How To Choose the Best All-in-One Heartworm and Flea Pill for Dogs

Natalie Stilwell, DVM, MS, PhD
By Natalie Stilwell, DVM, MS, PhD on Jul. 22, 2019
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Flea and heartworm prevention are two important components of ensuring your dog’s lifelong health.

Fleas can cause skin allergies, transmit tapeworm infection and infest the home environment, while heartworms can cause serious damage to a pet’s heart and lungs.

The Companion Animal Parasite Council recommends year-round prevention of both heartworms and fleas for dogs throughout the United States, even in northern areas.

Currently, there are three all-in-one heartworm and flea pills for dogs, so how do you decide which is best for your pet? And what are the benefits to using these products? Here are some tips you can use to pick the right one.

Ask Your Veterinarian

Your veterinarian can help you decide if the all-in-one heartworm and flea pill is most appropriate option for your dog. They can also help you choose the best product to ensure full protection.

Your veterinarian will be familiar with your dog’s individual needs, as well as the specific parasite risks in your geographic area, so they can give the most effective recommendation.

Review Your Options

Each of the three heartworm and flea pills available (Trifexis, Sentinel and Sentinel Spectrum) require a prescription from your veterinarian, who will first need to perform a heartworm test to ensure that your dog is not already infected. Giving heartworm prevention to an infected dog can cause a severe reaction or even death.


Trifexis is a flavored tablet that contains two ingredients (spinosad and milbemycin oxime) for combined protection against heartworms, fleas and some intestinal parasites (hookworms, roundworms and whipworms).

Milbemycin oxime kills heartworm larvae that are circulating in the blood by impairing their neurologic function. It also works to kill adult hookworms, roundworms and whipworms.

Milbemycin oxime will not kill adult heartworms, however, which is why it’s important that your dog tests negative for heartworm infection before the medication is administered.

The other active ingredient, spinosad, kills the adult fleas on your dog before they can lay eggs. Similar to milbemycin oxime, spinosad targets the parasite’s nervous system. Research studies have shown that spinosad kills all adult fleas within four hours after dosing. 

Trifexis is approved for dogs who are at least 8 weeks old and 5 pounds. This all-in-one product should be given once a month with food.

Vomiting is the most commonly reported side effect and may be seen more frequently in puppies younger than 14 weeks old.


Like Trifexis, Sentinel is a flavored tablet containing milbemycin oxime for prevention of young heartworms, hookworms, roundworms and whipworms.

For protection against fleas, Sentinel contains lufenuron instead of spinosad. This ingredient prevents flea eggs from hatching or developing into adults by impairing the production of chitin, a crucial component of the insect’s exoskeleton.

Even though lufenuron disrupts the flea’s life cycle, this product does not kill adult fleas. This is an important consideration if your dog is flea-sensitive or allergic, in which case, he may still suffer a reaction from flea bites.

If you have a flea-sensitive dog, you may need to supplement another product for adult flea control. 

Sentinel is administered on a monthly basis and is approved for dogs at least 4 weeks old and over 2 pounds. It should be given with food for maximum effectiveness.

Sentinel Spectrum

The final all-in-one product to consider is Sentinel Spectrum. In addition to milbemycin and lufenuron, this product also contains a third ingredient (praziquantel) to prevent tapeworm infections.

Sentinel Spectrum is administered on a monthly basis and is approved for dogs at least 6 weeks old and over 2 pounds. It should be given with a full meal to ensure full absorption of the medication.

Benefits of an All-in-One Heartworm and Flea Pill for Dogs

These all-in-one products offer many advantages.

For one thing, you only have to administer a single medication to protect against several parasites, which makes it less likely that you’ll forget a dose. Your pets will appreciate having to take fewer medications as well.

It can also be more affordable to purchase one medication instead of several.

Tablets are also less messy compared to topical formulations. They don’t need to soak in before your dog swims or gets a bath, and they can’t be rubbed or shaken off after administration.

Topical products may also be toxic if ingested and must soak into the skin for at least two hours before you can allow your pup to be around children and other pets.

Consider the Product’s Limitations

Understanding the limitations and side effects of an all-in-one heartworm and flea pill for dogs can help you have an informed discussion with your veterinarian. Here’s what you should be aware of.

Side Effects in Dogs With the MDR1 Gene Mutation

The three all-in-one oral products all contain milbemycin oxime, which is known to cause side effects in certain dog breeds possessing the genetic mutation MDR1. These include certain herding breeds (such as the Collie, Australian Shepherd, Shetland Sheepdog and Old English Sheepdog) and the Long-Haired Whippet.

Your veterinarian can request a DNA test to see if your dog has this mutation. 

Cautions for Pregnant and Lactating Dogs

The Trifexis label states that the product should be used with caution in pregnant and lactating dogs.

While Sentinel and Sentinel Spectrum did not cause any side effects during lab studies, these products are not officially licensed for use in pregnant and lactating dogs.

No Protection Against Ticks

Finally, none of the oral all-in-one products offer protection against ticks. Ask your veterinarian for advice on choosing a specific tick product.

Alternatively, you can may want to consider using a topical all-in-one product that offers flea, heartworm and tick coverage, such as Revolution. 

No matter which flea and heartworm product you choose, always follow the instructions closely to ensure that you give the correct dose on schedule. Always contact your veterinarian if your dog shows any signs of discomfort or sickness after administration.

Featured Image:

Natalie Stilwell, DVM, MS, PhD


Natalie Stilwell, DVM, MS, PhD


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