Pets and consumer apps
Flea and Heartworm Prevention: What Monthly Really Means
Flea and heartworm prevention explained: what monthly means, why year-round, dogs versus cats, missed doses and a routine you can copy. Ask your vet.
Monthly flea and heartworm prevention means giving your pet the product your vet chose on the same date every month, all year. The American Heartworm Society recommends year round heartworm prevention for dogs and cats, and says a late dose can leave a pet unprotected. If you miss one, call your vet.
Photo by Krista Mangulsone, CC0 1.0, via Wikimedia Commons.
Key points
- "Monthly" is a rhythm: one dose on a steady date, with the product and dose chosen by your vet.
- The American Heartworm Society (AHS) recommends heartworm prevention 12 months a year for dogs and cats.
- Puppies must start heartworm prevention as early as possible and no later than 8 weeks of age, per the AHS dog guidelines.
- The Companion Animal Parasite Council (CAPC) says every pet in the home must be treated before a flea problem comes under control.
- After a missed dose, call your vet before you do anything else.
This is general information, not veterinary advice. Ask your vet.
A monthly dose is easy to forget because nothing visible happens on the day you skip it. This post covers what the guidelines mean by monthly, how dogs and cats differ and how to build a routine. The heartworm parts follow American guidance. If you live outside the US, ask your vet whether heartworm applies where you are.
What does monthly flea and heartworm prevention mean?
It means one dose of a vet recommended product on a regular date each month. The AHS says oral and topical heartworm preventives are labeled for 30 day dosing, and many flea products are monthly too.
Heartworm is a parasite spread by mosquitoes. According to the AHS, larvae mature in the mosquito for 10 to 14 days and then enter a new pet through the bite, and the worms take about six months to reach adulthood. Monthly preventives act on the larvae a pet picked up earlier, which is why the date matters. The AHS also says preventives are highly effective but not 100 percent effective, and a pet can spit out a pill or rub off a topical.
Fleas are different. They are insects, and a flea problem involves your home as well as your pet, which is why the RSPCA says to treat both. CAPC notes that in temperate climates 90 to 95 percent of fleas emerge within 21 to 35 days, and that adults can wait in the cocoon for up to 30 weeks in cool, humid conditions. One missed month can start something that takes a long time to clear.
Some products cover more than one parasite. The AHS guidelines mention broad spectrum products with activity against intestinal and external parasites. Your vet will tell you whether you need one reminder or two.
Why do vets say flea and heartworm prevention should be year round?
Mosquito seasons vary, and a steady routine is easier to keep than a seasonal one. The AHS says to give heartworm preventive 12 months a year.
The AHS canine guidelines say continuous dosing is also a partial safeguard if a dose is delayed or missed, and that year round broad spectrum products likely help compliance. For fleas, CAPC says all dogs and all cats should be treated year round and throughout life.
In the UK the RSPCA says flea season is usually April to November, but fleas can be a problem all year, and in a centrally heated home you may need to treat your pet and home all year round. Ask your vet which product suits your pet.
How is prevention different for dogs and cats?
Both need it, but cats have no treatment option for heartworm, so prevention is the only protection. Dog and cat products are not interchangeable, and some dog products are highly toxic to cats.
On cats, the AHS says the cat is an atypical host and that medications used to treat dogs cannot be used in cats. It recommends year round prevention for all cats, indoor cats included, because mosquitoes can enter homes. It also strongly recommends annual testing. The RSPCA warns that permethrin in some dog flea treatments is highly toxic to cats, so use only a product your vet recommends for your cat.
On dogs, the AHS says puppies must start heartworm prevention as early as possible and no later than 8 weeks of age. A dog over 7 months, or one with no test in the past 12 months, must be tested before a preventive is dispensed. Annual testing continues after that. A puppy started after 8 weeks of age must be tested 6 months after the first dose, and yearly after that. Your vet sets the actual schedule.
| Dogs | Cats | |
|---|---|---|
| Heartworm prevention | Year round (AHS) | Year round (AHS) |
| Testing | Annual antigen and microfilaria test (AHS) | Annual testing strongly recommended (AHS) |
| Starting age | Puppies no later than 8 weeks (AHS) | Ask your vet |
| Flea control | Year round (CAPC) | Year round (CAPC), using a product your vet chooses for cats |
Should I give a monthly or an injectable preventive?
Ask your vet. For dogs, the AHS guidelines list monthly oral or topical products and also an injection that protects for 6 or 12 months.
A monthly product means you remember 12 times a year. An injection means the clinic gives it and you remember the visit. The AHS says the 6 month version is for dogs 6 months and older and the 12 month version for dogs 12 months and older. Which suits your dog is your vet's call.
What if I miss a dose of heartworm or flea prevention?
Call your vet and ask. Do not guess what to give next. The vet may want to test your pet, and when depends on how late the dose was.
The AHS canine guidelines say a dog must be tested for heartworm after known noncompliance with prevention, which makes the next step a veterinary decision and not a home fix. For fleas, CAPC says infestations may take several months to control and every pet in the home must be treated.
How can I build a monthly routine?
Pick one date, put it on a repeating reminder and record what you gave. Here is a template you can copy:
- Choose a date that is easy to remember, such as the 1st or your pay date.
- Write the product name your vet prescribed next to each pet.
- Set a repeating reminder for that date, at a time you are home.
- Tick it off the same day and note the date in your pet's record.
- At each vet visit, bring the record and ask when the next heartworm test is due.
- If a date slips, call the vet's office that day.
My app Pawlarm, which is on the App Store for iPhone, works on the same idea, so I will be exact. When you add a dog, Pawlarm creates two monthly reminders, Flea & tick prevention and Heartworm prevention. For a cat it creates one, Flea & parasite prevention. It does not add a heartworm reminder for cats, but you can add your own. The reminders fall on the day of the month you set the pet up, at 9:00 in the morning, and they sit in a Coming up list. The notification has Done and Snooze buttons, and every reminder can be edited, switched off or added to. Each reminder note tells you to use the product your vet prescribed. Pawlarm never picks a product or a dose, and it cannot prevent anything itself; it only reminds you. There is no account and your pet's details stay on your phone. See Pawlarm on the App Store if you want that. A calendar entry does the job too.
Frequently asked questions
Can I stop flea and heartworm prevention in winter?
The American Heartworm Society recommends heartworm prevention 12 months a year, and CAPC says dogs and cats should have flea control year round. The RSPCA says fleas can be a problem all year in centrally heated homes. Ask your vet what suits your area before you stop anything.
What do I do if I forget a monthly dose?
Call your vet. The AHS says a dog must be tested after known noncompliance with prevention, and that preventives are not 100 percent effective. Your vet will decide whether your pet needs a test, when, and how to restart. Do not guess the next dose yourself.
Do indoor cats need flea and heartworm prevention?
Ask your vet. The AHS recommends year round heartworm prevention for all cats, and says mosquitoes can enter homes. For a cat in a low incidence area or kept indoors, it calls a macrocyclic lactone product a reasonable year round measure. Fleas are a separate question, and your vet can match the product to your cat.
Are flea and heartworm preventives the same product?
Sometimes. The AHS guidelines mention broad spectrum products with activity against both internal and external parasites, but not every product does both, and dog and cat products differ. Your vet will tell you whether you need one reminder or two, which product suits each pet and how often to give it.
How I wrote this
On 15 October 2026 I read the American Heartworm Society canine and feline guidelines (PDFs posted June 2025) and its heartworm basics page, the CAPC flea guideline and the RSPCA flea page. I also checked in Pawlarm's source code which monthly reminders it creates for dogs and for cats.
This is general information, not medical or veterinary advice. Ask your doctor or vet.
By John Osakwe, founder of Etest Tech Hub. I have built apps and websites since 2015, including Etest CBT, FizzFree and Pawlarm.
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Etest CBT, FizzFree and Pawlarm are my own apps. Each has a page with what I built and how.
John Osakwe
John Osakwe has built 100+ apps and websites since 2015, including Etest CBT, FizzFree and Pawlarm. More about John.