Short answer
How often is heartworm prevention given?
It depends on the form: chewables and topicals are usually given once a month, and injectable preventives last six or twelve months. The American Heartworm Society recommends prevention 12 months a year and a heartworm test every 12 months for dogs. Cats have no approved treatment, so prevention matters even more. Follow your vet’s instructions on which product to use and when to start.
Typical intervalPer your vet · usually monthly + yearly test
This guide is about keeping records and reminders only. How often, how much and for how long are decisions for your vet — always follow your vet’s instructions.
How often: it depends on the form
How often you give heartworm prevention depends on the kind of preventive. The American Heartworm Society (AHS) lists three forms:
| Form | Usual interval | Who gives it |
|---|---|---|
| Chewable or tablet | Once a month | You, at home |
| Topical (spot-on) | Once a month | You, at home |
| Injection | Every 6 or 12 months, depending on the product | Your vet |
The exact interval comes from the product and your vet. Some products also cover fleas or intestinal worms; others only prevent heartworm. If you switch products, check whether the interval changes too.
Why year-round
AHS recommends giving prevention 12 months a year, not just in summer. Heartworm is spread by mosquitoes, and because infected mosquitoes come indoors, AHS points out that indoor pets are at risk as well as outdoor ones. In the US, heartworm disease has been diagnosed in all 50 states.
Guidance elsewhere can differ: ESCCAP, the European parasite council, recommends a monthly or long-acting injectable preventive during the mosquito season in areas where heartworm is endemic. That’s exactly why the schedule for your pet should come from your vet, who knows the local risk — then follow your vet’s instructions.
Whatever the schedule, AHS warns that missing just one monthly dose, or giving it late, can leave a dog unprotected.
A yearly test, even on prevention
Dogs should be tested for heartworm every year, even if they’re on prevention all year. Both AHS and the Companion Animal Parasite Council (CAPC) recommend annual testing for dogs, and AHS’s 2025 testing guide notes it now recommends both an antigen test and a microfilaria test each year. The reason: preventives are highly effective but not 100 percent, and a dog can spit out or vomit a pill without anyone noticing.
Before starting prevention, AHS advises:
- Puppies under 7 months can start prevention without a test, then be tested later.
- Dogs over 7 months that weren’t on prevention should be tested first, then retested on the schedule your vet sets.
- Puppies and kittens should start as early as the product label allows, and no later than 8 weeks of age.
Do cats need heartworm prevention?
Yes. AHS points out that there is no approved treatment for heartworm infection in cats, so prevention is the only way to protect them. For screening, AHS prefers using both an antigen and an antibody test; CAPC recommends testing cats before starting a preventive and then as your vet advises, and the latest AHS guidelines encourage routine screening for cats as well. Indoor cats can still be bitten by mosquitoes that get inside.
Common mistakes
- Stopping in winter or giving it only in summer, unless your vet has told you to — AHS recommends year-round prevention.
- Giving it a few days late or skipping a month. That can leave a gap. Call your vet; AHS advises restarting right away and retesting six months later.
- Not noticing a pill came back up. Watch your dog after a dose, and tell your vet if it vomited.
- Splitting doses or sharing between pets. Product size and use come from your vet — don’t improvise.
- Getting the injection and forgetting the next one. Log the date and product the same day and confirm the interval.
- Switching products without recalculating. Moving from a monthly chewable to an injection, or between brands, can change the next due date.
How to remember
Give monthly prevention on a fixed date, such as the 1st of every month, and keep one shared household log: whoever gives it logs it right away, so it never gets doubled or skipped. If two people share the care, check the log before giving a dose rather than asking around. Injections come around only once or twice a year, which makes a reminder even more useful.
Track the annual heartworm test as its own item, or book it alongside the yearly check-up and vaccines. If your product doesn’t cover intestinal worms or fleas, plan deworming and flea control separately. With a spot-on, check the label for how soon you can bathe before or after applying it — see how often to bathe a dog.
Keep track of it with Againday
A monthly dose is easy to second-guess (“did we already do it this month?”), and an injection that lasts six or twelve months is even easier to lose track of.
- Add a new item called “Heartworm prevention” and set How Often to “Your vet’s interval”.
- Set How Often to match the schedule your vet gave you, not a number from this page.
- Give it the tag “Pets” so you can filter it with related items.
- Set its reminder time to 8:00 PM. You’ll get one notification on the day it’s due.
- Put it in a shared list such as “Home”. Whoever does it marks it done, and everyone sees who did it.
- When it’s done, tap the cycle ring to mark it done. Forgot to log it? Add a past date from the item’s page.
- Add a note to each entry, for example: “Ate it all, checked that it stayed down”.
Againday is free for 3 items, with shared lists, widgets and reminders included. Againday Pro is one purchase, not a subscription.
Frequently asked questions
- Can I stop heartworm medicine in winter?
- The American Heartworm Society recommends against it — it advises giving prevention 12 months a year. In Europe, ESCCAP instead recommends prevention during the mosquito season in areas where heartworm occurs, so ask your vet what applies where you live.
- How often is the heartworm injection given?
- Every six or twelve months, depending on the product — those are the two injectable options the American Heartworm Society describes. Log the date and product when it’s given, and confirm the next due date with your vet.
- Does my dog need a heartworm test if it’s on prevention all year?
- Yes. The American Heartworm Society says preventives are highly effective but not 100 percent, and late or missed doses or a pill that gets spat out can leave a gap, so dogs should be tested every year.
- What should I do if I missed a month?
- Call your vet. The American Heartworm Society’s advice is to restart monthly prevention right away after talking to your vet and retest six months later, because heartworms need to be about seven months old before an infection shows up on a test.
Sources
Intervals vary by product, model and situation. When the manual or a professional says otherwise, follow them.
- Heartworm Basics — American Heartworm Society
- A guide to heartworm testing (May 2025) — American Heartworm Society
- CAPC Guidelines: General Guidelines for Dogs and Cats — Companion Animal Parasite Council (CAPC)
- ESCCAP Guideline 01: Worm Control in Dogs and Cats (7th edition, June 2025) — European Scientific Counsel Companion Animal Parasites (ESCCAP)