Ask the Vet: Help, I Forgot to Give My Dog His Monthly Heartworm Medication!

Dr. Ivana Crnec

If you forgot to give your dog his monthly heartworm medication, you are likely very worried about this mishap, and are wondering whether there may be any scary consequences. Rest assured, you are not alone. Many dog owners forget giving their dogs their monthly heartworm pill.

Forgetting to give your dog his monthly heartworm medication can happen. This is why many brands of heartworm pills have started providing stickers to attach to your calendar as a reminder. However, life can sometimes get in the way and it's easy to forget. Veterinarian Dr. Ivana shares important information on what to do if you forgot to give your dog's monthly heartworm medication. 

A Serious Condition

Heartworm disease in dogs is a serious and life-threating condition that can trigger havoc in the dog’s organism. It is caused by the pesky parasitic worm known as Dirofilaria immitis and transmitted by the even peskier mosquitoes. 

To make matters worse, the treatment of heartworm disease is challenging, costly and not always 100 percent efficient. That is why vets put the accent on preventing the condition rather than treating it. As the saying goes "an ounce of prevention is worth a pound of cure."

 The simplest and most efficient way of preventing heartworm disease in dogs is therefore by giving them monthly heartworm medications. 

A Matter of Convenience 

Giving your dog his monthly heartworm pill can be challenging when life gets in the way, but is this dose frequency of once per month written in stone?

If following the package instructions or your trusted vet’s advice, chances are you are giving your dog his heartworm medication on a monthly basis. As a responsible dog owner, you will therefore stick to that monthly use firmly. However, you probably did not know that the monthly use is planned more for owner convenience and not for addressing the dog’s actual needs. 

Simply put, remembering to give a pill once per months is easy – you can mark the calendar or set a reminder on your smartphone.

The actual situation is that heartworm medications can be given every 45 days. That means every one and a half months. It is true that this is harder to remember. When the frequency is decreased it becomes easier to forget that you have an obligation. 

No matter how serious dog parenting is for dog owners they can still miss a dose when practicing the once-per-month method. So imagine how easy it would be to miss a dose when there is more than a month timeframe between two administrations!

Help, I Forgot to Give My Dog His Monthly Heartworm Medication!

So, let us answer the big question dog owners may be wondering about: what to do if you forgot to give your dog his monthly heartworm medication?

 If you forget to give your dog his monthly heartworm medication, and you are a couple of days late, do not worry. 

Namely, you have a grace period that lasts 45 days. As long as you give the heartworm medication within those 45 days your dog will be completely fine. 

 Do I Really Need to Give My Dog Heartworm Medicine Every Month?

The next question that comes naturally considering the above listed statements, is should I start giving my dog his heartworm medication every 45 days instead of every 30? 

Well, honestly speaking, this is a complicated question with no easy answer. When given at 45 days, heartworm medications are still potent enough to kill most microfilaria. However, you are walking on the borderline which may turn out to be risky, especially in the long run.

In a nutshell, forgetting a dose and giving it within the 45 days grace period is safe, but using this timeframe as a regular guideline does not ensure complete efficacy.

Consider the Guarantee

What is more, many heartworm prevention manufacturers guarantee their products will keep your dog heartworm free as long as the products are used as directed on the label.

 The directions on the label are clear – they state monthly use is advisable. If your dog contracts the disease while on preventative medications, the manufacturer offers to cover the treatment costs. Once again, this is valid for dogs receiving prevention meds every month, year-round and to dogs that are annually tested for heartworms.

 Therefore, if you give the medications every 45 days, the manufacturer’s guarantee is not valid for you and your dog.

All in all, while there are no proven medical drawbacks on delaying the medication administration for up to 45 days (every now and then), keep in mind that most vets recommend adhering to their prescribed schedules, that is giving it every 30 days.

What if I Missed the 45 Day Grace Period?

First of all, no judging – keeping track of everything in life can be challenging. Therefore, missing the 45 days grace period can occur even to the most responsible dog parent. It happens.

 If it has been more than 45 days since your dog received its heartworm prevention medication call your trusted vet.

Being over 45 days late is problematic. That is because the tiny larvae that were supposed to be killed are now growing larger. When the medication is used regularly, every 30 days, it kills the young larvae living in the skin thus preventing them from migrating through the blood to the heart. 

When the medication is delayed for over 45 days it can no longer be capable of killing them. This is because it takes young larvae around 51 days to molt into adults and prevention meds are not powerful against adults. 

Consequently, the young larvae will grow larger and when large enough migrate and inhabit their final destination – the heart. Once inside the heart, they will develop to their full parasitic potential thus causing havoc in the affected dog’s body.

Therefore, we cannot emphasize enough how important calling you vet is in such situations. In most cases, the vet will suggest giving the heartworm prevention medication as soon as possible and then continue giving it following your old schedule. Simply put, fix the error right away and then stick to the usual routine.

Then the vet will recommend having your dog tested for heartworms. This test can be performed at least six months after you missed a dose.

 Dogs cannot be tested for heartworms earlier because blood tests cannot detect infections early infections. Waiting can be dreadful, but performing the test earlier will give negative results and those may be falsely negative. 

Simply put, the heartworms need a 6 months period before they become detectable. That is also the reason why young puppies are not routinely tested for heartworms before reaching the age of 6 months.

The Importance of Consulting With a Veterinarian

It's worthy of emphasizing that the question of "what to do if you forgot to give your dog his monthly heartworm medication?" ultimately does not have a one-size-fits-all answer. This is where the vet steps in. 

Which step should be undertaken, after missing a dose depends on several different factors. The vet will consider those factors and make a well-educated decision. What will the vet suggest depends on:

  • Your living area
  • Your dog’s travelling to certain areas
  •  The type of heartworm prevention used
  • When the lapse occurred
  • How many doses were missed
  • How long was the longest interval between two meds.

Last but not least, we must mention that performing the heartworm blood test, at the right timing, is of paramount importance. A dog with adult heartworms can develop serious problems if given heartworm preventatives. 

About the Author

Dr. Ivana Crnec is a graduate of the University Sv. Kliment Ohridski’s Faculty of Veterinary Medicine in Bitola, Republic of Macedonia.

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