Medication Guide • Reviewed by Dr. Tiffany Delacruz, DVM
Trifexis for Dogs
Trifexis is a prescription monthly chewable that prevents heartworm, kills fleas, and controls intestinal worms in dogs.
What Trifexis is used for in dogs
- Monthly heartworm prevention
- Flea kill and prevention
- Roundworm, hookworm, and whipworm control
How it works
Spinosad kills fleas within 30 minutes; milbemycin oxime prevents heartworm and intestinal worms.
Available formulations
Beef-flavored chewable in multiple weight ranges, monthly
Side effects to know
Common side effects
- • Vomiting (most common)
- • Decreased appetite
- • Lethargy
These usually resolve on their own. If they persist past the first few doses, ask your vet.
Call your vet immediately if you see
- ⚠ Severe vomiting
- ⚠ Neurologic signs (rare, especially in MDR1 breeds at high doses)
Drug interactions
- • High-dose ivermectin (use cautiously)
- • Other macrocyclic lactones
When not to use
- • Heartworm-positive dogs (test first)
- • Puppies under 8 weeks or 5 lbs
Monitoring during treatment
Annual heartworm test before refill. Give with food to reduce vomiting.
Need a Trifexis refill?
RexVet is the first 501(c)(3) non-profit online vet service in the U.S. Talk to a licensed veterinarian by video, get your dogs's Trifexis prescription, and have it shipped through RexVetRx — all for $64.99 per visit.
Book a vet visit — $64.99Trifexis for Dogs
Frequently asked questions
Should I give Trifexis with food?
Yes — Trifexis is more effective and less likely to cause vomiting when given with a full meal.
Trifexis vs Simparica TRIO?
Both prevent heartworm and intestinal worms; Simparica TRIO uses an isoxazoline for ticks AND fleas, while Trifexis covers fleas only (no ticks). Many vets prefer TRIO for tick coverage.
Why does my dog vomit on Trifexis?
Trifexis has a higher GI side effect rate than some alternatives. Giving with a substantial meal reduces this; some dogs do better on a different product.
Can RexVet prescribe Trifexis?
Yes — with a current negative heartworm test on file, RexVet can ship Trifexis through RexVetRx for monthly delivery.
Does Trifexis kill ticks?
No — Trifexis covers fleas only. For tick protection you need an isoxazoline (Simparica, NexGard, Bravecto) or a separate tick product.
Medical review by Dr. Tiffany Delacruz, DVM
Chief Executive Officer & Lead Veterinarian, RexVet. Licensed in Florida, New York, and Virginia.
Last fact-checked: 2026-05-19
Sources
This page is an educational reference and does not replace veterinary advice. Always consult a licensed veterinarian before starting, changing, or stopping medication for your pet.