Quick Verdict
Heartgard works. The ivermectin + pyrantel combination has a 30-year safety record and near-perfect heartworm prevention when dosed monthly. The issue is not efficacy -- it is price. US vet pricing ($8-14 per dose) is significantly above what the same product costs through licensed international pharmacies.
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Quick Answers
- Heartgard Plus contains ivermectin + pyrantel -- kills heartworm larvae and intestinal worms
- Near 100% efficacy when given monthly -- clinical studies show 99.6% prevention
- Beef-flavoured chewable -- most dogs take it willingly
- Safe for collies and herding breeds at standard doses
- Canada Pet Care saves 25-40% vs US vet price
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What Heartgard actually does
Heartworm disease is transmitted by mosquitoes. When a mosquito bites an infected animal and then bites your dog, it deposits microfilariae -- immature heartworm larvae -- into the bloodstream. Left untreated, these develop into adult worms over 6 months and lodge in the heart and pulmonary arteries. Treatment at that stage is expensive, risky, and hard on the dog.
Heartgard Plus (ivermectin + pyrantel) works by killing the larvae before they mature. It does not prevent mosquito bites or kill adult worms -- it eliminates the larvae deposited in the previous 30 days. This is why monthly dosing is non-negotiable. Miss a month and larvae from that period can survive and develop.
Efficacy data
From Boehringer Ingelheim's FDA submission: 99.6% efficacy against experimentally induced heartworm infection when dosed monthly. In 27 years of post-market surveillance, there have been isolated reports of breakthrough infections -- but almost all involved documented compliance failures (missed or late doses) or geographic areas with suspected ivermectin-resistant heartworm strains (primarily Mississippi River delta region).
For most dogs in most of the US, monthly Heartgard Plus provides effectively complete protection.
Heartgard vs Heartgard Plus
Original Heartgard contains ivermectin only -- it prevents heartworm but does not treat intestinal parasites. Heartgard Plus adds pyrantel pamoate, which also kills roundworm and hookworm. The Plus version costs marginally more and covers significantly more. There is almost no reason to choose original Heartgard over Plus.
Safety and side effects
Ivermectin at heartworm prevention doses (6 mcg/kg) is well-tolerated by the vast majority of dogs, including herding breeds with the MDR1/ABCB1 gene mutation that makes them sensitive to higher ivermectin doses. The heartworm prevention dose is substantially below the threshold that causes problems in sensitive breeds.
Reported side effects are uncommon and typically mild: vomiting, diarrhoea, lethargy. Serious adverse events are rare at label doses.
The price problem
A 6-month supply of Heartgard Plus for a medium dog (26-50 lbs) costs $50-85 at a US vet or pet store. The same 6-pack from Canada Pet Care typically runs $35-55 -- 25-40% less for the identical product made by the same manufacturer.
Canada Pet Care is a licensed Canadian pharmacy. They source from regulated supply chains. The product is not counterfeit, grey-market, or reformulated. The price difference reflects US distribution markups, not product quality.
Should you switch to Simparica Trio?
If your dog is on monthly Heartgard plus a separate monthly flea treatment, Simparica Trio consolidates both into one tablet at a similar combined cost. It is worth pricing out. If your dog only needs heartworm prevention and does not need flea/tick coverage (indoor dog, low-exposure area), Heartgard Plus remains the simpler and often cheaper option.
For flea and tick coverage separately, see our Bravecto vs Simparica Trio comparison.
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Pros & Cons
Pros
- 30-year safety record with near-perfect efficacy
- Safe for MDR1-sensitive herding breeds at prevention doses
- Beef-flavoured -- high voluntary acceptance
- Covers roundworm and hookworm (Plus version)
- Wide availability through licensed international pharmacies
Cons
- Monthly dosing required -- no quarterly option
- Does not kill adult heartworms (treatment, not cure)
- Does not cover flea or tick -- separate product needed
- Resistance emerging in some Southern US regions
- US retail price significantly above international pharmacy price
Comparison Table
| Product | Active Ingredient | Heartworm | Flea/Tick | Dose Frequency | 6-Month Cost (CPC) |
|---|---|---|---|---|---|
| Heartgard Plus | Ivermectin + Pyrantel | Yes | No | Monthly | $35-55 |
| Simparica Trio | Sarolaner + Moxidectin + Pyrantel | Yes | Yes | Monthly | $75-90 |
| Interceptor Plus | Milbemycin + Praziquantel | Yes | No | Monthly | $38-58 |
Bottom Line
Heartgard Plus is effective and well-tolerated. Buy it through Canada Pet Care instead of your vet -- same manufacturer, same product, 25-40% less. Or consider Simparica Trio, which covers heartworm plus flea and tick in one tablet for a similar monthly cost.