Merck Manual

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Larry M. Bush

, MD, FACP, Charles E. Schmidt College of Medicine, Florida Atlantic University

Reviewed/Revised Jun 2023

Botulism is a rare, life-threatening poisoning caused by toxins produced by the anaerobic bacteria Clostridium botulinum.

  • Botulism toxins, usually consumed in food, can weaken or paralyze muscles.

  • Botulism may begin with dry mouth, difficulty swallowing and talking, double vision, and inability to focus the eyes or with digestive symptoms such as diarrhea, vomiting, and abdominal cramps.

  • Doctors examine samples of blood, stool, or tissue from a wound, and electromyography may be done.

  • If people think they may have botulism, they should go to the hospital immediately.

  • An antitoxin is used to prevent or slow the effects of the toxin.

  • Careful food preparation and storage prevent botulism that is caused by eating contaminated food.

In very small doses, botulism toxin types A and B can be used to relieve muscle spasms and to reduce wrinkles.

Did You Know...

  • The toxins that cause botulism are the most potent known poisons.

  • In very small doses, toxin types A and B can be used to control muscle spasms and to reduce wrinkles.

Causes of Botulism

Clostridium botulinum bacteria form dormant cells called spores. These dormant spores are present widely in the environment including in soil and in river and sea water. Like seeds, spores can exist in a dormant state for many years and they are highly resistant to destruction, for example, by heat. When moisture and nutrients are present and oxygen is absent (as in the intestine or in sealed jars or cans), the spores develop into active bacteria and produce toxins. Some toxins produced by Clostridium botulinum are not destroyed by stomach acid or the intestine’s protective enzymes.

Clostridium botulinum is common in the environment, and spores can be transported by air. Occasionally, cases of botulism result from ingesting or inhaling small amounts of soil or dust that contain spores. Also, the toxin can be used as a weapon Toxins as Weapons "Toxin" is often loosely used to refer to any poison but technically refers only to a poisonous chemical produced by an organism (although some toxins can now also be produced synthetically)... read more . Spores can also enter the body through the eyes or a break in the skin.

Botulism has rarely occurred after botulinum toxin injection (onabotulinumtoxinA) for cosmetic reasons, such as for wrinkles, or medical reasons, such as for migraine headaches.

However, most cases result from eating food that contains the toxins.

There are different forms of botulism, which have different causes. The most common forms are

A rare form of botulism is called intestinal toxemia botulism. It can occur if the spores get into a person's intestines and produce the toxin there. When intestinal toxemia botulism occurs in infants, it is called infant botulism Infant Botulism Infant botulism is a potentially life-threatening infection that causes muscle weakness. It develops in infants who eat food containing spores of Clostridium botulinum bacteria. (See... read more . In children 1 year of age and older and in adults, the botulism is called adult intestinal toxemia botulism. These two types of intestinal toxemia botulism differ from foodborne botulism, which is more common and occurs when people ingest food contaminated with the toxin itself. It is not clear how spores get into the intestine in intestinal toxemia botulism. In infants, swallowing honey or being around contaminated soil may be the source. In adults, those who have had surgery on their stomach or intestines, have inflammatory bowel disease Overview of Inflammatory Bowel Disease (IBD) In inflammatory bowel diseases, the intestine (bowel) becomes inflamed, often causing recurring abdominal pain and diarrhea. The 2 primary types of inflammatory bowel disease (IBD) are Crohn... read more , or have been on antibiotics have a greater risk of developing intestinal toxemia botulism.

Foodborne botulism

Foodborne botulism occurs when people eat food contaminated with botulinum toxin, produced by Clostridium botulinum bacteria. The toxin enters the bloodstream from the small intestine and is carried to nerves. Food may be contaminated if it is inadequately cooked before it was stored.

The most common sources of foodborne botulism are

  • Home-canned foods, particularly foods with a low acid content, such as asparagus, green beans, beets, and corn

Other sources include chopped garlic in oil, chili peppers, canned tomatoes, foil-wrapped baked potatoes that have been left at room temperature too long, and home-canned or fermented fish. However, about 10% of outbreaks result from eating commercially prepared foods—most commonly, vegetables, fish, fruits, and condiments (such as salsa). Less commonly, botulism results from eating beef, milk products, pork, poultry, or certain other foods.

Refrigerating food does not make food safe because Clostridia can produce some toxins at typical refrigerator temperatures.

Wound botulism

Wound botulism occurs when Clostridium botulinum contaminates a wound or is introduced into other tissues. Inside the wound, the bacteria produce toxins that are absorbed into the bloodstream.

Injecting drugs with needles that are not sterilized can cause this type of botulism, as can injecting contaminated heroin into a muscle or under the skin (skin popping).

Symptoms of Botulism

The various forms of botulism cause many of the same symptoms:

  • Dry mouth

  • Blurred or double vision

  • Drooping eyelids

  • Difficulty focusing on nearby objects

  • Pupils that do not constrict normally when exposed to light

  • Slurred speech

  • Difficulty swallowing

Nerve damage by the toxins affects muscle strength but not sensation. The mind usually remains clear.

Typically, after strength is lost in the muscles of the face and head, strength is then gradually lost in the muscles of the arms and legs and in the muscles involved in breathing. Muscles become progressively weaker. Paralysis of breathing muscles can cause death if mechanical ventilation Mechanical Ventilation Mechanical ventilation is use of a machine to aid the movement of air into and out of the lungs. Some people with respiratory failure need a mechanical ventilator (a machine that helps air get... read more (use of a machine to assist breathing) is not provided.

In foodborne botulism, symptoms develop suddenly, usually 18 to 36 hours after toxins enter the body, although symptoms can start as soon as 4 hours or as late as 8 days after ingesting the toxins. The more toxin ingested, the sooner people become sick.

The first symptoms of foodborne botulism are often nausea, vomiting, stomach cramps, and diarrhea. Over time, many people become constipated. These digestive symptoms usually occur before muscles are affected.

People with wound botulism do not have digestive symptoms.

Diagnosis of Botulism

  • When possible, tests to detect toxins in food, blood, or stool

  • Sometimes electromyography

Doctors suspect botulism based on symptoms. However, other disorders can cause similar symptoms, so additional information is needed.

For foodborne botulism, a likely food source provides a clue. For example, when botulism occurs in two or more people who ate the same food prepared in the same place, the diagnosis is clearer. The diagnosis is confirmed when the toxins are detected in the blood or when the bacteria or toxins are detected in a sample of stool. Toxins may also be identified in food that was eaten.

For wound botulism, doctors ask whether people have had an injury that broke the skin. Doctors may inspect the skin for puncture marks suggesting use of an illicit drug. The diagnosis is confirmed when the toxins are detected in the blood or when the bacteria are detected in a culture of tissue from the wound.

Electromyography Electromyography Diagnostic procedures may be needed to confirm a diagnosis suggested by the medical history and neurologic examination. Imaging tests commonly used to diagnose nervous system (neurologic) disorders... read more Electromyography (electrical stimulation of muscles and recording of their electrical activity) may be useful. In most cases of botulism, electromyography shows abnormal muscle responses after electrical stimulation.

Sometimes determining whether botulism developed from a wound or from food is impossible.

Treatment of Botulism

  • Sometimes activated charcoal to prevent absorption of toxin consumed in food

  • Antitoxin

  • For breathing problems, use of a mechanical ventilator

If people think they may have botulism, they should go to the hospital immediately. If botulism is diagnosed, they are hospitalized and closely monitored.

Laboratory tests to confirm the diagnosis may be done, but treatment cannot be delayed until the results are known. To help eliminate any unabsorbed toxin, doctors may give activated charcoal by mouth or through a tube passed through the nose or mouth and into the stomach.

Vital signs (pulse, breathing rate, blood pressure, and temperature) are measured often. If breathing problems begin, people are transferred to an intensive care unit and may be temporarily placed on a mechanical ventilator Mechanical Ventilation Mechanical ventilation is use of a machine to aid the movement of air into and out of the lungs. Some people with respiratory failure need a mechanical ventilator (a machine that helps air get... read more . Such treatment has reduced the percentage of deaths due to botulism to less than 10% currently.

If people have wound botulism, the wound is thoroughly cleaned, and dead tissue is removed. Then antibiotics, such as penicillin and metronidazole, are given by vein (intravenously).

Some people who recover from botulism feel tired and are short of breath for years afterward. They may need long-term physical therapy.

People do not develop immunity to Clostridium botulinum toxin after an infection, so they can become infected again.


Antitoxin is a substance that blocks the action of the toxins. It is given as soon as possible after botulism has been diagnosed.

Antitoxin can be given to adults and children but is not recommended for infants under 1 year of age; a different antitoxin is available for infant botulism Infant Botulism Infant botulism is a potentially life-threatening infection that causes muscle weakness. It develops in infants who eat food containing spores of Clostridium botulinum bacteria. (See... read more . It is most likely to help if given within 72 hours of when symptoms begin.

The antitoxin may slow or stop further physical deterioration, so that the body can heal itself over a period of months. However, the antitoxin cannot undo damage already done. Also, some people have a serious allergic reaction (anaphylactic reaction Serum sickness Venomous snakes in the United States include pit vipers (rattlesnakes, copperheads, and cottonmouths) and coral snakes. Severe envenomation can cause damage to the bitten extremity, bleeding... read more Serum sickness ) to the antitoxin, which is derived from horse serum, or they may develop another type of reaction called serum sickness Treatment Treatment .

Prevention of Botulism

  • Thorough cooking or heating of food

  • Appropriate storage and handling of food

The spores of Clostridium botulinum are highly resistant to heat and may survive boiling for several hours. However, the toxins are readily destroyed by heat.

Stored foods can cause botulism if they were inadequately cooked before they were stored. The bacteria can produce some toxins at temperatures as low as 37.4° F (3° C), a typical refrigerator temperature, so refrigerating food does not automatically make it safe.

The following measures can help prevent foodborne botulism:

  • Cooking food at 176° F (79.9° C) for 30 minutes, which almost always destroys toxins

  • Discarding canned foods that are discolored or smell spoiled or are swollen or leaking

  • Following the home-canning instructions from the Centers for Disease Control and Prevention (CDC)

  • Refrigerating homemade oils infused with garlic or herbs and throwing away any of these unused oils after 4 days

  • Keeping potatoes that have been baked in aluminum foil hot until served

If people are unsure whether a can should be discarded, they can check it when they start to open it. Before making the first puncture, they can place a few drops of water in the spot to be punctured. If water is expelled rather than sucked into the can when the can is punctured, the can is contaminated and should be discarded. If there is any question whether a can of food is safe, it is better to discard it than risk getting botulism.

Any food that may be contaminated should be disposed of carefully. Even tiny amounts of toxins ingested, inhaled, or absorbed through the eye or a break in the skin can cause serious illness. Skin contact should be avoided as much as possible, and hands should be washed immediately after handling the food.

If a wound becomes infected, promptly seeking medical attention can reduce the risk of wound botulism.

More Information

The following English-language resources may be useful. Please note that THE MANUAL is not responsible for the content of these resources.

  • Centers for Disease Control and Prevention (CDC): Home-canning instructions for preventing botulism and information about how to recognize contaminated food and how to properly dispose of it

  • Infant Botulism Treatment and Prevention Program: Web site or call 510-231-7600: Provides information about treatment, prevention, and support groups

Drugs Mentioned In This Article

Generic Name Select Brand Names
Botox, Botox Cosmetic
No brand name available
Flagyl, Flagyl ER, Flagyl RTU, LIKMEZ, MetroCream, MetroGel, MetroGel Vaginal, MetroLotion, Noritate, NUVESSA, Nydamax, Rosadan, Rozex, Vandazole, Vitazol
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