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How Long Does Heartworm Medicine Stay in a Dog's System?

Heartworm Medicine

If you give your dog a monthly heartworm pill, you may be wondering how long heartworm medicine stays in a dog's system. Perhaps you cannot recall whether you gave your dog his heartworm pill last week and you are afraid to give another and cause an overdose, or perhaps you are wondering whether your dog is having side effects from a double dose you accidentally gave. Since heartworm pills are given every month, you may assume that the heartworm medicine stays in your dog's system for a whole month, but surprisingly, it doesn't stay around for so long.

Apoquel for dog allergies

Just 24 Hours

The active ingredients of heartworm medication are commonly ivermectin (Heartguard) or milbemycin oxime (Interceptor). Despite the fact the heartworm pill is given monthly to dogs, the heartworm drug stays in a dog's system for about 24 hours, explains veterinarian Dr. Z. Many dog owners may therefore wonder: "If heartworm medicine stays only in the dog's body for 24 hours, how is it possible that it can protect the dog for an entire month?"

This is a very good question. In order to understand the answer, we must take a closer look at how heartworm preventives work. Many owners think heartworm medicine keeps the dog protected for a whole month, while in reality, heartworm medicine works backwards.

In other words, heartworm medications work by killing the larval stages of heartworm that have developed in the past 30 days. Every month you give your dog a heartworm pill you are therefore killing any potential larva that may have developed the previous month.

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About Added Ingredients

And what about heartworm pills that contain other ingredients? Many heartworm pills nowadays not only work in killing the larvae stages of heartworms but also kill intestinal parasites. So what about a heartworm medicine that contains also pyrantel pamoate on top of ivermectin or milbemycin oxime, such as Heartguard Plus, how long does that stay in a dog's system? In this case as well, both drugs are rapidly cleared from the body, and usually within 24 hours there are low appreciable levels of both of these drug in a dog's system, explains Dr. Kara. 

A different story are heartworm medications that also contain added ingredients to kill fleas. For example Trifexis contains milbemycin oxime to kill heartworms but also spinosad to kill fleas. In this case though, the spinosad component lingers for longer, hanging around the dog's body for three to four weeks, adds Dr. Kara.

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About Giving More than Once

One main concern of dog owners is giving a dog too much heartworm medicine. This can happen when dog owners make a mistake, such as forgetting they gave their dog a heartworm medicine a week ago and then giving it again a week later or perhaps a dog owner giving it and then another family member giving it again, perhaps a week later.

In this case, there should be really no harm, even if the dose of heartworm was already given last week. In this case, once again, one must consider that heartworm medicine stays in a dog's system for about 24 hours, adds Dr. Z.

And what about a product like Trifexis, which contains spinosad which is known for hanging around longer? Even though this can sound worrisome, the good news is that Elanco, the company that produces this medication has done a toxicity study and it was found that a dog can safely ingest one dose of Trifexis as often as once a day without causing any harm, explains veterinarian Dr. Matt. 

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About Double Dosages

Even dogs who receive two heart worm pills in just one day, don't seem to be at any particular risk. The amount of medication in heartgard is very low, and a double dose shouldn't cause any problems, other than perhaps a small chance for digestive upset, but even that is unlikely, explains veterinarian Dr. Christian. Doses 10 times the dose found in a heartworm medicine like heartguard, are sometimes even used to treat some conditions in dogs.

And what about dogs receiving a double dose of Trifexis? In this case, once again there is good news. The study by Elanco found that even giving 3 doses of Trifexis at one time was shown to be safe, further adds Dr. Matt.  Perhaps, at the most one may see some gastrointestinal distress such as vomiting, diarrhea, loss of appetite.

Warning: the above situations may be different for collie breeds and some other breeds, which have the "MDR1" gene mutation considering that they are particularly sensitive to invermectin drugs. If you own a collie, Australian shepherd or other breeds with the MDR1 gene mutation, and suspect exposure to invermectin, please consult with your vet immediately. Also, consult with your vet if your dog has an underlying health problem or if you're seeing worrisome symptoms.


"Heartguard is formulated very, very safely so it would take ten times the dose ingested to cause any type of illness... unless he has a history of sensitivity to this medication'" ~Dr. Altman  DVM

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