Monthly heartworm preventatives do kill heartworms, but only the youngest ones. They wipe out immature larvae that entered your dog’s body within the previous 30 days, long before those larvae can grow into the foot-long adult worms that cause serious disease. If your dog already has adult heartworms living in the heart and lungs, monthly preventatives alone won’t reliably eliminate them. That requires a separate, more intensive treatment.
What Monthly Preventatives Actually Kill
When a mosquito bites your dog, it deposits tiny heartworm larvae just under the skin. Those larvae begin migrating through tissue and developing through several stages. Monthly preventatives work by killing the third-stage and fourth-stage larvae that have been in your dog for less than about 30 days. At that point, the larvae are still small and highly vulnerable. The drugs are 100% effective against these young stages in susceptible heartworm strains.
This is why timing matters so much. If you skip a month or give the dose late, larvae that entered your dog during the gap have extra time to mature. Once they develop past that early window, they become progressively harder to kill with preventative doses. Older fourth-stage larvae and young adult worms are far less susceptible. Think of monthly prevention less as a shield that blocks infection and more as a monthly cleanup crew that eliminates any larvae picked up since the last dose.
Why Preventatives Can’t Reliably Kill Adults
Adult heartworms are a completely different challenge. They live in the pulmonary arteries and heart, can reach 12 inches long, and survive for five to seven years. The low doses of drug in monthly preventatives aren’t enough to kill worms at this stage. Some veterinarians have tried a “slow kill” approach, keeping a heartworm-positive dog on monthly preventatives and waiting for the adults to eventually die off. The American Heartworm Society recommends against this. It can take a year or much longer to see results, the timing of worm death is unpredictable, and the worms continue damaging the heart and lungs the entire time.
How Adult Heartworms Are Treated
There is only one FDA-approved drug for killing adult heartworms in dogs. It’s an injectable arsenical compound that disrupts the worms’ ability to process energy, essentially starving them. The preferred protocol involves three injections spread over about two months: one initial injection, then a rest period of at least one month, followed by two more injections given 24 hours apart. This staged approach is both safer and more effective than giving just two injections. The three-dose protocol kills about 98% of adult worms, compared to roughly 90% with two doses.
The reason for spacing the injections is deliberate. The first injection kills around half the worms. Giving the dog’s body time to absorb those dead worms before killing the rest reduces the risk of a dangerous complication: dead worm fragments breaking loose and blocking blood vessels in the lungs. This is called pulmonary thromboembolism, and it’s the primary danger during treatment.
Exercise Restriction During Treatment
After each injection, your dog needs strict rest for six to eight weeks while the worms decompose and the body clears the debris. This means crate rest, leash walks only for bathroom breaks, and no running, jumping, or roughhousing. Physical activity increases heart rate and blood flow, which pushes worm fragments into smaller vessels in the lungs more forcefully. Dogs that don’t get adequate rest after treatment are at risk of fatal blockages. This rest period is one of the hardest parts of treatment for dog owners, but it’s not optional.
The Susceptibility Gap
There’s an awkward window during treatment where some worms in your dog’s body can’t be reached by either type of medication. Monthly preventatives kill larvae younger than about 30 days. The adult-killing injections work on mature worms. But worms in between, those that are too old for the preventative to handle and too young for the injectable drug to target, fall into what’s called the susceptibility gap. This gap lasts roughly two to three months. It’s one reason the full treatment protocol takes time: your vet may start your dog on monthly preventatives weeks before the first injection, giving younger worms time to either be killed by the preventative or mature enough to be killed by the injectable.
Resistance Is an Emerging Concern
Monthly preventatives have historically been 100% effective against heartworm larvae when given on schedule. That’s no longer guaranteed everywhere. Resistant heartworm strains have been identified in the southern United States, where heartworm transmission is heaviest. In 2023, the first case of a resistant strain was documented in Europe, in a dog that had been imported from the U.S. This doesn’t mean preventatives are useless. They still work against the vast majority of heartworm strains. But it does mean that year-round prevention combined with annual testing is more important than ever, even for dogs that never miss a dose.
Prevention vs. Treatment: The Bottom Line
Monthly heartworm medicine kills larval heartworms before they can establish an infection. It does not reliably kill adult worms already living in your dog. If your dog tests positive, treatment requires a specific injectable drug given over a multi-month protocol, along with weeks of strict rest. The full treatment timeline, including the preventative phase, susceptibility gap management, injections, and recovery, typically spans several months from start to finish. Prevention costs a fraction of what treatment does, both financially and in terms of what your dog goes through.