Spinosad plus Milbemycin oxime

PetMD Editorial
Written by:
PetMD Editorial
Published: December 21, 2012

Drug Info

  • Common Name: Trifexis
  • Generics: No generics available at this time
  • Drug Type: Heartworm preventative and flea control
  • Used For: Heartworm prevention, treatment and control of hookworms, roundworms and whipworms as well as flea prevention
  • Species: Dogs
  • Administered: Tablets
  • How Dispensed: Prescription Only
  • Available Forms: 5-10 lbs, 10.1-20 lbs, 20.1-40 lbs, 40.1-60 lbs & 60.1-120 lbs
  • FDA Approved: Yes, for dogs


Trifexis is indicated for the prevention of heartworm disease and treat and control hookworms, roundworms and whipworms. Trifexis also kills fleas and is indicated for the treatment and prevention of flea infestations.

Dosage and Administration

Trifexis should be given according to your veterinarian’s instructions. It should be given orally once a month at the minimum dose of 13.5mg/lb spinosad and 0.2mg/lb milbemycin oxime body weight. For heartworm prevention, give once monthly for at least 3 months after exposure to mosquitoes. Trifexis for dogs should be given with food for maximum effectiveness. Please monitor your dog for one hour after dosing to ensure your dog does not vomit; if vomiting occurs, please redose with another full dose as the pill may have been vomited up. 

Missed Dose?

If a monthly dose of Trifexis is missed, give the dose as soon as possible with food and continue the monthly dose on the new schedule to minimize the opportunity for the development of adult heartworm infections and flea infestation. Do not give two doses of Trifexis for dogs at once.

Possible Side Effects

Side effects from Trifexis are rare when given at the recommended dose. Possible side effects include:

  • Vomiting
  • Lethargy
  • Decreased appetite
  • Skin irritation (redness, scabs, or scratching)

Please contact your veterinarian immediately if any of these signs are observed. Puppies less than 14 weeks of age may experience a higher rate of vomiting.


Treatment with fewer than 3 monthly doses after the last exposure to mosquitoes may not provide complete heartworm prevention.

Prior to administering Trifexis, dogs should be tested for existing heartworm infection. Use with caution in breeding females. The safe use of Trifexis in breeding males has not been evaluated. Use with caution in dogs with pre-existing epilepsy.

Do not give to dogs that are allergic to either spinosad or Milbemycin oxime.


Store at temperatures between 68° and 77°F. Brief periods of 59° - 86°F are permitted. Keep out of reach of pets and children.

Drug Interactions

There are no known contradictions to the use of Trifexis. Please consult with your veterinarian if your dog is taking any other medications.

Signs of Toxicity/Overdose

Overdose of Trifexis may cause:

  • Vomiting
  • Salivation
  • Tremors
  • Decreased Activity
  • Coughing
  • Vocalization

If you suspect or know you dog has had an overdose, please contact your veterinarian, an emergency vet clinic, or the Pet Poison Helpline at (855) 213-6680 immediately.

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