Written By hannah hollinger
Published: 08/07/2020Updated: 08/30/2021
Trifexis for Dogs | Wag!

Trifexis is a 3-in-1 chew formulated to protect your dog against heartworms, fleas, and adult hookworms, whipworms, and roundworms. The drug is given in a palatable, beef-flavored tablet once a month. 

Puppies and dogs 8 weeks and older who weigh at least 5 pounds can be given this medication. To learn about the correct dosage, efficacy, and side effects of Trifexis, read on.


Trifexis for dogs is available in 5 strengths, dependent on the weight of your dog. Each strength has a specific amount of spinosad and milbemycin oxime. Spinosad acts to cause flea paralysis and flea death. Milbemycin oxime protects against hookworm, whipworm, and roundworm. Spinosad + milbemycin works to prevent heartworms.

  • 5-10 lbs – spinosad 140 mg, milbemycin oxime 2.3 mg

  • 10.1-20 lbs – spinosad 270 mg, milbemycin oxime 4.5 mg

  • 20.1-40 lbs – spinosad 560 mg, milbemycin oxime 9.3 mg

  • 40.1-60 lbs – spinosad 810 mg, milbemycin oxime 13.5 mg

  • 60.1-120 lbs – spinosad 1620 mg, milbemycin oxime 27 mg

The recommended minimum dosage is 13.5 mg/lb for the spinosad and 0.2 mg/lb milbemycin oxime per body weight. If your dog is over 120 pounds, your vet will advise the appropriate combination of tablets. For heartworms, give the Trifexis once a month for 3 months after exposure to mosquitoes.

Dosage Instructions

The requirement for parasitic protection is one tablet per month, given with food. Observe your dog for one hour after giving the tablet. If vomiting occurs, contact the vet, who will most likely advise a redosing.

For flea protection and treatment of infection, start the medication one month before flea season begins and continue each month until the end of flea season. If necessary, due to a year-round flea season, Trifexis for dogs can be given continually on a monthly basis.

For heartworms, give monthly within the first month of exposure to mosquitoes. It is essential that treatment be continued at least 3 months after the last seasonal exposure to mosquitoes. Dogs can be given Trifexis year-round if you live in a climate with an ever-present mosquito population.

For treatment of worm infestations, Trifexis is recommended during all seasons. Intestinal parasites are easily contracted at the park, on a walk, and in your own yard.


Trifexis for dogs is effective against all three parasitic types. In a controlled study, Trifexis was 100% effective in eradicating adult fleas (C. felis), and on day 37, it proved to be between 97% and 98%. This once-a-month medication begins working to kill fleas in 30 minutes with complete elimination within 4 hours of giving the tablet to your dog.

As for whipworm, roundworm, and hookworm, the results for efficacy are as follows:

  • Hookworms (A. caninum): 99.8% after 7 days

  • Whipworms (T.vulpis) between 99.6% and 100% after 7 days

  • Roundworms (T. leonina) 93.4% after 7 days

  • Roundworms (T. canis) between 99.6 and 100% after 7 days

Heartworm (D. immitis) protection for dogs given Trifexis was 100%. This efficacy number was reached after three consecutive monthly doses.

Side effects

Typical side effects noted in Trifexis for dogs are:

  • Vomiting

  • Decreased activity

  • Itching

  • Decreased appetite

  • Diarrhea

If vomiting occurs within one hour of administration, redose with a full dose. If you have concerns, contact your vet. Discuss the possibility of breaking the tablet in half, giving each half of the tablet 12 hours apart to avoid the occurrence of vomiting. 


Before administering Trifexis, dogs should be tested for heartworm to rule out an existing infection. When considering any medication, discuss any current vitamins, holistic or homeopathic medicines, or supplements your dog is taking. Other considerations are:

  • Pregnant or nursing females should not be given Trifexis. Studies have observed possible loss of litter, vomiting, and poor weight gain in puppies.

  • Breeding males have not been evaluated.

  • Dogs with pre-existing seizure disorders such as epilepsy should not be given Trifexis.

  • There may be a higher incidence of vomiting in puppies less than 14 weeks of age. 

Drug interactions 

Although spinosad is felt to be safe when used with other flea and heartworm medications, serious adverse effects have been noted with the use of ivermectin (at high doses required to eliminate parasites like demodectic mange) and spinosad alone. The two components of Trifexis for dogs are spinosad + milbemycin oxime. 

Allergic reactions and sensitivity

Caution should be taken to not purchase non-FDA approved medications on the Internet as sometimes large animal formulations are sold as safe for dogs. This can be the case with ivermectin in particular, which in high doses, causes serious adverse effects when combined with Trifexis.

Frequently asked questions

How do I know if my dog has fleas?

If your dog has a flea infestation, you may see:

  • Itching

  • Hair loss

  • Visible fleas, particularly on the stomach and between the legs

  • Flea dirt will turn red when the specks are dropped on a paper towel

  • Restlessness

  • Scabs and redness of the skin

  • Rubbing and chewing at the skin

How do I know if my dog has worms?

Dogs who have roundworms, whipworms, or hookworms may display the following symptoms: 

  • Weight loss

  • Increased appetite

  • Pale mucous membranes

  • Stools may be soft, bloody, or tarry

  • Diarrhea

  • Cough

  • Vomiting

  • Lethargy

  • Swollen abdomen

  • Abdominal pain

  • Roundworms may appear like spaghetti in the stool

How do I know if my dog has heartworms?

Dogs with heartworms may not show symptoms until the later stages of the illness:

  • Mild persistent cough

  • Reluctance to partake in activity

  • Weight loss

  • Fatigue after moderate activity

  • Loss of appetite

Do I need to give Trifexis with food?

Trifexis should not be given on an empty stomach. Doing so increases the chance of vomiting. Taking with food also speeds up the absorption of the medication into the bloodstream.

Does Trifexis treat existing heartworms?

Trifexis is a preventative drug. Adult heartworms are often not discovered until the disease has advanced significantly.

What if I see worms in my dog’s stool during the month after giving Trifexis?

It is normal to possibly see worms being expelled. This means the medication is working. Address concerns with your veterinarian.

Do I need a prescription for Trifexis?

Yes, a prescription is required. The veterinarian will ensure that you receive the correct dosage, safe for your dog, based on weight. Your vet can answer questions you may have on the proper usage of this effective medication.

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