Dewormer guide — which worm, which drug
Different worms need different drugs. This guide covers what each worm looks like, what kills it, and when to skip the OTC and call a vet. Fecal testing once or twice a year catches most infections before they show symptoms.
The five worms you\'ll see
| Worm | Appearance | How transmitted | Effective drug |
|---|---|---|---|
| Roundworm (Toxocara canis / cati) | Spaghetti-like, 5–15 cm, in vomit or poop | Mother-to-puppy (in utero); ingestion of contaminated soil | Pyrantel pamoate (OTC); fenbendazole (OTC, Panacur C / Safe-Guard) |
| Hookworm (Ancylostoma) | Tiny — rarely visible; diagnosed by fecal egg count | Larvae penetrate skin or ingested in soil | Pyrantel (OTC); fenbendazole (OTC) |
| Tapeworm (Dipylidium) | Flat, white segments around anus or in poop, look like rice | Ingesting a flea (the most common route) | Praziquantel (OTC + Rx); not pyrantel |
| Whipworm (Trichuris) | Not visible; diagnosed by fecal | Ingestion of contaminated soil; eggs survive years in environment | Fenbendazole (OTC); milbemycin (Rx) |
| Heartworm (Dirofilaria) | Not visible; diagnosed by blood antigen test | Mosquito bite | Prevention only with ivermectin / milbemycin (Rx). Treatment of established infection is complex and expensive (multi-month protocol). |
OTC vs prescription — when to use which
Over-the-counter dewormers (pyrantel pamoate, praziquantel combinations) are appropriate for:
- Routine adult-pet "just in case" deworming when fecal testing isn\'t available
- Confirmed light roundworm or tapeworm infection
- Pre-purchase deworming of a rescue pet before vet introduction (with caveat: see your vet within a week regardless)
Prescription dewormers (milbemycin / Interceptor, moxidectin / Advantage Multi, ivermectin combinations) are needed when:
- You need heartworm prevention (always prescription in the US)
- OTC treatment has failed twice — resistance is increasingly documented
- Pet is pregnant or nursing — most dewormers have specific safety profiles for these states
- You need broad-spectrum monthly prevention bundled with flea / tick coverage
Note: fenbendazole (Panacur C / Safe-Guard) is sold over-the-counter in the US at feed and pet stores — it's the first-line OTC drug for whipworm and giardia. Earlier versions of this guide listed it as prescription-only; that was incorrect.
- Heartgard Plus (ivermectin + pyrantel) — covers heartworm, roundworm, hookworm
- Interceptor Plus (milbemycin + praziquantel) — heartworm, roundworm, hookworm, whipworm, tapeworm
- Sentinel Spectrum (milbemycin + lufenuron + praziquantel) — heartworm, roundworm, hookworm, whipworm, tapeworm; lufenuron sterilises flea eggs only (does not kill adult fleas — pair with an adulticide if there's an active infestation)
- Trifexis (milbemycin + spinosad) — heartworm, roundworm, hookworm, whipworm, fleas
- Revolution Plus for cats (selamectin + sarolaner) — heartworm, roundworm, hookworm, fleas, ticks, ear mites
- Bravecto Plus for cats (fluralaner + moxidectin) — heartworm, roundworm, hookworm, fleas, ticks
OTC dewormers you might consider
For routine adult-pet roundworm + tapeworm coverage when fecal testing isn't handy. Heavy infections or unfamiliar worms need a vet.
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Puppy / kitten deworming schedule
Standard CAPC protocol:
- 2 weeks: first dose (pyrantel)
- 4 weeks: second dose
- 6 weeks: third dose
- 8 weeks: fourth dose — usually around the first vaccine visit, so the vet handles it
- Monthly until 6 months: often as part of monthly heartworm preventative which also kills intestinal worms
- Lifetime: twice-yearly fecal testing + dewormer if positive
Routine prevention is cheaper than treatment
A year of monthly heartworm preventative (which also covers most intestinal worms): ~$120–$180. A year of twice-yearly fecal tests + targeted dewormer if needed: ~$80–$140. Treatment of established heartworm disease: $1,000–$2,000+, plus weeks of restricted activity.
Sources: Companion Animal Parasite Council (CAPC) Guidelines for the Diagnosis, Treatment, and Control of Parasites in Cats and Dogs (2024). Bowman DD. Georgis\' Parasitology for Veterinarians, 11th ed. Drake J et al. "Hookworm resistance in dogs." Parasit Vectors (2021).
Frequently asked questions
What worms can my pet get?
Roundworms (Toxocara) — most common, especially puppies and kittens. Spaghetti-like. Hookworms (Ancylostoma) — attach to intestinal wall, can cause anemia. Tiny. Tapeworms (Dipylidium, Echinococcus) — flat, segmented; segments shed in poop look like rice grains. Whipworms (Trichuris) — long-living, hard to eliminate. Heartworm (Dirofilaria) — mosquito-transmitted, lives in heart/pulmonary arteries; lethal if untreated.
How do I know my pet has worms?
Sometimes visible: tapeworm segments around the anus or in poop (look like rice grains); roundworms in vomit or stool (spaghetti-like). Other signs: dragging hindquarters (anal-gland or worm irritation), pot belly in puppies, dull coat, weight loss despite normal appetite, persistent diarrhoea, blood in stool. Many infections show NO outward signs — routine fecal testing at vet visits is the only reliable detection.
Can I just use OTC dewormer?
Fecal test first when possible — different worms need different drugs, and treating with the wrong one delays effective treatment AND drives drug resistance (the #1 cause of CAPC-reported pyrantel failure is repeated OTC monotherapy at the wrong dose for the wrong worm). OTC is second-line: pyrantel + praziquantel (Sentry WormX Plus) covers light roundworm + tapeworm infections in adult pets when a fecal test is not accessible. Fenbendazole (Panacur C / Safe-Guard) is OTC and broad-spectrum (whipworm, roundworm, hookworm, giardia) — useful when whipworm is suspected. Puppies, kittens, pregnant or nursing pets, or any pet with persistent diarrhoea / blood in stool / weight loss: see a vet, not OTC.
Are worms dangerous to humans?
Yes — some are zoonotic. Roundworms can cause visceral larva migrans in children who ingest contaminated soil. Hookworms can cause cutaneous larva migrans (skin penetration). Echinococcus tapeworms (less common, foxes / sheep area) can cause serious hydatid disease in humans. Hygiene: pick up poop promptly, wash hands after pet contact, keep sandboxes covered, and deworm pets routinely.
How often should I deworm?
Standard CAPC (Companion Animal Parasite Council) recommendation for adult pets: routine fecal testing 2× per year + dewormer if positive. Many vets also recommend year-round monthly preventative for dogs in heartworm-endemic areas (which covers most US intestinal worms as a bonus). Puppies and kittens: more frequent — at 2, 4, 6, 8 weeks then monthly until 6 months.
My pet still has worms after treatment. What went wrong?
Three usual causes: (1) reinfection from a still-contaminated environment — yard/litter/bedding wasn't cleaned. (2) Wrong dewormer for the species — pyrantel doesn't kill tapeworms; praziquantel doesn't kill whipworms. (3) Resistance — increasingly documented for some hookworm species. After two failed OTC rounds, see a vet for fecal egg count and a targeted prescription.