Internal deworming in dogs helps to control roundworms and other parasites that may live in the digestive tract or other organs. However, there is no one-size-fits-all schedule: age, lifestyle, diet and contact with the outside world all affect the risk.
This guide explains how dogs pick up roundworms, how they are diagnosed and what might happen after treatment. For external parasites — fleas and ticks — the approach is different; see ‘Flea collar or tablet: which to choose’. The decision on which medication to use and how often it should be administered should be made in consultation with your vet.
What are roundworms in dogs?
‘Roundworms’ is the common name given primarily to intestinal ascarids such as Toxocara canis and Toxascaris leonina. Adult worms live in the small intestine and may resemble thick threads or spaghetti.
Many infected adult dogs show no symptoms. In puppies or in cases of severe infection, symptoms may include a distended abdomen, stunted growth, a swollen spleen, diarrhoea, vomiting, weight loss or visible worms in the stools.
How do dogs get roundworms?
Routes of transmission include:
- ingestion of eggs present in soil, paws, objects or food contaminated by faeces;
- ingestion of rodents, prey, offal or contaminated raw meat;
- transmission from the mother dog to her puppies during pregnancy or whilst nursing.
The eggs need to mature in the environment before they become infectious and can persist in the soil for a long time. Clearing up faeces promptly (keeping some biodegradable bags to hand is helpful) and washing your hands reduces the risk of contamination.
How is the infection confirmed?
The vet will assess the dog’s age, diet, travel history, access to the outdoors, hunting habits and previous treatments. A faecal analysis looks for eggs or other signs of the parasite. In some cases, antigen tests, PCR tests or repeat samples may be necessary.
A single negative faecal test does not always rule out infection: the dog may not yet be passing eggs, or may be passing them in small quantities.
How do you treat a dog for worms?
The medication should be chosen based on the suspected or identified parasite, as well as the animal’s weight, age and state of health. Some products only eliminate certain stages of the life cycle, so it may be necessary to repeat the treatment or carry out a follow-up faecal examination.
Do not use human medicines, products intended for other species or improvised doses. Just because two products are sold as ‘dewormers’ does not mean they target the same parasites.
How often should a dog be treated for parasites?
ESCCAP recommends a risk-based plan. A dog that lives mainly indoors and has no contact with prey or raw meat may require less frequent treatment. Dogs that hunt, eat prey or raw meat, live in kennels, travel or live with vulnerable people may require more frequent monitoring.
Where the risk cannot be determined, ESCCAP recommends at least four treatments or faecal examinations per year. Puppies require a specific schedule due to maternal transmission. Your vet should adapt these guidelines to suit your pet.
How long does it take for the worming treatment to take effect?
There is no single answer. The time taken depends on the active ingredient, formulation, parasite and stage of the life cycle. Some medicines start to work within hours; in other cases, the result is assessed over several days or through a follow-up examination.
Eliminating existing parasites does not guarantee permanent protection. The dog may be re-exposed after treatment.
What happens after taking the worming treatment?
- Worms or fragments may appear in the stools.
- No worms may be visible, even when the treatment is working.
- Temporary digestive changes may occur, depending on the product.
- Persistent symptoms may be due to re-infection, another parasite or another illness.
Contact your vet if your pet experiences repeated vomiting, blood in the stools, a very distended or painful abdomen, pale gums, weakness, refusal to drink or eat, difficulty breathing or a rapid deterioration in condition — especially in a puppy.
Frequently asked questions
Does a dog that lives indoors need to be treated for worms?
It may need to. The risk tends to be lower, but eggs can be brought in via soil, footwear or objects. The frequency of treatment should reflect the actual level of exposure.
Does not seeing any worms after treatment mean the treatment has failed?
No. The parasites may have been broken down in the gut and are no longer visible.
Can roundworms affect people?
Yes. Toxocariasis is a zoonosis. The main route of transmission is the accidental ingestion of eggs present in contaminated soil or on contaminated hands, not simply petting the dog. We explain in detail in ‘Who should give their pets worming treatment?
Sources
- ESCCAP — worm control in dogs and cats
- CAPC — recommendations on roundworms
- CDC — toxocariasis
- MSD Veterinary Manual — gastrointestinal parasites in dogs
For routine care alongside worming, we have compiled the options in the dog hygiene section.
Informative content, revised in July 2026. This is not a substitute for a veterinary assessment.