Is deworming safe?

The short version

How often to deworm, at a glance

Guideline frequencies for roundworms and tapeworms. For dogs and cats the figures come from ESCCAP’s worm-control guideline, for horses from the 2024 AAEP guidelines and for donkeys from The Donkey Sanctuary.

• Puppies

  From 14 days old, every 2 weeks until 2 weeks after weaning; then monthly up to 6 months of age if the risk of infection continues (puppy playgroups, for example).

• Kittens

  From 3 weeks old, every 2 weeks until 2 weeks after weaning; then monthly up to 6 months of age if the kitten roams outdoors.

• Nursing mother

  At the same time as the first treatment of her litter.

• Adult dog, indoors only

  1–2 times a year, or faecal testing at the same frequency.

• Adult dog with regular outdoor access

  4 times a year, or faecal testing at the same frequency.

• Adult dog that eats slugs, snails or grass

  4–12 times a year against roundworms and lungworms, and 4–12 times a year against tapeworms.

• Adult dog that hunts, eats prey or offal, or is fed raw meat

  4–12 times a year against roundworms and lungworms, and monthly against tapeworms.

• Adult cat, indoors only

  1–2 times a year, or faecal testing at the same frequency.

• Adult cat that roams outdoors

  4–12 times a year against roundworms, and 4–12 times a year against tapeworms.

• Adult horse

  1–2 baseline treatments a year, plus 1–2 more only for horses that faecal egg counts show to be high shedders. Not at fixed intervals.

• Donkey

  Small strongyles treated on the basis of faecal egg counts, plus a yearly treatment against encysted larvae; many products are not licensed for donkeys.

Your vet places your animal in a risk group, and homes with small children, elderly or immunocompromised people can call for monthly treatment. The list is a starting point for that conversation, not a replacement for it. Flea, tick and heartworm control follow separate schedules, covered below.

Dogs and cats

Why “once a year” isn’t enough

The most common compromise owners make is one or two dewormings a year. ESCCAP addresses it directly: “Studies have shown that 1–3 annual dewormings do not provide sufficient protection.” For roundworms, it adds that annual or twice-yearly treatments “do not have a significant impact on preventing patent infection” across a population.

The reason is timing. A dewormer kills the worms present on the day it’s given; it doesn’t protect against the next infection. After a dog or cat swallows infective roundworm eggs, or eats an infected rodent, it takes “a little over four weeks” before the new worms are adult and shedding eggs of their own. Treat every three months and there is still a window. Treat once a year and the window lasts most of the year. That’s why, when the risk is unknown and can’t be ruled out by testing, ESCCAP recommends at least four treatments a year, and monthly treatment in higher-risk situations, such as a pet that roams freely and shares a home with small children.

Puppies and kittens are a special case, because they are often infected through their mother’s milk or, in puppies, before birth. That’s why the early schedule is every two weeks, and why the nursing mother is treated at the same time as her litter: mothers often develop an infection themselves at this stage.

Around the world

Europe, the US and the tropics: the same parasites, different defaults

The three main expert councils for dogs and cats, ESCCAP in Europe, CAPC in the US and TroCCAP in the tropics, agree on the basics: treat puppies and kittens early and often, and never let adult protection lapse for long. They differ on the default for adults, mostly because of how common the parasites are where they work.

• Puppies

  Europe (ESCCAP)From 2 weeks, every 2 weeks until 2 weeks after weaning.

  United States (CAPC)From 2 weeks, every 2 weeks until year-round broad-spectrum control begins.

  Tropics (TroCCAP)From 2 weeks, every 2 weeks until 8 weeks, then monthly.

• Adult default

  Europe (ESCCAP)Risk-based: 1–12 times a year; at least 4 if the risk is unknown.

  United States (CAPC)Year-round broad-spectrum control covering heartworm, intestinal parasites, fleas and ticks.

  Tropics (TroCCAP)Monthly.

• Faecal testing

  Europe (ESCCAP)An accepted alternative to routine treatment, if done as often as treatment would be.

  United States (CAPC)At least 4 times in the first year, then at least twice a year for healthy adults.

  Tropics (TroCCAP)At least once every 3 months, to check that the control plan is working.

In other words, the American and tropical defaults are more frequent than the European one, not less. None of the three councils suggests once a year for an adult dog with outdoor access.

The risk of skipping

What parasites do, to animals and to people

Roundworms affect people too. ESCCAP describes Toxocara roundworms as having “an elevated zoonotic potential”: if a person swallows infective eggs, the larvae can migrate through the body (larva migrans), with serious consequences for health. The eggs are passed in faeces and “can survive in the environment for years”. Deworming a dog or cat also protects the sandpit, the garden and the family, which is why ESCCAP singles out homes with small children.

How common is human exposure? A 2019 meta-analysis of 250 studies covering more than 265,000 people in 71 countries estimated that 19% of the world’s population carries antibodies to Toxocara, a sign of past exposure. The figure varies widely between WHO regions: 8.2% in the Eastern Mediterranean, 10.5% in Europe, 22.8% in the Americas (12.8% in North America), 24.2% in the Western Pacific, 34.1% in South-East Asia and 37.7% in Africa. Exposure isn’t the same as disease, but every exposure starts with eggs passed in the faeces of an infected animal, and pet dogs and cats are the animals we can treat.

Tapeworms that can kill people. The dog tapeworm Echinococcus granulosus causes cystic echinococcosis in humans: slow-growing cysts, most often in the liver and lungs. The fox tapeworm E. multilocularis, which dogs occasionally carry (and cats very seldom), causes alveolar echinococcosis, which is progressive and fatal if left untreated. The World Health Organization estimates that more than a million people are affected by echinococcosis at any one time, and in parts of Argentina, Peru, East Africa, Central Asia and China, cystic echinococcosis can affect as many as 5–10% of people. In the EU and EEA, 731 confirmed cases were reported in 2022: 41% caused by the dog tapeworm, 25% by the fox tapeworm and 34% by an unidentified species. Reported cases are only a small part of the picture: the same ECDC report cites an ultrasound survey that estimated about 45,000 people with cystic echinococcosis in rural areas of Romania and Bulgaria. Dogs pick up the dog tapeworm by eating raw offal from infected livestock, mainly sheep, and the fox tapeworm by eating infected rodents. WHO lists deworming dogs with praziquantel at least four times a year among the control measures for cystic echinococcosis, and ESCCAP recommends that, where these tapeworms are endemic, dogs with access to offal or livestock carcasses be treated at least every six weeks, and dogs that may hunt and eat small prey monthly.

Heartworm: in Romania, not only in America. ESCCAP’s vector-borne disease guideline describes Dirofilaria immitis as endemic across southern Europe and in Slovenia, Romania and Bulgaria. Spread by mosquitoes, it can cause “severe and potentially fatal disease in dogs and cats”. In dogs it often starts with a chronic cough and can progress to heart failure. In cats, the American Heartworm Society notes that “just one or two worms can make a cat very ill”, and the first sign can be sudden collapse or sudden death. In the United States, heartworm “has been diagnosed in all 50 states”, and the AHS recommends testing every 12 months and prevention 12 months a year. In Europe, prevention is a macrocyclic lactone given monthly through the mosquito season: in southern Europe, ESCCAP advises protection from May until the end of November, and year-round in hyperendemic areas. Treatment, by contrast, is difficult. Dogs need a staged course of injections plus strict rest to reduce the risk of clots in the lungs, and there is no licensed drug to kill adult heartworms in cats.

Fleas and ticks spread disease. In Europe, ticks transmit babesiosis, Lyme disease and ehrlichiosis, among others. Fleas carry the Dipylidium tapeworm and Bartonella henselae, the cause of cat scratch disease. ESCCAP’s ectoparasite guideline advises tick protection for the whole period in which ticks are active, and year-round where the risk of tick-borne disease is high or ticks survive indoors.

The risk of treating

What the side effects really are

Here is the honest picture, taken from official product documentation rather than anecdotes.

For routine dewormers, adverse reactions are rare. Take a widely used dog tablet that combines milbemycin oxime and praziquantel. The UK summary of product characteristics for its chewable form lists every adverse event (vomiting, diarrhoea, drooling, loss of appetite, lethargy, hypersensitivity, and neurological signs such as unsteadiness, tremors or convulsions) under a single frequency category: “very rare”, meaning fewer than 1 in 10,000 animals treated, including isolated reports. Mild stomach upset is the most likely problem. Serious reactions do happen, but they are exceptional.

Flea and tick tablets carry a known neurological warning. In September 2018 the US Food and Drug Administration warned that some dogs and cats given isoxazoline flea and tick products (the class that includes fluralaner, afoxolaner, sarolaner and lotilaner) had experienced muscle tremors, ataxia and seizures. It asked manufacturers to add this to the labels, while stating that the products remain safe and effective for the majority of animals, and asked vets to review each animal’s medical history with the owner before choosing one. If your animal has ever had a seizure, make sure your vet knows.

Three situations where the risk is real and avoidable:

1. Collies and related breeds. Many herding dogs carry a mutation in the MDR1 gene that makes them sensitive to some antiparasitic drugs. Washington State University puts it at about 70% of Collies and 50% of Australian Shepherds, and it also occurs in mixed breeds. The common heartworm preventives have been judged safe for these dogs at label doses, and the Milbemax product information warns that the safety margin is lower in Collies and related breeds, so the recommended dose must be strictly observed. The danger lies in higher doses and in products not made for dogs, such as livestock ivermectin. A simple genetic test settles the question.
2. Dog products on cats. ESCCAP warns that highly concentrated synthetic pyrethroids registered for dogs only “are toxic for cats”. Never use a dog spot-on on a cat unless it is also licensed for cats, and take care in homes with both.
3. Wrong dose, wrong weight, wrong species. Doses are set by weight band and by species. Weigh the animal, use a product licensed for that species and weight band, and never improvise a pet dose from a farm product. If an animal gets into a whole pack of flavoured chewable tablets, treat it as a possible poisoning (see Your pet ate WHAT? Here’s what to do next).

Put the two sides together. The documented risk of a correctly chosen, correctly dosed dewormer is a rare, usually mild reaction. The documented risk of skipping it includes infections that pass to children, a tapeworm disease that affects more than a million people worldwide, and a heartworm infection that is hard to treat in dogs and has no licensed cure in cats.

Horses

Here, less often is the point

Horses are the species where the old advice has been reversed. For decades, horses were dewormed at regular intervals of weeks or months, often rotating products. The result is drug resistance. In the 2024 AAEP guidelines, resistance of small strongyles (cyathostomins) to benzimidazoles and pyrantel is described as widespread, and for foal roundworms (Parascaris), resistance to macrocyclic lactones “should be assumed until documented otherwise”.

The AAEP says plainly: “Do not deworm with fixed intervals year-round” and “Abandon rotational deworming.” The goal is to keep horses healthy, not parasite-free, because trying to eradicate parasites leads to “accelerated development of parasite drug resistance”.

• Adult horses: one or two baseline treatments a year with a macrocyclic lactone, plus praziquantel against tapeworms at least once a year for horses on green pasture, and one or two extra treatments only for horses whose faecal egg counts show they are high shedders.
• Foals: the main target is roundworms. The first treatment is at about 2–3 months of age and the second at about 5 months, with a first faecal egg count at about 5–6 months.

The European guideline takes the same line. ESCCAP’s equine guideline describes selective treatment of adult horses, in which only those that consistently shed many eggs above a threshold (200 eggs per gram, for example) are treated, and says the 4–8-weekly treatments once recommended for foals and yearlings should now be avoided.

That doesn’t make skipping treatment safe. The AAEP lists real dangers: acute larval cyathostominosis, “described with a 50% case-fatality rate in the United Kingdom”; Strongylus vulgaris, “considered the most pathogenic equine gastrointestinal helminth”; and roundworm impactions of the intestine in foals. For horses, the right amount is set by testing, not by the calendar.

Donkeys

Not small horses

Donkeys follow similar principles, but the differences matter. The Donkey Sanctuary recommends treating small strongyles on the basis of faecal egg counts. There is no recognised threshold, but it considers about 300 eggs per gram reasonable in most cases, and it recommends a once-yearly treatment against encysted larvae. Donkeys are the reservoir host for lungworm (Dictyocaulus arnfieldi): they rarely show signs themselves, but the infection can cause severe coughing in horses that share their pasture, as ESCCAP’s equine guideline also notes. Donkeys also appear susceptible to liver fluke.

Many dewormers aren’t licensed for donkeys, so they have to be prescribed by a vet, and The Donkey Sanctuary does not recommend combination dewormers, because there is no data on their use in donkeys. This is one area where borrowing the horse plan is a mistake.

In practice

Five habits that remove most of the risk

1. Weigh the animal before buying the product. Doses are set by weight band.
2. Use a product licensed for that species, and never a dog product on a cat.
3. Tell your vet the breed and the history: herding breeds, previous seizures, pregnancy, illness.
4. Match the frequency to the lifestyle, not to habit. A hunting farm dog and an apartment cat need different plans.
5. For horses and donkeys, test before treating. A faecal egg count costs less than resistance.

The source

“Is this product safe for my animal?”

Deworming questions are exactly the kind that get answered badly online: which product, what dose, for which species, how long before the milk or meat can be used. Lares Pecoris answers them from official documents, among them the ESCCAP and AAEP guidelines cited here and product information sheets, and every answer comes with its source. The dose calculator takes the species, the weight and the medicine and shows the dose from its source document, with contraindications and, for food-producing animals, withdrawal periods. When a question isn’t covered by the sources, Lares says so instead of guessing (more in how we build the corpus).

Questions

Frequently asked questions

Sources

1. ESCCAP Guideline 01: Worm Control in Dogs and Cats, 7th edition, June 2025.
2. ESCCAP Guideline 03: Control of Ectoparasites in Dogs and Cats, 7th edition, January 2022.
3. ESCCAP Guideline 05: Control of Vector-Borne Diseases in Dogs and Cats, 5th edition, December 2024.
4. ESCCAP Guideline 08: A Guide to the Treatment and Control of Equine Gastrointestinal Parasite Infections, 2nd edition, March 2019.
5. American Association of Equine Practitioners. AAEP Internal Parasite Control Guidelines, revised May 2024.
6. The Donkey Sanctuary. Endoparasites in donkeys, quick reference guide for professionals, February 2020.
7. Companion Animal Parasite Council (US). General Guidelines for Dogs and Cats, last updated April 2025.
8. Tropical Council for Companion Animal Parasites (TroCCAP). Canine guidelines: general considerations.
9. American Heartworm Society. Heartworm Basics.
10. Rostami A, Riahi SM, Holland CV, et al. Seroprevalence estimates for toxocariasis in people worldwide: a systematic review and meta-analysis. PLOS Neglected Tropical Diseases 2019;13(12):e0007809. doi:10.1371/journal.pntd.0007809.
11. World Health Organization. Echinococcosis (fact sheet), 17 May 2021.
12. European Centre for Disease Prevention and Control. Echinococcosis. Annual Epidemiological Report for 2022. Stockholm: ECDC; 2024.
13. Veterinary Medicines Directorate (UK). Summary of Product Characteristics: Milbemax 12.5 mg/125 mg chewable tablets for dogs, revised May 2025. Cited as an example of a common dog dewormer, not as a recommendation of a brand.
14. US Food and Drug Administration. Animal Drug Safety Communication: FDA alerts pet owners and veterinarians about potential for neurologic adverse events associated with certain flea and tick products, 20 September 2018 (updated 2019).
15. Washington State University, Washington Animal Disease Diagnostic Laboratory. MDR1 in dogs.

All sources accessed 25 October 2026.