Acute diarrhoea is usually caused by a viral or bacterial infection (gastroenteritis) and often resolves on its own. In adults it typically improves within 2-4 days and clears within a week; in children it can last a little longer. The main risk is dehydration, so drink plenty of fluids and use oral rehydration solutions if needed.
Eat as normally as you can tolerate once nausea settles, and rest. Most people do not need specific medicine, though pharmacists can advise on rehydration and, in some cases, anti-diarrhoeal products for adults.
See a doctor if diarrhoea lasts more than 7 days, keeps coming back, or you recently travelled abroad. Seek urgent care or call your local emergency number if there is blood or black colour in the stool, severe abdominal pain, a high fever, persistent vomiting, or signs of serious dehydration such as very little urine, dizziness, confusion, or a rapid heartbeat. Babies, older adults and people with weak immune systems should be assessed sooner.