Acute watery diarrhoea: possible causes and warning signs

Recent meals, sick contacts, travel, new medication, abdominal symptoms and the appearance of the stool help narrow the cause. The immediate priority is replacing lost fluid and salts while watching for dehydration or other warning signs.

Viral gastroenteritis commonly presents with watery diarrhoea, nausea, cramps and sometimes fever. Food poisoning can present with diarrhoea and vomiting after contaminated food. Giardiasis can present with prolonged loose, foul-smelling or greasy stools, especially after exposure to unsafe water.

Food intolerance can present with diarrhoea, bloating and wind after a trigger food. Crohn's disease, coeliac disease and an overactive thyroid can also present with diarrhoea, but their pattern is often persistent or recurrent rather than a brief isolated episode. Antibiotics, laxatives, magnesium-containing products and some other medicines can loosen stools.

Take frequent small sips. An oral rehydration solution replaces water and salts more effectively than plain water when losses are substantial; mix it exactly as directed. Continue light foods as tolerated and avoid alcohol. Babies, older adults and people with frailty or chronic illness can dehydrate quickly.

Wash hands with soap and water, clean shared bathroom surfaces and avoid preparing food for others while unwell. Do not use leftover antibiotics. Before using an anti-diarrhoeal medicine, check with a pharmacist or clinician, particularly if there is blood in the stool, fever, pregnancy, significant medical illness or treatment for a child.

Seek urgent medical care for fainting, confusion, severe weakness, very little urine, inability to keep fluids down, severe or worsening abdominal pain, a rigid or swollen abdomen, black stool, or visible blood. These features can signal serious dehydration, bleeding or abdominal disease.

Contact a clinician promptly if diarrhoea follows recent antibiotics or a hospital stay, starts after foreign travel, comes with high fever, continues without improvement, or repeatedly returns. Early advice is also appropriate for pregnancy, immune suppression, kidney disease, a very young child or a frail older person.

Tell the clinician when symptoms began, how often you are passing stool, whether it contains blood or mucus, and whether vomiting, fever, pain or reduced urination is present. Mention recent travel, unusual food or water, sick contacts, antibiotics and all regular medicines. Ask whether stool or blood tests are appropriate.