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Internal worms can cause disease in pets and some can be transmitted to humans. Although practical measures like washing hands and removing faeces from the environment play a key role in controlling parasitic diseases, parasiticide medication also plays an important role. Parasiticide medicines that are used to control internal worms are called wormers or anthelmintics or endoparasiticides. By the end of this module you will:

  • make sense of the range of non-prescription wormers by understanding about the active ingredients
  • understand what is needed for a worming product to be a veterinary medicine
  • know how to report a side effect 
  • know what to check before selling a wormer
  • understand what is different about prescription wormers.

The types of internal worm that most commonly affect cats and dogs in the UK are roundworm, tapeworm and lungworm.

  • Roundworm. The most important roundworms are dog and cat roundworm (Toxocara canis and Toxocara cati). These can cause severe illness in puppies and kittens. The worms may be carried by adult cats and dogs in a dormant form and eggs may be shed in the faeces from time to time, with the risk of spreading the disease to humans. Hookworm and whipworm are other types of roundworm but measures to control Toxocara usually control these too.
  • Tapeworm. There are three main types of tapeworm - Dipylidium caninum, which is spread by fleas and lice and is controlled through effective flea control; Taenia, which is picked up through eating raw meat or carcases of infected animals but which doesn’t usually cause problems in cats and dogs;  and Echinococcus (the most serious typewhich can cause severe disease in humans. It seems to be present in only a few parts of the country and is picked up by dogs through eating carcases or raw meat especially from sheep and cattle. 
  • Lungworm. There are are several different lungworms that can affect cats and dogs. They include the cat lungworm Aelurostrongylus abstrusus and the dog lungworm Oslerus osleri. The most significant in dogs is Angiostrongylus vasorum (strictly a type of heartworm) which is carried by slugs, snails and frogs and which can cause serious disease in dogs. 
  • All cats and dogs need regular roundworm protection.
  • Regular flea protection will also control the tapeworm Dipylidium caninum in dogs.
  • Protection against other tapeworms is needed if the pet hunts and eats prey or carcasses; and if a dog lives in, or travels to, an area where Echinococcus granulosus is present, the tapeworm drug praziquantel should be used.
  • If a dog regularly eats slugs or snails or grass (on which there might be small slugs and snails), it may need protection against lungworm. The owner should discuss this with a vet. 

There are dozens of different brands of non-prescription wormers, but if you look more closely, they all contain one or two out of 8 different active ingredients (drugs). That’s still quite a big number, but when you group the ingredients by the job they do, the range becomes much simpler.

Drugs active against roundworm




Drugs active against roundworm and tapeworm

fenbendazole (not Dipylidium). Also active against cat lungworm (Aelurostrongylus abstrusus and the dog lungworm Oslerus osleri)

febantel (not Dipylidium)

nitroscanate (not Echinococcus)

Drugs active against tapeworm

dichlorophen (not Echinococcus)



A product might contain two or three drugs. For example, Beaphar Multiwormer contains piperazine (against roundworm) and dichlorophen (against tapeworm). Febantel and pyrantel are often included together in products because their effects are additive. For example, Drontal Plus contains febantel and pyrantel (against roundworm, hookworm and whipworm) and praziquantel (against tapeworm).

Worming drugs work by interfering with the nervous or metabolic systems of worms leading to their death.

Any product that claims to treat or prevent worm infections in animals must be licenced as a veterinary medicine by the Veterinary Medicines Directorate or the European Medicines Agency, which are government medicines regulators. To get a licence a company has to show proof of the quality and safety of the product and show evidence that it kills the relevant worms in tests in which the product has been used as recommended in cats and dogs. A licensed medicine will have a Vm (veterinary medicine) number on the package.

There are several ways of using wormers. Most non-prescription wormers are in the form of tablets. Several are available an oral paste, suspension, syrup or granules, which are more suitable for kittens and puppies. There are spot-ons for cats containing the tapeworm drug praziquantel.

All medicines can cause side effects (adverse effects). For example, wormers sometimes cause digestive tract disorders such as vomiting. The package leaflet should list the reported side effects.

The trials that are done to get a marketing licence for a veterinary medicine test the safety of the product as well as how effective it is. But trials can usually only detect the most common effects. Rarer side effects often only show up when a medicine has been marketed for several years and used in lots of animals. Medicines available without a prescription have been in use for many years and so their side effects are usually well known. Knowing about rarer effects depends on people (SQPs, nurses, pharmacists, vets, pet owners) reporting them to the medicines regulator. Anyone can report a side effect to the Veterinary Medicines Directorate via the website:

There are certain things you should check before selling a worming product:

  • what parasites the product is licensed to cover. Although different products might have the same ingredients, they may have different licensed uses.
  • what species it is intended for
  • that the product is suitable for the animal's age 
  • that the dose is correct (for the animal's weight)
  • how often it should be used
  • the expiry date
  • if a cat or dog hunts or eats raw food it should have more frequent roundworm prevention (monthly rather than 3-monthly) and have a wormer to control tapeworm
  • if the cat or dog lives with a person who might be particularly susceptible to roundworm infection, this is another reason for using more frequent roundworm prevention (monthly rather than 3-monthly)
  • praziquantel is the only wormer that is active against all types of tapeworm, including the dangerous Echinococcus species.
  • if a dog regularly eats slugs, snails or grass (on which there might be small slugs or snails), the owners should consider getting veterinary advice about lungworm protection.

Wormers available only on prescription from a vet have the following features:

  • Broader parasite coverage. Some prescription products protect against fleas and/or ticks as well as worms. This might be more convenient for the owner.
  • Protection against dog lungworm (Angiostrongylus vasorum). There are no non-prescription products licensed to protect against this type of lungworm.
  • Protection against parasites that are found abroad. If the pet is travelling to another country, the owner should consult a vet for advice about parasite control.
  • Wormers in the form of spot-ons. These might be more suitable if oral treatment is difficult.


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Test yourself

To do the CPD questions and receive your certificate, just hit the play button on the video below and enter your name and email address before you start.