I forgot my dog's heartworm pill. Can I give two doses to catch up?

No. Don't give two at once. The Merck Veterinary Manual's guide for dog owners stresses giving "the prescribed dose of medication at the correct time" and says that if you miss a dose, you should contact your veterinarian about giving the medication. Call the clinic that prescribed it and have five facts ready: the date of the last dose, how many doses were missed, the product name, your dog's current weight and any recent travel. If you've already given two, call right away.

My dog's heartworm pill is a few days late. Is it still OK to give?

Only the vet who knows your dog can say. The American Heartworm Society says that missing just one dose of a monthly medication, or giving it late, can leave your dog unprotected. Even for a few days, call the clinic that prescribed it with the date of the last dose and the product name, and ask whether to give it now and when the next one should be. Write down what they decide, with the date.

I missed two months of heartworm prevention. Should my dog be tested right away?

The vet decides whether and when to test. A test right after the gap can't show an infection from it yet: the Merck Veterinary Manual says tests turn positive at the earliest about 5 months after infection. For two missed months, the American Heartworm Society tells owners to consult their vet, restart monthly prevention right away and retest 6 months later. Its guidelines for vets add a test before restarting. Keep the date of the last dose and the date you restarted.

Does my dog need heartworm medicine in the winter?

For most dogs, yes. The American Heartworm Society, the FDA and the Companion Animal Parasite Council all recommend year-round prevention, and the Merck Veterinary Manual's owner guide says year-round treatment is recommended for most dogs but to check with your vet about the best schedule. If your vet chooses a seasonal schedule, the AHS guidelines note that some products' labels may require continuing for up to 6 months after transmission typically stops, so ask which month is the last.

I forgot my dog's flea and tick medicine. What should I do?

First check how long the product is meant to last: VCA Animal Hospitals notes that some flea products are designed to be given once a month and others once every three months. Then ask the clinic when the next dose should be. If the product also prevents heartworm, treat it as a missed heartworm dose and call with the date of the last one. In Australia, RSPCA Australia advises searching your dog for paralysis ticks daily, even if your dog is on a tick product.

Forgot to Give Your Dog Their Heartworm Pill? What Your Vet Will Ask

It's the 22nd, and you're wiping down the kitchen counter when the calendar on the fridge catches your eye. On the 15th, in small letters, it says "heartworm".

"Did you give it this month?" you ask. Your partner thinks for a second. "I gave it last month. I thought you had this one." So what day was last month's? Neither of you can say for sure.

You'll usually catch a missed daily pill by evening. A monthly one can go a week before anyone notices. Neither of you takes your dog's prevention lightly. It slipped because it only comes around once a month.

If you forgot to give your dog their heartworm pill, don't give two to catch up. Get five facts in front of you: the date of the last dose, how many doses were missed, the product name, your dog's current weight and any recent travel. Then call the clinic that prescribed it and ask whether to give it now and whether your dog needs a test. Those three steps are the whole answer.

Search online and you'll find charts sorted by days missed that don't agree with each other. This post walks through what your vet will ask and decide at each length of gap, then gets to what a household can fix: a monthly dose that nobody clearly owns. For a medicine your dog takes every day, like a heart pill, see our post on a missed daily dose. This post is about dogs; for a cat's preventive, ask the clinic that prescribed it.

This is a guide to keeping records, written by Memofu's developer, not a veterinarian. It is not veterinary advice. Whether to re-dose or test is a question for the vet who prescribed the preventive.

When to call your vet right away

Your dog seems unwell after you restart prevention. The U.S. Food and Drug Administration (FDA) warns that if a dog already has heartworm offspring (microfilariae) in its bloodstream, a preventive may make them die suddenly, triggering a shock-like reaction and possibly death. If your dog seems off after that dose, call right away.

Signs that could mean heartworm disease. The American Heartworm Society (AHS) lists a mild persistent cough, reluctance to exercise, fatigue after moderate activity, decreased appetite and weight loss. It names a sudden onset of labored breathing, pale gums and dark bloody or coffee-colored urine as signs of caval syndrome, a life-threatening blockage that few dogs survive without prompt surgery. If your clinic is closed, call an emergency vet.

You already gave two doses. Don't wait to see what happens. Call, and tell them the product and when each dose was given.

Your dog seems fine. The AHS says there are few, if any, early signs when a dog is infected, so looking healthy after a missed dose doesn't rule it out. If you're unsure, that's reason enough to call.

What should you do first if you forgot your dog's heartworm pill?

Don't give two at once to catch up. Call the clinic that prescribed it, and let them decide whether to give it now, when the next dose should be and whether your dog needs a test.

The Merck Veterinary Manual's guide for dog owners, written by veterinary cardiologist Clarke Atkins, puts it plainly: "If you miss a dose, you should contact your veterinarian regarding administration of the medication." The same guide stresses giving the prescribed dose at the correct time, and two at once is not the prescribed dose. If you've already given two, call right away, as in our guide to preventing double doses.

If you notice at night and the clinic is closed, write the five facts down tonight and call when it opens. If your dog shows any of the signs in the box above, call an emergency vet instead of waiting.

Online, the advice for the same gap changes from page to page. Writing for veterinarians in dvm360, Atkins said there is "no one-size-fits-all answer to this question" and listed what a vet should weigh instead: how common heartworm is locally, whether the dog traveled and how many doses were missed. The FDA's page for owners agrees: "The timing and frequency of heartworm tests depend on many factors."

The answers differ because the facts differ from dog to dog. So gather the facts before you call.

What will your vet ask when you call?

Five things: the date of the last dose, how many doses were missed, the product name, your dog's current weight and any recent travel. The date matters most.

Fact	What the vet uses it for	Where to find it
Last dose	How long your dog was unprotected, and when to test	Your record, the calendar, photo timestamps
Doses missed	How long the gap was	Doses left in the box
Product name	Which label and schedule apply	The box, the pharmacy label, the invoice
Weight	Whether the prescribed size still fits	Your last weigh-in
Recent travel	Whether your dog was somewhere higher-risk	Trip dates

The date of the last dose. The AHS explains that in as little as 51 days, heartworm larvae can molt into a stage that preventives can't effectively eliminate, which is why it says preventives must be given strictly on schedule. Test timing counts from a date too, and you can't count from "sometime in the middle of last month."

Doses missed. The FDA lists whether the owner forgot to give prevention, and for how long, among the factors that set test timing. What's left in the box can give you the count.

The product name. The AHS says oral and topical preventives are given monthly and the injectable every 6 or 12 months, so with the injection there's no monthly dose to miss. Some products also cover fleas and ticks. The FDA lists switching from one type of preventive to another as a factor in test timing too. Keep the box in hand when you call.

Your dog's current weight. The AHS says the dosage is based on body weight, not age, and that a young dog can gain enough to move up to the next dosage range within weeks. It asks owners to have their pet weighed at every visit.

Recent travel. The FDA lists recent travel to an area where heartworm is more common, and the length of the local heartworm season. Mention a summer road trip, and whether your dog is on year-round or seasonal prevention.

If nobody wrote the date down, piece it together: what's left in the box, the timestamp on a photo taken around dose time, the date the clinic or pharmacy dispensed it. If you still can't pin it down, don't guess. Say "sometime in the first half of last month, we're not sure." The vet can work with that too.

A few days, a week or two weeks late: can you still give the heartworm pill?

There's no number of days that's safe for every dog; as Atkins put it, there's no one-size-fits-all answer. Whether to give it late, and when the next dose should be, is for the prescribing clinic to decide, and the fact they'll need most is the date of the last dose. Even a day or two late, don't decide on your own that it's fine.

The AHS says, "If you miss just one dose of a monthly medication—or give it late—it can leave your dog unprotected." Preventives, it explains, work by treating heartworms that already infected the dog within the past month or longer.

Can one missed dose really lead to heartworm? The AHS guidelines tell vets it can: a dog can become infected because of just one skipped or delayed dose, particularly in highly endemic areas.

How much a gap matters depends on when and where. In Atkins's dvm360 example, the risk is much lower when a dose is missed once in Ohio in February than when a three-month lapse happens there in summer, and "just a two-week lapse in summer can result in infection in the Mississippi Delta." That's why the vet wants the date, and where your dog has been.

On the call, the clinic may decide:

Whether to give the dose now.
Whether the next dose goes back to the usual day or is counted from today.
Whether your dog needs a test later, and when.

Write down what they decide, with the date:

Oct 22, 9 pm · heartworm chewable · 7 days late (due Oct 15) · called the clinic · next dose: ___ (clinic's date) · logged by Sam

That line answers the vet's most important question the next time you call, and at the next heartworm test.

Missed a month of heartworm prevention: should your dog be tested right away?

Whether to test, and when, is your vet's decision. But a test right after a missed month can't yet show whether your dog was infected during that gap.

The Merck Veterinary Manual says heartworm tests turn positive at the earliest about 5 months (antigen) and 6.5 months (microfilariae) after infection. Because antigen tests detect only adult female worms, it gives 7 months after the last possible exposure as a reasonable time to test, and applies the same reasoning after a lapse in prevention.

Skip one monthly dose, and by the time you notice, it may be close to two months since the last one, past the 51 days the AHS mentions. To decide which date to count from, the vet needs the exact date.

What the household can do is keep the dates: the last dose, the day you restarted and the test date the clinic set. At your dog's next heartworm test, you want to be able to say, "We missed September and restarted on October 22, as the clinic told us."

Two months or more without prevention: can you just restart with the pills you have?

Don't restart on your own, and don't reach for leftover doses from an old box before you call. The vet may want a test first.

The FDA explains why: giving a preventive to a dog infected with adult heartworms "may be harmful or deadly" (see the shock-like reaction in the box above). The AHS guidelines tell veterinarians that after missed doses, the dog must be tested before restarting and retested 6 months later.

For owners, the AHS answers this exact question on its site: "I have missed 2 months of heartworm prevention for my dog. What should I do?" Its reply: "You need to consult your veterinarian, and immediately re-start your dog on monthly preventive—then retest your dog 6 months later." Heartworms, it explains, must be about 7 months old before the infection can be diagnosed. Whether to test before restarting, and with which product, is the vet's decision.

Leftover doses need one more check: a dog who has grown since that box was dispensed may need a different size.

Does your dog need heartworm prevention all year?

The AHS, the FDA and the Companion Animal Parasite Council (CAPC) all recommend year-round prevention. Your vet sets your dog's schedule, and if it's seasonal, the last doses count as much as the first.

The AHS asks owners to "think 12": test every 12 months and give a preventive 12 months a year, because heartworm has been diagnosed in all 50 states, risk factors are impossible to predict, and infected mosquitoes can come inside. The FDA says, "Year-round prevention is best!" The Merck Veterinary Manual calls dosing all year in all climates the safest approach, since transmission is year-round in tropical and subtropical latitudes and hard to predict elsewhere. The AHS guidelines add that year-round dosing is a partial safeguard when a dose is missed.

If your vet does choose a seasonal schedule, the AHS guidelines say it must begin at least one month before transmission is expected to start and, depending on the product, may need to continue for up to 6 months after transmission typically stops to meet some products' label requirements. The doses after the mosquitoes are gone still matter: as the AHS explains, each one treats heartworms that infected the dog within the past month or longer. Ask your clinic which month is this year's last, and put it on the calendar now.

What if you forgot your dog's flea and tick medicine?

How long a dog can go without flea and tick medicine depends on the product, so first check how long yours is meant to last. Then ask the clinic when the next dose should be. If it also covers heartworm, treat it as a missed heartworm dose.

VCA Animal Hospitals notes that some flea products are designed to be given once a month and others once every three months, and that some are combined with heartworm prevention. If the missed dose was one of those, call with the date of the last dose, as above.

CAPC recommends treating all dogs for fleas year-round, with prevention tailored to the individual pet. In Australia, RSPCA Australia calls a thorough search of your dog's skin and coat at least once a day the most essential protection against tick paralysis, even with a tick product applied, and notes that paralysis ticks may be found all year round in the north. Mention any ticks or fleas you found during the gap.

Five things to agree on before next month's heartworm dose

A monthly dose is easy to forget to begin with, and in a shared household it slips even more easily when nobody clearly owns it. Below are five things to agree on. The first three matter most: name one person, give it on the same day every month, and mark it given where everyone can see.

The Merck owner's guide points out that because the most common preventives are given only once a month, many owners may forget. The AHS guidelines say the data suggest owner compliance remains the biggest factor in the "failure" of preventives, and that "Reminder systems must be implemented" to help owners buy and give doses on time. Two studies of veterinary-clinic sales records, by researchers at Zoetis (which also sells an injectable preventive), found that owners who got monthly heartworm preventives from the clinic bought only about seven months' worth a year, in Australia (counting repeat buyers) and in the United States. Purchases aren't the same as doses given, and doses bought elsewhere weren't counted, but it's a big gap.

Daily and monthly medicines go wrong in opposite ways. With a daily pill, the risk is two people both giving it; that's our post on preventing double doses. With a monthly preventive, each person assumes the other gave it, and nobody does, as in the opening scene. A yearly vaccine slips the same way when its date lives in one person's head; see how to keep track of your dog's vaccinations.

So the fix isn't "try harder." It's deciding who gives it, on which day, and where it gets written down. Go over this once, out loud, with everyone who might give the dose.

Name one person. "Whoever notices" is how nobody does it. Decide who covers when that person is away.
Pick the same day every month. The 1st or the 15th, whatever the household will remember, and set a heartworm prevention reminder for it. On a seasonal schedule, write this year's first and last months on the calendar.
Mark it given right away, where everyone can see. Date and name. The Merck owner's guide suggests posting the dose dates on the fridge door and checking each one off as it's given.
However many phones ring, keep one place to check. Reminders on more than one phone are fine, as long as everyone checks the same record before giving the dose. When every phone goes off at once, two people may both give it, so check the record first, then give it and mark it given.
Write down the misses too. A late dose, a missed one and what the clinic said, in a line like the one above.

Paper works, as long as everyone can see it. Its limit is the front door: it can't answer "did anyone give it?" the weekend your dog stays with your parents.

Where Memofu fits

Memofu has care reminders for vaccine and preventive dates. Here's exactly what they do for a monthly preventive today, and what they don't.

"Heartworm prevention" and "Flea & tick prevention" repeat monthly. Pick either when you add a reminder; it's already set to repeat monthly. For a product given every three months, change it to every 3 months.
On the day, owners and co-owners get a notification. It arrives at the time you set, or after quiet hours for anyone who has them on. It can reach several phones at once, so before giving the dose, check the list to see whether it's already marked done, and by whom. The reminder doesn't stop a second dose. If notifications are off on a phone, it shows only in the app.
Viewers don't get care reminders and can't mark them done. Only owners and co-owners can add or change them, too, so whoever gives the preventive should be a co-owner.
It records who marked it done. The list shows when it was last done and by whom, and Past care lists every completed reminder and who did it. That's where to look when the vet asks for the last dose. The date it keeps is when someone tapped Done, and it can't be set to an earlier day, so tap it as the dose goes in.
A missed month stands out. A reminder that isn't marked done moves to Overdue, with how many days overdue it is. It doesn't send a second notification; the Overdue list is where it shows. If it's marked done late, the next date stays on the usual day of the month, so if the clinic moves the schedule, change the reminder's date.
Some things it doesn't know or decide. It doesn't know your season: a monthly reminder repeats until someone changes or deletes it, so on a seasonal schedule, delete it after the last dose (the repeats stop and past records stay). A dose nobody marked done isn't in the record. Whether to re-dose or test is the vet's decision.
A health link can show the preventive dates. It's free, opens in any browser with no account and expires after 1, 7 or 30 days. Under Vaccines & preventives, it shows "Done" and upcoming "Due" dates, not who marked them. A passed date with nothing marked shows "Not marked done", which can't tell a skipped dose from one nobody checked off.

Care reminders and every health record, including the full history, are free on every plan, and so are health links. The vet-ready PDF is Premium. A full export is free on every plan too, for good. There are no ads, no data sales and no public feed. You can see care reminders on our features page and what is and isn't paid on our pricing page.

A calendar on the fridge works too. One person in charge, the same day each month, and the date and name written down the moment it's given: keep those three and any tool will do.

One last reminder: this is a guide to keeping records, not veterinary advice. When you realize a dose was missed, what you need is the date of the last one and your vet's phone number.

Sources

Heartworm Basics, American Heartworm Society
Heartworm Prevention for Dogs, American Heartworm Society
Canine Guidelines for the Prevention, Diagnosis, and Management of Heartworm (Dirofilaria immitis) Infection in Dogs, American Heartworm Society, revised 2024
Keep the Worms Out of Your Pet's Heart! The Facts about Heartworm Disease, U.S. Food and Drug Administration (Center for Veterinary Medicine)
Heartworm Disease in Dogs, Clarke E. Atkins, Merck Veterinary Manual, Dog Owner Version
Heartworm Disease in Dogs, Cats, and Ferrets, Merck Veterinary Manual
Heartworm prevention: "Oops, I missed a dose!", Clarke Atkins, dvm360, 2016
Heartworm, Companion Animal Parasite Council
Fleas, Companion Animal Parasite Council
Flea Control in Dogs, VCA Animal Hospitals
Retrospective Analysis of Heartworm (Dirofilia immitis) Prevention Medication Compliance and Economic Value in Dogs in Veterinary Practices in Australia, Mwacalimba et al., Frontiers in Veterinary Science, 2021 (funded by Zoetis; authors employed by Zoetis)
Retrospective analyses of heartworm (Dirofilaria immitis) disease and ectoparasite preventive medication compliance in veterinary practices in the USA, Mwacalimba et al., Parasites & Vectors, 2023 (funded by Zoetis; authors employed by Zoetis)
How can I protect my dog or cat from tick paralysis?, RSPCA Australia