Antihelmintics - Medicines for Worms

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Worms are parasites that enter the body and are able to live and feed off the person they have entered. The most common worm infection in the UK is threadworms. Medicines to treat worm are sometimes called antihelmintics. Mebendazole (trade name Vermox®) is the most commonly prescribed medicine for worms in the UK.

Worms (sometimes called helminths) are parasites that enter the body and are able to live and feed off the person they have entered. There are many types of worms and they are usually divided into three groups:

  • Roundworms - the most common roundworm infections are thought to be caused by threadworms, ascaris, hookworms, and trichuris. Roundworm eggs usually enter the body by the person eating food or drinking water that contains eggs. The eggs live and hatch in the intestine (gut).
  • Tapeworms - these worms live in the gut. People usually become infected by eating raw or uncooked meat.
  • Flukeworms - these worms can live in the blood vessels, gut, lungs, or liver. You usually become infected with fluke worms by swimming or washing in fresh water that contains flukeworms. An infection with flukeworms causes a tropical disease call schistosomiasis.

Most worm infections occur in warm tropical countries or developing countries where food and water may not be clean. It is thought that millions of people around the world are infected with worms at any one time. In the UK the most common worm infection is threadworms - this is a type of roundworm and is sometimes referred to as a pinworm. Threadworms are spread by being in contact with someone who is already infected with threadworms. Other types of worm infections can occur if you live in the UK; however, they do not occur very often. People who get these infections have usually been travelling in countries where these types of worm infections are common.

The rest of this leaflet discusses the medicines that may be used to treat worm infections. There are separate leaflets called Roundworms, Threadworms and Schistosomiasis.

Note: ringworm in not an infection caused by worms. It is a name given to a type of fungal skin infection.

There are several medicines that may be used to treat worm infections - they are sometimes called antihelmintics. Mebendazole (trade name Vermox®) is the most commonly prescribed medicine for worms in the UK. This is because it is used to treat threadworms - the most common worm infection in the UK. 

Other medicines that may be prescribed for the various types of worm infections include: levamisole, niclosamide, praziquantel, albendazole, diethylcarbamazine, ivermectin, and tiabendazole. These medicines are not licensed for use in the UK and are specially imported to treat more unusual worm infections. They are usually prescribed by doctors who specialise in treating worm infections.

The type of medicine prescribed will depend upon the type of infection. See below for a list of medicines and examples of the worms they are used to treat (in the UK):

  • Mebendazole - used to treat threadworms, roundworms, whipworm, and hookworm infections.
  • Levamisole - used to treat roundworms.
  • Niclosamide - used to treat tapeworms.
  • Praziquantel - used to treat tapeworms and schistosomiasis.
  • Albendazole - used to treat hookworms, cutaneous larvae migrans, and strongyloides infections.
  • Diethylcarbamazine - used to treat filaricides infections.
  • Ivermectin - used to treat filaricides, cutaneous larvae migrans, and strongyloides infections.
  • Tiabendazole - used to treat cutaneous larvae migrans.

Most medicines used to treat worm infections kill worms by either starving or paralysing them; for example:

  • Mebendazole, albendazole, and tiabendazole work by preventing the worms from absorbing the sugars they need for survival. They kill the worms, but not the eggs.

  • Praziquantel, and ivermectin work by paralysing the worms in the bowel. This allows them to be easily dislodged and removed from the gut in the faeces. 

In general, the length of treatment is usually quite short (at most a few days), but will depend on what type of worm you have and which medicine has been prescribed. For example, for threadworms - mebendazole is usually given as a single one-off dose and this is repeated two weeks later. For whipworm, or common roundworm infections, mebendazole is given twice a day for three days. 

It is not possible to list all the side-effects of these medicines in this leaflet. If you want more information specific to your medicine, see the information leaflet that came with your medicine.

In general, medicines for worms are well tolerated and very rarely cause serious side-effects. The side-effects most commonly reported with these medicines are stomach upsets such as cramps, diarrhoea and nausea.

There are very few people who cannot take a medicine for worms. If for some reason one medicine has caused a side-effect or there is a reason you cannot take one, your doctor can choose a different type of medicine that will suit you.

For adults and children aged over two years, you can only buy mebendazole to treat threadworms from your local pharmacy. If you need to treat threadworms in a child younger than two years old, discuss this with your doctor.

If you think you have had a side-effect to one of your medicines, you can report this on the Yellow Card Scheme. You can do this online at the following web address:

The Yellow Card Scheme is used to make pharmacists, doctors and nurses aware of any new side-effects that your medicines or any other healthcare products may have caused. If you wish to report a side-effect, you will need to provide basic information about:

  • The side-effect.
  • The name of the medicine which you think caused it.
  • The person who had the side-effect.
  • Your contact details as the reporter of the side-effect.

It is helpful if you have your medication and/or the leaflet that came with it with you while you fill out the report.

Original Author:
Dr Tim Kenny
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Peer Reviewer:
Dr Tim Kenny
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13923 (v1)
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