Revolution (selamectin) begins killing intestinal worms within hours of application, with most roundworms and hookworms dying within 24 to 72 hours. The exact timeline depends on the type of worm, since Revolution works differently against intestinal parasites than it does against heartworm larvae.
How Revolution Kills Parasites
Once you apply Revolution to the skin on the back of your pet’s neck, the active ingredient (selamectin) absorbs through the skin and enters the bloodstream. In cats, blood levels peak about one day after application. In dogs, it takes closer to three days to reach peak concentration.
Selamectin targets the parasite’s nervous system by locking open specific channels in nerve cells, flooding them with chloride ions. This permanently shuts down the nerve signals that control muscle movement, leaving the worm paralyzed. Because the binding is irreversible, affected parasites cannot recover. They die and pass out of the digestive tract, often without you noticing.
Roundworms and Hookworms
For intestinal worms, the killing process starts as soon as selamectin reaches effective levels in the blood and gut tissue. In cats, where absorption is fastest, most adult roundworms and hookworms are dead or dying within the first 24 to 48 hours. Dogs may take slightly longer, closer to 48 to 72 hours, because selamectin takes about three days to reach its highest concentration.
You might see dead worms in your pet’s stool during this window, though many pets pass them without any visible evidence. A heavy worm burden can sometimes cause mild, temporary digestive upset as the parasites are expelled. If your pet has a significant infestation, a second dose 30 days later is typically needed to catch any larvae that have matured since the first treatment.
Heartworm Larvae
Revolution’s role with heartworms is prevention, not treatment of an existing adult infection. Each monthly dose kills the immature heartworm larvae (picked up from mosquito bites in the previous 30 days) before they can develop into adults that lodge in the heart and lungs. This is why the 30-day dosing schedule matters: after that window, the larvae mature past the stage where selamectin can reliably kill them.
Revolution does not kill adult heartworms. If your pet already has an adult heartworm infection, a separate treatment plan is needed to clear those adults. However, continuing monthly Revolution in an infected pet prevents new larvae from developing while the existing infection is managed.
What Revolution Covers
Revolution is labeled for several parasites beyond intestinal worms:
- Cats: roundworms, hookworms, heartworm prevention, fleas, and ear mites
- Dogs: heartworm prevention, fleas, ear mites, sarcoptic mange, and the American dog tick
It’s worth noting that Revolution is not a broad dewormer for every type of intestinal parasite. It does not treat tapeworms or whipworms. If your pet has been diagnosed with those, a different medication is required.
Dosing Schedule and Duration
One application of Revolution provides 30 days of protection. The recommended dose is a minimum of 2.7 mg per pound of body weight, and the product comes in pre-measured tubes sized by your pet’s weight range. Cats and dogs must be at least six weeks old to receive it.
For ongoing protection against heartworm larvae and reinfection with intestinal worms, Revolution should be applied every 30 days year-round (or as directed by your vet based on your region’s parasite risk). Missing a dose or stretching the interval past 30 days leaves a gap where heartworm larvae can survive long enough to become untreatable with a preventive alone.
Why Worms May Seem to Persist
If you’re still seeing worms in your pet’s stool a week or more after applying Revolution, it doesn’t necessarily mean the product failed. A few things can explain this. Larvae that were migrating through body tissues at the time of treatment may have matured into adults after the drug’s initial kill, especially with roundworms that have a complex life cycle involving migration through the lungs. Reinfection from contaminated soil or feces in the environment is also common, particularly in multi-pet households or areas with stray animals. In these cases, a follow-up dose at the next 30-day interval usually clears the new crop of worms. Persistent or heavy infections after two consecutive doses are worth discussing with your vet, as a fecal test can confirm whether the parasites have actually been eliminated.