Medication Guide • Reviewed by Dr. Tiffany Delacruz, DVM
Heartgard for Dogs
Heartgard is a monthly chewable that prevents heartworm disease in dogs. Heartgard Plus also kills roundworms and hookworms.
What Heartgard is used for in dogs
- Monthly heartworm prevention
- Roundworm control (Heartgard Plus)
- Hookworm control (Heartgard Plus)
How it works
Ivermectin disrupts the nervous system of immature heartworm larvae (microfilariae), killing them before they mature into adult heartworms. It does not kill adult heartworms.
Available formulations
Beef-flavored chewable, monthly
Heartgard Plus (with pyrantel)
Side effects to know
Common side effects
- • Rare — most dogs tolerate it with no symptoms
These usually resolve on their own. If they persist past the first few doses, ask your vet.
Call your vet immediately if you see
- ⚠ Neurologic signs (especially in dogs with MDR1 gene mutation — common in Collies and herding breeds)
- ⚠ Severe vomiting or ataxia
Drug interactions
- • Minimal at preventive doses
When not to use
- • Dogs that have not been tested for heartworm — starting prevention in a heartworm-positive dog requires careful veterinary management
- • Dogs with documented ivermectin sensitivity (test MDR1 gene if breed is at risk)
Monitoring during treatment
Annual heartworm test is required before each year's prescription is dispensed.
Need a Heartgard 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 Heartgard prescription, and have it shipped through RexVetRx — all for $64.99 per visit.
Book a vet visit — $64.99Heartgard for Dogs
Frequently asked questions
How long is Heartgard good for once given?
Heartgard provides 30 days of heartworm protection. Give it on the same day each month for consistent coverage.
What if I forget a dose?
Give it as soon as you remember. If more than 6-8 weeks have passed since the last dose, talk to your vet — a heartworm test may be needed before continuing.
Is Heartgard safe for Collies?
Heartgard is safe for Collies at preventive doses. The MDR1 gene concern relates to high doses of ivermectin used for other parasites — not the small preventive dose in Heartgard.
Can RexVet prescribe Heartgard?
Yes, after a current negative heartworm test. RexVet ships monthly through RexVetRx.
Why do I need a heartworm test every year?
Federal labeling and best veterinary practice require it. Giving preventive to a heartworm-positive dog can trigger a severe reaction — the test rules that out before refill.
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.