctor

NICE CKS describes diarrhoea as three or more loose or liquid stools a day, or more often than is normal for the individual. The second part of that definition matters: someone who normally opens their bowels once a day and now goes twice with soft stool may feel something has changed even if the numbers look small.

Duration shapes everything else. CKS uses less than 14 days for acute diarrhoea, more than 14 days for persistent diarrhoea and more than 4 weeks for chronic diarrhoea. In the first group infection dominates, whereas in the last the search widens to the gut lining, absorption and sometimes the thyroid or bowel cancer.

The NHS says the most common causes of diarrhoea and vomiting are a stomach bug and food poisoning. CKS adds that acute diarrhoea is usually caused by a bacterial or viral infection, with other causes including medication, anxiety, food allergy and acute appendicitis. Most people recover at home within a week.

Clues to the cause include who else is unwell, what you ate, recent travel, recent antibiotics or a stay in hospital, and whether there is fever or vomiting. The NHS notes that medicines can cause diarrhoea (the leaflet will say if so), as can a food intolerance or allergy, and that diarrhoea can accompany irritable bowel syndrome, anxiety, appendicitis, inflammatory bowel disease, coeliac disease, diverticular disease and radiation treatment in the pelvic area.

CKS lists irritable bowel syndrome, diet, inflammatory bowel disease, coeliac disease and bowel cancer as causes of chronic diarrhoea. The table sets out how they tend to present, drawing on NHS pages for each.

Losing fluid through loose stools is the commonest way in which diarrhoea becomes dangerous. The NHS gives dark, smelly pee or peeing less than usual as signs, and says to speak to a pharmacist if you are an older person, have a weakened immune system or have other health conditions that raise your risk of dehydration. Pharmacists may recommend oral rehydration powder that you mix with water, or loperamide to stop diarrhoea for a few hours in people who need that, though loperamide is not suitable for children under 12.

The NHS recommends sipping water or squash, avoiding fruit juice and fizzy drinks, which can make diarrhoea worse, eating when you feel able and avoiding fatty or spicy foods. It also asks you to stay off work or school until you have not been sick or had diarrhoea for at least 2 days, to wash hands with soap and water, and not to use a swimming pool until at least 48 hours after symptoms stop.

The NHS diarrhoea and vomiting page sets out the following thresholds.

• Call 111 if you keep being sick and cannot keep fluid down.

• Call 111 if there is bloody diarrhoea or bleeding from the bottom.

• Call 111 if diarrhoea has lasted more than 7 days or vomiting more than 2 days.

• Call 111 if signs of dehydration persist after using oral rehydration sachets.

• Call 999 or go to A&E if you vomit blood or have vomit that looks like ground coffee.

• Call 999 or go to A&E for a sudden, severe tummy ache, a sudden severe headache, a stiff neck with pain looking at bright lights, confusion, severe difficulty breathing, or blue, grey, pale or blotchy skin.

• Black poo, as the NHS bowel cancer page notes, deserves an urgent GP appointment or NHS 111 call, and black, sticky, very smelly poo is a 999 or A&E sign on the NHS stomach ulcer page.

CKS says the assessment should establish onset, duration, frequency and severity, identify red flag symptoms, look for the underlying cause and look for complications such as dehydration. Expect questions about blood or mucus, night-time diarrhoea, weight loss, travel, antibiotics, hospital stays, medicines, alcohol, food triggers and family history of bowel disease or cancer.

A stool sample is recommended in acute diarrhoea in particular circumstances, for example if you are systemically unwell and need admission or antibiotics, there is blood or pus in the stool, you are immunocompromised, you have recently had antibiotics, a proton pump inhibitor or hospital care, or you have diarrhoea after foreign travel. For chronic diarrhoea, CKS says blood tests should be requested in all people: full blood count, urea and electrolytes, liver function, calcium, vitamin B12 and folate, ferritin, thyroid function, ESR and CRP, and testing for coeliac disease. Depending on the findings, a person may be admitted, referred on a suspected cancer pathway or referred routinely.

For short episodes the NHS suggests staying at home, resting and taking paracetamol if you have discomfort. Washing clothing or bedding soiled with poo or vomit separately on a hot wash, cleaning toilet seats, flush handles, taps and door handles daily, not preparing food for others and not sharing towels helps stop the spread to the household.

If you have diarrhoea that lasts beyond a couple of weeks, keep a short diary of what you ate, how often you went, stool appearance and any pain, and take your medicine list to the appointment. That record often shortens the diagnostic process more than people expect.