The common symptoms of food poisoning are diarrhoea, abdominal pain or cramps, nausea, vomiting and fever, although the exact combination depends strongly on the causative organism or toxin. Some people experience predominantly vomiting, some predominantly diarrhoea, and others have both. Headache, chills, muscle aches, weakness and general malaise can accompany gastrointestinal symptoms in particular infections. Symptoms can range from mild and self-limited to severe illness requiring medical treatment. Because many unrelated diseases can produce similar gastrointestinal symptoms, symptoms alone do not always identify the specific cause.
Diarrhoea occurs when the intestine fails to absorb water normally, secretes excess fluid or becomes inflamed. It may be watery, loose or, with certain invasive infections, bloody. Abdominal cramps can result from increased intestinal contractions and inflammation. Vomiting is a protective gastrointestinal reflex involving coordinated contraction of abdominal and respiratory muscles and relaxation of the upper gastrointestinal tract. Nausea is the unpleasant sensation that often precedes vomiting but can occur independently. Fever reflects an immune response and is more characteristic of some infections than others.
The timing of symptoms provides useful but imperfect clues about the cause. Some preformed bacterial toxins can cause prominent nausea and vomiting within a few hours, whereas Salmonella symptoms may begin several hours to several days after exposure. Campylobacter commonly has an incubation period of several days, while norovirus typically produces symptoms about 12 to 48 hours after exposure. Consequently, the last food eaten immediately before symptoms began is not necessarily the food responsible.
The character of the diarrhoea can also differ between pathogens. Watery diarrhoea is common in illnesses caused by secretory mechanisms or viruses, whereas inflammatory or invasive infections may produce blood, mucus, fever and more pronounced abdominal pain. Bloody diarrhoea is clinically important because it can occur with organisms capable of causing intestinal tissue injury, including certain strains of E. coli, Campylobacter and Salmonella. Severe diarrhoea that persists, particularly when accompanied by blood or substantial abdominal pain, should not simply be assumed to be an uncomplicated stomach bug.
Vomiting and diarrhoea create the principal immediate complication of many foodborne illnesses: dehydration. Water is lost directly in stool and vomit, while electrolytes such as sodium and potassium can also be lost. Early dehydration can manifest as thirst, dry mouth and reduced urine output; increasing dehydration can cause dizziness, weakness and pronounced fatigue. Severe fluid loss can impair circulation and organ function. People who cannot keep fluids down are particularly vulnerable because they cannot replace losses adequately by drinking.
Some symptoms indicate a need for prompt medical assessment rather than routine home management. These include bloody diarrhoea, persistent diarrhoea, high fever, repeated vomiting that prevents adequate fluid intake and clear signs of dehydration. In the CDC guidance, severe features include bloody diarrhoea, diarrhoea lasting more than three days, fever above 102°F, frequent vomiting and dehydration. UK clinical advice similarly emphasises dehydration, persistent vomiting, blood in stools and prolonged symptoms as reasons to seek medical help.
The consequences can extend beyond the digestive tract in particular infections. Some strains of E. coli can lead to haemolytic uraemic syndrome, which can cause kidney failure; Salmonella and Campylobacter can occasionally cause complications outside the intestine; Listeria can cause invasive disease and is particularly significant during pregnancy. These complications are uncommon relative to uncomplicated food poisoning but explain why the term should not automatically be equated with a trivial illness. People in higher-risk groups, including young children, older adults, pregnant people and those with weakened immune systems, warrant particular attention.
Food poisoning symptoms overlap extensively with norovirus and other forms of acute gastroenteritis, so symptom pattern alone is often insufficient to establish a diagnosis. The exposure history, incubation period, presence of fever or blood, duration, other affected people and relevant travel or food history can help clinicians determine whether testing is warranted. Stool testing may be considered for severe, prolonged or epidemiologically important illness. In many uncomplicated cases, however, treatment focuses on hydration and supportive care while the illness resolves.
