Food Poisoning in Adults

food poisoning

Food poisoning occurs when food or water contaminated with harmful microbes (germs), toxins or chemicals is eaten or drunk. When we think of food poisoning, we usually think of the typical gastroenteritis - an infection of the gut (intestines) - that usually causes diarrhoea with or without vomiting.


Bacteria that can cause food poisoning include Salmonella, Escherichia coli (usually shortened to E. coli), Listeria, Shigella, Campylobacter and Clostridium perfringens.


Some viruses, such as norovirus or rotavirus, can contaminate food and cause food poisoning.


Parasites are organisms (living things) that live within, or on, another organism. Examples include Cryptosporidium, Entamoeba histolytica and Giardia parasites.

Toxins (poisons) and chemicals

Toxins can be produced by bacteria that contaminate the food. For example, the bacterium Staphylococcus aureus can contaminate ice cream and its toxins can lead to food poisoning. The bacterium Bacillus cereus can contaminate rice. If contaminated rice is reheated and eaten, the toxins produced can lead to food poisoning. Certain types of fish (including shark, marlin, swordfish and tuna) contain high levels of the chemical mercury.

How does food become contaminated?

Contamination of food can occur because of problems in food production, storage or cooking. For example:

  • Not storing food correctly or at the correct temperature. For example, not refrigerating food. This is particularly a problem for meat and dairy products.
  • Inadequate cooking of food (undercooking or not cooking to the correct temperature). Bacteria are often found in raw meat and poultry.
  • Contamination by someone preparing the food who has not followed food hygiene rules and has not washed their hands properly.
  • Contamination from other foods (cross-contamination). For example, not washing a board used to prepare raw meat before you cut a slice of bread using the same board.
  • Bacteria can also be present in unpasteurised milk and cheese. The pasteurisation process kills the bacteria.

How does water become contaminated?

Water can become contaminated with bacteria or other microbes usually because human or animal feces get into the water supply. This is particularly a problem in areas with poor sanitation.

How long does it take for food poisoning to develop?

For most cases of food poisoning, symptoms tend to come on within one to three days of eating the contaminated food. However, for some types of food poisoning, this 'incubation period' can be as long as 90 days.

What are the usual symptoms of food poisoning?

The main symptom is diarrhea, often with vomiting as well. Diarrhea is defined as 'loose or watery stools (feces), usually at least three times in 24 hours'. Blood or mucus can appear in the stools with some infections. Crampy pains in your abdomen (tummy) are common. Pains may ease for a while each time you pass some diarrhea. A high temperature (fever), headache and aching limbs sometimes occur. If vomiting occurs, it often lasts only a day or so, but sometimes longer. Symptoms of dehydration. Diarrhoea and vomiting may cause dehydration (a lack of fluid in the body). Consult a doctor quickly if you suspect you are becoming dehydrated.

Dehydration in adults is more likely to occur in:

  • Elderly or frail people.
  • Pregnant women.
  • People with severe diarrhoea and vomiting. In particular, if you are not able to replace the fluid lost with enough drinks.

How is food poisoning diagnosed and do I need investigations?

Most people will recognise food poisoning from their typical symptoms. If symptoms are mild, you do not usually need to seek medical advice or receive specific medical treatment.
Your doctor may ask you to collect a stool sample. This can then be examined in the laboratory to look for the cause of the infection.

When should I seek medical advice?

Seek medical advice in any of the following situations, or if any other symptoms occur that you are concerned about:

  • If you suspect that you are becoming dehydrated.
  • If you are vomiting a lot and unable to keep fluids down.
  • If you have blood in your diarrhoea or vomit.
  • If you have severe abdominal pain.
  • If you have severe symptoms, or if you feel that your condition is getting worse.
  • If you have a persisting high fever.
  • If your symptoms are not settling; for example, vomiting for more than 1-2 days, or diarrhoea that does not start to settle after 3-4 days.
  • If you are elderly or have an underlying health problem such as diabetes, epilepsy, inflammatory bowel disease, kidney disease.
  • If you have a weakened immune system because of, for example, chemotherapy treatment, long-term steroid treatment, HIV infection.
  • If you are pregnant.

What is the treatment for food poisoning?

Symptoms often settle within a few days or so as your immune system usually clears the infection. Occasionally, admission to hospital is needed if symptoms are severe, or if complications develop (see below).

Fluids - have lots to drink. As a rough guide, drink at least 200 mls after each bout of diarrhoea (after each watery stool). This extra fluid is in addition to what you would normally drink. If you vomit, wait 5-10 minutes and then start drinking again, but more slowly. For example, a sip every 2-3 minutes, but making sure that your total intake is as described above.

Eat as normally as possible


Anti-diarrhoea drugs are not usually necessary. The safest and most effective is loperamide. The adult dose of this is two capsules at first. This is followed by one capsule after each time you pass some diarrhoea up to a maximum of eight capsules in 24 hours

Note: do not give anti-diarrhoea drugs to children under 12 years. Also, do not use anti-diarrhoea drugs if you pass blood or mucus with the diarrhoea or if you have a high fever.

Stop the spread of infection to others

Wash your hands thoroughly after going to the toilet. Ideally, use liquid soap in warm running water, but any soap is better than none. Dry properly after washing. Don't share towels and flannels. Don't prepare or serve food for others. Regularly clean the toilets that you use.

Food handlers: if you work with food and develop diarrhoea or vomiting, you must immediately leave the food-handling area.

Are there any complications that can occur from food poisoning?

Dehydration and salt (electrolyte) imbalance in your body. This is the most common complication. Reactive complications such as arthritis (joint inflammation), skin inflammation and eye inflammation (either conjunctivitis or uveitis).

Spread of infection to other parts of your body such as your bones, joints, or the meninges that surround your brain and spinal cord.

Irritable bowel syndrome is sometimes triggered by a bout of food poisoning.

Lactose intolerance can sometimes occur for a period of time after food poisoning. This is known as 'secondary' or 'acquired' lactose intolerance.

Haemolytic uraemic syndrome is another potential complication

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