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What to Do if you Forget to Give your Dog its Monthly Heartworm Pill

Dog's Monthly Heartworm Pill

Heartworm disease in dogs is a serious disease caused by mosquitoes and dog owners may be concerned when they forget to give their dogs their monthly heartworm pill. The concerns after all are not unfounded, heartworm disease can wreck havoc to a dog's heart and respiratory system and an ounce of prevention is worth a pound of cure. Heartworm disease can be challenging and costly to treat, especially when caught late; however, the good news is that, if you forgot to give your dog's heartworm pill on time, there are high chances you're still on time to remedy the situation if you follow your vet's guidelines.


A Word about About Dose Frequency

Most package instructions and most veterinarians advise their clients to give their dogs heart worm pills once a month, but the truth is that this once-a-month plan is mostly for owner convenience. Because let's face it, it's not like parasites go around carrying an iPhone with a calendar and schedule, points out Dr. Peter Dobias.

Giving a pill once a month is easier to remember and owners can mark their calendars with handy stickers that can placed on the desired day of the month so they can remember that date every month.

Regardless, the truth is that heartworm pills can be given as far out as every 45 days. However, the problem with giving the heart work pill within 45 days is that, with the dosages so far apart, it can be easier for dog owners to forget compared to once a month. If dog owners already have a hard time remembering to give it monthly, one may imagine how confusing it may feel giving a heartworm pill every 45 days!

So if you forgot to give your dog the heartworm pill and are just a few days late, you should be fine as long as you give it as soon as you remember and are within the" 45 day grace period." Does this mean though that you should start giving the month heartworm pill every 45 days?

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Well, while at 6 weeks or around 45 days, heartworm preventives may still be capable of killing most microfilaria, giving it at this interval would be risking it since you're right at the border of it working or not working, explains Dr. Ralston.

On top of that, consider that most manufacturers of heartworm prevention offer a guarantee that covers treatment costs should the dog contract the disease. This guarantee is valid as long as the heartworm preventive is given year-round as directed by the label and annual testing is carried out. So if you aren't giving your dog heartworm medication monthly, you're missing out on this as the guarantee is not valid, explains veterinarian Dr. Chris Bern.

So if you decide to go the 45-day route, consider that most vets do not recommend it, and it's your responsibility to ensure you strictly adhere to this schedule.

"You can administer heartworm preventive every 45 days instead of every 30 days, but only if this interval is strictly adhered to. If it’s difficult to keep track with a reminder calendar, then your dog may need to stay on the medication every month." ~Dr. Jean Dodds

Being too Late


While being a few days late over the monthly regimen, problems start when you are too late and are over the 45 days grace period. At this point, your best bet is to contact your veterinarian.

What's the problem with being over 45 days? The problem is that now the tiny heartworm larvae are growing larger and larger. When the heartworm pill is given every 30 days, it is capable of killing all the young larvae still living in the skin before they have a chance of travelling in the blood towards the heart, but if you wait just a little over a couple of weeks, they will grow and the heartworm pill won't longer be able to kill them anymore.

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At this point, the heartworms are free to start living in the heart and develop to their full potential: foot-long adult parasites that wreck havoc in the dog's body.

So if you forgot to give your dog his heartworm pill, give your vet a call. Your veterinarian will likely suggest you give the pill right away and then continue giving the medication on the day you’ve always given it. You will then have to get your dog heartworm tested in 6 months if the missed dose is more than two weeks late. Why wait though 6 months later to get the dog tested?

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The reason why a dog cannot be tested immediately is because heartworm blood tests cannot pick up an infection that has occurred in less than 6 months, so there's no point in testing earlier than that, explains veterinarian Dr. Fiona. This is also the reason why puppies are not tested when they are under 6 months of age.

"Unfortunately, in as little as 51 days, immature heartworm larvae can molt into an adult stage, which cannot be effectively eliminated by preventives." American Heartworm Society

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Importance of Consulting With Your Vet

"Help, I forgot to give my dog's heartworm pills two months in a row, now I missed two months, what should I do?" In this case, the American Heartworm Society still suggests to immediately re-start giving the monthly heartworm and then retesting the dog 6 months later, but the American Heartworm Society points out the importance of talking with the vet. Why is this important? There are several reasons and one important one is that your vet may want to take further precautions based on several factors.

The fact is that there is no one-size-fits-all answer to the question "my dog missed a dose of heartworm pill, what should I do?" There are several factors to take into consideration such as the location where the dog lives, if the dog has recently traveled to certain locations, the type of preventive being used (some heartworm products can kill larvae up to a couple of months old), when the lapse occurred and how many doses of hearwtorm preventive were missed.

These are important factors considering that a one-time missed dose in Ohio in February is less risky than just a two-week lapse in the summer in the Mississippi Delta, explains Dr. Clarke Atkins, a veterinarian specializing in cardiology. If the lapse is two months or longer or if the dog resides or visited a highly endemic area, on top of re-instituting the preventive and having a heartworm test done in the next 6 months, the vet may recommend giving doxycycline for one month so to kill the larvae and even some immature adults.

Testing your dog for heartworms is extremely important because giving your dog a heartworm preventive when he has adult heartworms,can cause serious problems and your dog needs proper treatment.


  • DVM360, Heartworm prevention: "Oops, I missed a dose!"
  • A Vet's Guide to Life: Scrimping On Heartworm Prevention?

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