Alternating constipation and diarrhoea: causes, red flags and when to get it checked

The bowel responds to what you eat, your stress levels, hormones and the bacteria it carries. When its rhythm is disturbed, hard stool and loose stool can alternate within days. In constipation, watery stool may seep past a blockage of hard stool, giving a false impression of diarrhoea.

• Irritable bowel syndrome (mixed type): cramps, bloating, urgency and incomplete emptying, often lasting for months or years, triggered by food or stress.

• Constipation with overflow diarrhoea, particularly in older or less mobile people.

• Diet changes: low fibre, high fat or irregular meals, dehydration and food intolerances.

• Medicines: opioids, iron, calcium channel blockers and antidepressants (constipation), alongside laxatives and metformin (diarrhoea).

• Diverticular disease, with episodes of cramping and altered stool.

• Coeliac disease, inflammatory bowel disease or an underactive or overactive thyroid.

• Pelvic floor dysfunction or rectal prolapse with incomplete emptying.

• Bowel cancer, which may present with a change in bowel habit, blood, weight loss and tiredness.

Blood in stool, unintentional weight loss, anaemia, a persistent change over six weeks in someone over 50, a lump in the abdomen, night-time diarrhoea, symptoms starting after 50 for the first time, a family history of bowel cancer or inflammatory bowel disease and severe or worsening pain are warning signs.

• Call 999 or go to A&E: severe abdominal pain with distension and vomiting, or inability to pass wind or stool.

• Urgent GP referral: new bowel habit change over six weeks with rectal bleeding, weight loss or anaemia, particularly over 50.

• GP within weeks: alternating bowel habit with pain lasting more than a few weeks.

• Pharmacist or routine GP: occasional swings with diet or travel.

The doctor asks how long, what the stools look like, pain pattern and relief, bloating, urgency, blood, weight change, appetite, diet, medicines, stress, family history and periods. They examine the abdomen, perform a rectal examination if appropriate and check for anaemia.

• Blood tests: full blood count, ferritin, CRP, coeliac screen and thyroid function.

• Faecal calprotectin and FIT: exclude inflammation and screen for bowel cancer risk.

• Colonoscopy: for red flags, anaemia, abnormal stool tests or persistent symptoms.

• CT colonography or flexible sigmoidoscopy: alternatives in some cases.

• Anorectal tests: for suspected pelvic floor dysfunction.

Eat regular meals, drink enough fluids, increase soluble fibre gradually (oats, ispaghula), stay active and keep a symptom diary. Peppermint oil may help cramps. Try to build a regular toilet routine after breakfast and do not strain. Avoid overuse of laxatives and anti-diarrhoeals.

The NHS route is a GP with dietetics or gastroenterology referral as needed. Private colorectal and gastroenterology clinics offer direct access to endoscopy and tests.